Altitude Life - Progetto di una passerella pedonale fra due grattacieli

Testo completo

(1)Dipartimento dell’Energia, dei Sistemi, del Territorio e delle Costruzioni C.d.L.M. Ingegneria Edile-Architettura. Tesi di Laurea. ALTITUDE LIFE Progetto di una passerella pedonale fra due grattacieli. A.A. 2017-2018 Relatori: Prof. Ing. P. Croce Prof. Ing. J. Wong Correlatori: Prof. Ing. G. Buratti Prof. Arch. E. Bascherini Ing. D. Shook Ing. J. Zhang Controrelatore: Prof. Ing. Z. Jiang. Candidato: Federico Caldi.

(2)

(3)

(4)

(5) ABSTRACT Currently in big cities, metropoleis, towns, an evident trend leads buildings towards a vertical expansion. In downtowns, skyscrapers seem to compete each other to achieve the highest point in the sky, to dominate the skyline in order to be seen as much far as they can. In the common idea this should show how much power, how big, how rich an owner or owners of a skyscrapers are. However, this is not all; in fact, sometime a complex of towers are connected together (horizontal expansion), composing one big connected organism. Such links can be accomplished simultaneously to the skyscraper or after. The title of this dissertation, “Altitude Life”, is not just referred to buildings which are linked to each other but it is even a concept, an idea, a utopia. When in a near future the space will be “crowded with constructions, cars, public transports”, people, how could cities be livable for its citizens? Which subterfuges could be adopted? “Altitude life” is one of the possible answers. In this futuristic scenario a lot of big metropoleis appear on more levels. Through pedestrian walkways, that run until the boundaries of cities, buildings in a determined area would be linked each other, depicting an image similar to a frame that science fiction movies nowadays show. No clear perimeters, the impossibility to define a unique shape since constructions have not their own forms anymore. The word “block”, often used overseas to quantify how far a certain place is, would lose its meaning and maybe the American cities could assume a more European shape where there is no perpendicularity between streets and the towns rise following the best pattern that matches with the surrounding environment. Overall, the purpose of this thesis is not to produce a European model to export in the American towns, but it is to provide the changes on two towers in terms of behavior and stresses when they are linked together and to design a skybridge which connects them. The two buildings are located in the downtown of San Francisco. This should show that it is feasible to make a footbridge among two existing towers even if they have different heights, intended uses, bearing structure types, footprints, materials, etc. However, it is not only a demonstration of what is and what is not possible because studying the correct position of the bridge the behavior of the linked buildings system under earthquake motions or wind excitations can sharply improve. In this way linking two towers could have as a double benefit because it would increase the structural solidity and would growth the safety of pedestrians, that now live in cities where the vehicular traffic are increasingly taking hold. The thesis is so organized. As first an introduction of the existing skybridge has been made. After dynamic studies of the two single and connected skyscrapers, doing the due simplifications, have been discussed. Before to jump in the bridge design process, 3 different solutions have been illustrated (listing advantages and disadvantages). To conclude a footbridge design and the related project drawings have been done. The bridge has been mainly designed according to the AASHTO LRFD Bridge provisions and AASHTO LRFD Pedestrian bridge directions. All of the data has been processed using SAP 2000 CSI (Computers and Structures Inc.) Software. Units of measure are consistent to the United States customary units and MKS system, in order to can be easily read in both cases. Finally a 3D model and some renderings have been made using Rhinoceros+Grasshopper and Cinema 4D+V-Ray software..

(6)

(7)

(8)

(9) 1.

(10) CONTENTS Introduction......................................................................................... 9 Existing Examples............................................................................... 11 Copenhagen Gate by S. Holl Architects.........................................................13 Cable-stayed bridge.....................................................................................................13. Linked Hibrid by S. Holl Architects..............................................................15 Truss bridge.................................................................................................................15. Petronas Twin Towers by C. Pelli Architects.................................................16 Two-hinged arch..........................................................................................................18. The Gate Towers by Arquitectonica...............................................................19 CCTV Headquarters by OMA........................................................................19 Bridge 93863 by P. Johnson and J. Burgee....................................................21 Minneapolis Skyway System......................................................................................21. Bellevue Skybridge by Sclater Partners.........................................................23 Copper Buildings by SHoP Architect’s..........................................................25 Marina Bay Sands by Moshe Safdie..............................................................25 Redevelopment del Grand Central by SOM..................................................27 Further examples............................................................................................28. Guide lines for the skyscraper designs.............................................. 33 Superstructure................................................................................................35 Substructure...................................................................................................42 Foundation options......................................................................................................43 The design process......................................................................................................44. Dynamic problem................................................................................ 49 Examples........................................................................................................61. Intervention site.................................................................................. 67 California history...........................................................................................69 San Francisco.................................................................................................71 History.........................................................................................................................71 Urban development transformations...........................................................................72. Intervention site..............................................................................................73. Pedestrian Bridge design philosophy................................................ 81 Ductility (ηD)...............................................................................................................83 Redundancy (ηR)..........................................................................................................84 Operational Importance (ηI)........................................................................................84 Strength Limit State....................................................................................................84 Service Limit State......................................................................................................84 Fatigue and Fracture Limit State.................................................................................85 Extreme Event Limit States.........................................................................................85. Loads..............................................................................................................85 Load factors and load combinations............................................................................85. Dead load (DC)..............................................................................................87 Live Load (PL)...............................................................................................87 Snow Load (IC)..............................................................................................88 Wind Load (WS)............................................................................................89 Wind design procedure (analytical method)...............................................................92. Other loads.....................................................................................................101 Earthquake action...........................................................................................102. 2.

(11) Seismic Hazard............................................................................................................102 Time History Analysis.................................................................................................105. Load combinations.........................................................................................106. Model designs...................................................................................... 111 Tower 1...........................................................................................................114 Tower 2...........................................................................................................115 Rigid link - Linked Buildings System (LBS).................................................116 Skybridge + Buildings System (SBS)............................................................116. Dynamic characterization.................................................................. 121 Modal Analysis..............................................................................................123 Single towers...............................................................................................................124 Linked buildings system (LBS)..................................................................................126. Skybridge plus buildings system (SBS).........................................................131 Modal time history Analysis.........................................................................135 LBS’ Response spectra................................................................................................135 SBS’ Response spectra................................................................................................139 LBS - Shear sensitivity................................................................................................144 Base shear....................................................................................................................145 SBS - Shear sensitivity................................................................................................148 SBS vs LBS.................................................................................................................152. Concluding remarks.......................................................................................154. Shape of the skybridge....................................................................... 155 Checks.................................................................................................. 163 Superstructure checks.....................................................................................165 Steel profiled decking and composite steel slab checks.................................166 Secondary cross beams checks.......................................................................178 Top chord checks............................................................................................191 Axial compression.......................................................................................................193 Bending stresses..........................................................................................................195 Shear stresses..............................................................................................................196 Fatigue stresses............................................................................................................199. Bottom chord checks......................................................................................201 Tensile stress...............................................................................................................203 Bending stresses..........................................................................................................205 Shear stresses..............................................................................................................207 Fatigue stresses............................................................................................................210. Vertical diagonal checks.................................................................................211 Axial compression.......................................................................................................213 Tensile stress...............................................................................................................215 Fatigue stresses............................................................................................................218. Horizontal diagonal checks............................................................................219 Axial compression.......................................................................................................221 Tensile stress...............................................................................................................223 Fatigue stresses............................................................................................................226. Cross elements checks....................................................................................227 Axial compression.......................................................................................................229 Tensile stress...............................................................................................................231 Fatigue stresses............................................................................................................234. Longitudinal floor beam checks.....................................................................235 Axial compression.......................................................................................................237 Tensile stress...............................................................................................................239 Bending stresses..........................................................................................................240 Shear stresses..............................................................................................................242. 3.

(12) Fatigue stresses............................................................................................................244 Axial compression....................................................................................................... Tensile stress............................................................................................................... Fatigue stresses............................................................................................................. Diaphragm checks..........................................................................................245 Axial compression.......................................................................................................247 Tensile stress...............................................................................................................249 Fatigue stresses............................................................................................................250. Built-up end member checks..........................................................................251 Tensile stress...............................................................................................................253 Axial compression.......................................................................................................254 Bending stresses..........................................................................................................256 Shear stresses..............................................................................................................258 Fatigue stresses............................................................................................................260. Connection and splice designs........................................................... 265 Top chord connections...................................................................................273 First step - bolt designs...............................................................................................273 Second step - end plate design....................................................................................275 Third step - shear checks.............................................................................................276 Fourth step - slip-critical checks.................................................................................277 Fifth step - resistance domain of the joint...................................................................279. Bottom chord connections..............................................................................281 First step - bolt designs...............................................................................................281 Second step - end plate design....................................................................................282 Third step - shear checks.............................................................................................284 Fourth step - slip-critical checks.................................................................................285 Fifth step - resistance domain of the joint...................................................................286. Diagonal connections.....................................................................................289 First step - bolt designs...............................................................................................289 Second step - end plate design....................................................................................290 Third step - resistance domain of the joint..................................................................292. Cross member connections............................................................................295 First step - bolt designs...............................................................................................295 Second step - end plate design....................................................................................296 Third step - resistance domain of the joint..................................................................298. Diaphragm connections .................................................................................300 First step - bolt designs...............................................................................................300 Second step - end plate design....................................................................................301 Third step - resistance domain of the joint..................................................................303. Longitudinal floorbeam connections .............................................................305 First step - bolt designs...............................................................................................305 Second step - end plate design....................................................................................306 Third step - resistance domain of the joint..................................................................308. Skybridge bearing designs................................................................. 313 Pot bearings....................................................................................................315 Design.........................................................................................................................316. Skybridge mounting phases............................................................... 323 Conclusions.......................................................................................... 329 List of Tables........................................................................................ 332 Table References.................................................................................. 336 List of figures....................................................................................... 337 Figure References................................................................................ 341 Bibliographic References.................................................................... 344 4.

(13) 5.

(14) 6.

(15) 7.

(16) 8.

(17) INTRODUCTION In this dissertation, a complete design approach of skybridges is illustrated. Such bridges can be defined as a pedestrian walkway which links two or more tall buildings together. That issue is made more difficult because of the features of both towers. Displacements of single buildings, modal shapes, base shears are some of the characteristics that need to be carefully studied. Also called Skyways, such structures are becoming a valid solution in big cities where the vehicular traffic are assumed a main role inside downtowns. Being located at an important height from the ground level, these links even provide a better overview of the city, especially because skyscrapers do not allow looking over them. Therefore, those connections should represent the effort of the people to recuperate the human dimension in the urban spaces. In literature several examples of Skybridges can be found, but they usually rise up with the tower buildings. Just few of them are built later and have a very short span (about 50 feet). Unlike to the short span of the existing examples, in this paper, a footbridge 118 feet long has been designed. However, in this thesis the necessary reinforcements for the existing structures, because of the bridge induced loads (as self-weight, wind load, seismic load), were not discussed. All of the work is carried out through SAP 2000 software, AutoCAD 2015 and Rhinoceros 5 and the relative layout programs.. 9.

(18) 10.

(19) EXISTING EXAMPLES. 11.

(20) 12.

(21) Nowadays, several skybridge examples connect two or more buildings. Their structural types, the deck features, the materials used are different each other, so it’s not possible to define an univocal skybridge model to follow. As a matter of fact, each pedestrian walkway rises with the purpose to be the greatest solution in the context where it is; not only by the engineering point but even by the architectural point, being a mixed complex of the two subjects. Before to dive into the design process of such bridges and in the study of their behavior, it’s of main importance to study under every viewpoint the existent examples, in order to become familiar to the problem. Every footbridge is different from the other because of the, for instance, height from the ground level, span length and other factors that can exclude a determined structural type in favor of another type. In the next sections, 10 diverse skybridge were analyzed under the architectural and engineering viewpoints, paying attention not just at the link but even at the buildings where it lie, just like as two piers or two abutments. Those examples are organized in two parts: as first the general information of the construction and its elements are provided, while in the second part the skybridge tipology is described (technical characteristics, advantages, disadvantages, span length, particular aspects, etc...). Copenhagen Gate by S. Holl Architects Winner of the international competition in 2008 (building works started in 2016), the Copenhagen Gate is an entirety consisting of two skyscrapers with irregular shape and 100 meters high. These two towers are placed at the harbor entering, of the city above mentioned, as a immaginary door for the tourists on a cruise and for the container ships. Such Picture is made stronger by two cable-stayed bridge that start from the two buildings, going to define the “architrave” of this immaginary enetering.. FIGURE 1.. View of Copenhagen Gate (left side) and one of the project drawings.. The cable-stayed bridge used to connect the two towers is 65 meters high and it is 150 meters long (one of the competition requests was the design of a pedestrian bridge with a specified length).. Cable-stayed bridge The geometry definition of this kind of bridges is a sensitive step, that is featured by several changes mainly due at economic and aesthetic factors (this one in the last decades). The Steven Hool firm chose a fan pattern of the cables, i.e. where the arrangement of the cables changes with the stay (they are not parallel as in the harp pattern), allowing numerous benefits, among which: - the total weight of the cables is substantially less than the harp pattern; - longitudinal inflections of the pylons remain moderate; - greater stability. However, on the other side, there is a problem to build the top of the pylon where cables converge. In fact, an ideal convergence of the cables cannot be concretely realized and for this reason it is necessary to amplify the dimensions of that connection. Doing that, an intermediate solution is obtained between. 13.

(22) the harp and fan patterns, which has the advantages of both the typology and removes the disadvantages.. FIGURE 2.. View of the skybridges and one of the towers.. The lateral suspension system with inclined stay planes is another characteristic that can be deduced from the project drawings presented in the competition. This bridge typology is the most frequently used for these reasons: - improve the stiffness and the stability of the structure; - reduce the displacements of the deck because it makes that the eccentric loads are absorbed by the stays; - get better the aerodynamic stability of long-span decks. This system has the following disadvantages: - the absence of a torsional rigid deck does not allow the reduction of the second order bending moments; - the tension of the stays is not uniformly distributed because the no-torsional rigid deck has a low spread of concentrated loads (cables subject to a larger fatigue load). Although it is hidden inside the buildings, the pylon configuration can be recognized. In fact, it depends either from the suspension system or the structural type. In this project just one vertical pylon is used because all of the cables converge a vertical line long. In this specific case, the deck build methods, that are not clear (supposedly built in steel)and the economy are important factors as well as the others discussed so far. In fact, if a steel deck can be lighter than an equivalent in concrete, on the other side, it can be very expensive. Thus, the self-weight reduction of the bridge should be followed by a saving in other parts of the structures (stays, pylon, foundations) to be competitive against a reinforced concrete deck.. 14.

(23) Linked Hibrid by S. Holl Architects Even this built by Steven Holl Architects, the Linked Hibrid (building works 2013 to 2018) is lie on a site near the ancient walls of the Beijing City. The project born with the intent to promote social interactions inside public spaces. To allow this, a system of skybridges spreads from the 12th to the 18th story, providing a spectacular view of the city. These pedestrian footbridges, which are reflected within the water mirrors, are located 50 meters above the ground level and are a length that fluctuates between 40 and 50 meters. The truss type is the structural type of such constructions. Those bearing structures are backward compared to the huge exterior windows, in order to allow a better dialog with the surrounding environment.. Design of Steel Structures. FIGURE 3.. Overview of the buildings complex.. Truss bridge. Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar. 7.7 Truss bridges. A structural type both economic and efficient, is made up by truss members. Such elements are mainly subject to tension or compression stress. These features make such types of bridge competitive with Design of Steel Structuresbridges that have orthotropic decks (for Prof. small S.R.Satish span), Kumar and Prof. A.R.Santha with box Kumar girders (for moderate span length)and with cable-stayed bridges (for long span). This is possible because the different types of truss can be used to cover a determined span length (for instance, Warren truss is recommended for short span it uses less material). 7.7because Truss bridges. Fig. 7.21 some of the trusses that are used in steel bridges. FIGURE 4.. Truss Girders, lattice girders or open web girders are efficient and. Some truss types.. economical structural systems, since the members experience essentially axial. hence the material is fully utilised. Members of the truss girder bridges The Steven Holl firm, in this project, chose theforces Pratandtruss. As it can be noticed by the Pictures, some can in be the classified as chord members and webthis members. skybridges do not present any diagonal elements middle frame. Probably is dueGenerally, to the the factchord members overall bending in thebetween form of direct that the skybridges have been thought as cantilevers thatresist meet together in moment the middle one tension tower and compression and web members carry the shear force in the form of direct tension and Fig. another one.of the trusses that are used in steel bridges 7.21 some The optimal value of the ratio span length andorwidth of the depends the magnitude of the compression. Duedeck to their efficiency, from truss bridges are built over wide range of Truss Girders, lattice girders or open web girders are efficient and live load which the bridge shall carry. As concern the compressed elements, it is recommended the spans. Truss bridges compete against plate girders for shorter spans, against economical structural systems, since the members experience essentially axial same slender in both planes, in order to do notbox have a principal instability direction. Finally, tense girders for medium spans and cable-stayed bridges for longfor spans. Some of forces and hence the material is fully utilised. Members of the truss girder bridges the most commonly used trusses suitable for both road and rail bridges are can be classified as chord members and web members. Generally, the chord illustrated in Fig.7. 21. members resist overall bending moment in the form of direct tension and compression and web members carry the shear force in the form of direct tension or compression. Due to their efficiency, truss bridges are built over wide range of Indian of Technology Madras spans. Truss bridges compete against plate girders forInstitute shorter spans, against. 15.

(24) members, particular attention towards fracture shall be kept in mind, when joints are designed.. Petronas Twin Towers by C. Pelli Architects Modern expression of the culture, the history and the economical growing of Malaysia, Petronas Towers have been built in Kuala Lumopur city (building works ended in 1996). Until 2014 they were the highest buildings of the World with a height of 450 meters. To give more importance at the towers, a space between them where a skybridge lies. Such bridge is about 170 meters high (between the 41st and 42nd story) and 58.4 meters long and it works just as a link, rising the vertical shape of the two skyscrapers. Because of the impressive height and length, it was chosen to use structural steel, lighter and easier to connect two parts together, to realize the skybridge. In the design process of the footbridge, the complex movements of the towers on the skybridge supports have been considered, included vertical motions due at the tower diverting, the aerodynamic response of the bridge legs with a diameter of 1,1 meters, the fatigue phenomenon, sliding movements, the response at a sudden loss of support and the motions of the exterior cladding. The bridge is supported by a two hinged arch system, where the two spherical bearings have been placed at the ends of the legs to allow the motion of the towers (to avoid an accumulation of tension in the hinges when strong winds occur). For instance, if the buildings sway in different directions, the two hinges rotate on the bearings, keeping the bridge stopped and in position while they move. . . . .

(25) .

(26)

(27)

(28)

(29) . FIGURE 5.. View of the Petronas Twin Towers and Skybridge details.

(30) . .

(31)

(32)

(33)

(34)

(35)

(36)

(37)

(38)

(39)

(40)

(41)

(42) . .

(43)

(44)

(45) .

(46)

(47)

(48)

(49)

(50)

(51)

(52)

(53)

(54)

(55)

(56)

(57)

(58)

(59)

(60)

(61) . To study the wind effects on the behavior of the structures, analytical models, aero-elastic models (1:400 scale) and wind tunnel test have been analyzed. Of particular interest was the response of the long, thin and cylindrical legs of the skybridge under specific wind phenomena. In fact, each leg is formed by a . with

(62)

(63) joined hollow structural section using bolted end plates. They are stiffly linked together a steel box.

(64)

(65) girder which carries the middle part of the pedestrian walkway. Therefore, the four legs are actually disconnected by the other parts of the link and by the towers. It’s interesting to notice that the skybridge has been separately designed from the towers, to study the wind effects, to reduce the amount of data and to make the construction easier. Furthermore, the four lower elements which link the skyscrapers, have been separately studied. The wider responses at the legs (modes 1, 7 and 9) correspond at the four legs which moved, in the first case all in the same way, in the second two in opposite of the two others and in the last case each leg moved in a different way. The differential movements between closer elements require more strength, so they have shorter periods than movements between more separated elements. Because the critic aerodynamic damping depends from the sensitivity of the skybridge at the

(66) . . . 16. ¡ .

(67) vortex shedding, to study this condition an aero-elastic model (scale 1:70) with the legs designed in 7 parts has been used. Of particular interest was the effect to merge the legs to study the vortex shedding response, taking as reference the dynamic behavior of the smokestacks. In the next Picture the main modal shapes (modes 1, 7 and 9) of the legs are showed. . .

(68) . FIGURE 6.. Main shapes of the skybridge legs. modal

(69) .

(70) . In. The phases of the skybridge lifting are particularly interesting. the next lines the nine main steps of

(71) the skybridge lifting are discussed: - the skybridge legs are lifted up one at atime by tower cranes. Once they are in position, control cables are used to lower them over their permanent bearings at level 29;

(72)

(73) are lifted individually;

(74) . - the two end block girder frames of the skybridge the blocks are installed about

(75)

(76)

(77)

(78) 100mm above their final position at level 41; they also retracted about 100mm into the tower to are

(79)

(80)

(81) provide sufficient clearance for the skybridge center section during lifting; - the four lifting jacks located at level 50 of both towers are connected on the bridge center, the other four lifting jacks located on level 48 of both towers are connected to the bridge ends; - the center section which

(82) weighs 325 tons is lifted about 11 meters and restrained, this is to allow the .

(83)

(84) upper 10 meters of the legs to be connected to the girder on the bridge;

(85)

(86)

(87)

(88)

(89)

(90) - after a final check, lifting of the center section commences;

(91)

(92) - at a minimum lifting speed of 12 meters per hour, the center section is gradually lifted to its final level; - steps seven to nine took about two weeks; a temporary connection secures the center section and the end block girder frame together to ensure there is no stress; - the legs are moved into place; when the legs are in their final position, the skybridge end blocks are lowered on their permanent bearings at Level 41; the center section is then lowered to meet the legs; - after the lifting system has been removed, the floors were concreted, the skybridge roofed; the maintenance equipment is set up on stainless steel rails on top of the bridge. At about 7.40 p.m., on 9 August 1995, after 32 hours of lifting operation, the skybridge was lifted to its final position at the 41st and 42nd levels of the Petronas Twin Towers. . . . . 17.

(93) FIGURE 7.. Nine steps of the skybridge lifting (starting from the left side at the top) .. Two-hinged arch The structure type used for Petronas’ skybridge is the two hinged arch. This kind of structures are made so that the acting loads are predominantly compressive. Such condition may be just for fixed loads while under live loads flexural stresses always occur. Being one time hyperstatic, that bridge tipology is featured by a reduction of the thrust than the three hinged arch. This is due at the elastic shortening of the arch. However, bending moments resulting from the dead load can affect the structure (axis line designed according to the loads of the funicular). The thrust fall is greater as much as the arch is lowered (arrow-span ratio). Their negative effects may be in part eliminated designing the axis line different from the pressure curves of the dead loads. Every arch type, being a structure mainly compressed, is subject to instability phenomenons in its plane and out of its plan. It’s important to notice that so far as in the deck does not exist joints, its bending stiffness is no longer negligible, the loads shall be brought in part from the deck and in part from the arch (as in the Petronas Twin Towers). In this case the arch designing process can be conducted considering the structure as a flate frame and using any automatic calculation program.. 18.

(94) The Gate Towers by Arquitectonica Gate Towers (ended in 2013), with their single shape, allude to a future where the skyscrapers will be so dense which all the services at the ground floor can be replicated in height. They are made up by a series of towers that work as columns to sustain the skybridge, a big architrave which creates a monumental portal. An oval courtyard which is located to the back side of the towers works as an entering. The towers are made of reinforced concrete. Even the flooring system is made of reinforced concrete while for the upper stories a system of post-tensioned slabs is used. Reinforced concrete columns and shear walls are used to transfer the floor loads at the foundations.. FIGURE 8.. Frontal view of Gate Towers.. The element symbol is surely the skybridge which is 238 meters high and about 300 meters long (the distance between one tower and another one is around 85 meters). The bridge links the three towers and goes cantilever over the last one for 45 meters (positioned from above using a bridge crane). The bearing structure of the bridge consists of two main truss, one on each side, carrying loads in the circumferential direction of the structure. Several computer models and in-depth checks have been made to study the structural motions, modal shapes, exterior cladding movements in order to define each possible wind and seismic loads combo. The portion of the skybridge up to the towers has been built above the 63rd story while the parts between the towers have been lifted from the podium at the 5th story. To put in position the skybridge, which has a weight of 750 tons, jacks located at the 64th level (238 meters) have been used. Such lifting is lasted, for each section, about three days with a speed of 18 meters for hour. All the jacks have been synchronized to ensure the safety of the lifting. The extreme precision and the accuracy have been key factors to ensure that the lifted portion of the skybridge fit perfectly in its place.. CCTV Headquarters by OMA Ended 2012, CCTV (China Central Television) is located in the Financial District of Beijing, China. Unlike the other skyscrapers that growth in height using just two dimensions, this building 240 meters high uses 3 dimensions instead. The innovative and iconic shape of the building (ring with two big inverted L bodies that meet at the top) ends in a perpendicular cantilever of 75 meters. The singular structure is completely realized in steel to decrease the total weight and improve its seismic behavior. The main support of the structure is provided by an irregular grid visible on the facade which it allows a clear expression of the stress load distributions. It is made up by a network of diagonals which it becomes denser in the areas where the stress is bigger and it is softer where the loads are lower. In that way, the facade assume the function of a clear show of the internal structure. This type of structure even allows creating new alternative loads distributions, providing when a repair is necessary a rapid removal of a column to make the due repairs. The ring shape of the building mixed to the inclined facades need huge internal spaces that brought the introduction of truss members in strategic points of the skyscraper. Such truss elements were introduced. 19.

(95) to cover the long span and bring greater loads. These transfer trusses are typically two stories deep and located in plant floors so as to be hidden from view and to minimize the impact on floor planning. The sizes of the transfer trusses mean that they could potentially act as outriggers linking the external tube to the internal steel cores - undesirable as this would introduce seismic forces into the relatively slender internal cores. The transfer trusses are thus connected to the internal cores and the external columns at singular ‘pin-joint’ locations only.. FIGURE 9.. View of Headquarters.. At 36 stories above the ground, the two leaning towers crank horizontally and cantilever 75 m outwards in the air to join together forming the continuous loop defining the building shape. This 75 m cantilever structure encompasses 13 stories and is known as the “overhang”. The overhang floor is supported by columns landing on transfer trusses. These trusses span the bottom two stories of the overhang in two directions connecting back to the external tube structure. Thus the overall overhang structure is ultimately supported off the external tube structure. As the design of the CCTV building is well outside the scope of the prescriptive Chinese Standards, a performance-based design approach was used. Performance targets for the building at different levels of seismic events were set by the Arup design team. The performance objectives set out that: - when it is subjected to the design frequent earthquake (level 1) with an average return period of 50 years (63% probability of exceedance in 50 years), the building shall not sustain structural damage; - under the design intermediate earthquake (level 2) with an average return period of 475 years (10% probability of exceedance in 50 years), the building may undergo repairable structural damage; - when it is subjected to the design rare earthquake (level 3) with an average return period of 2500 years (2% probability of exceedance in 50 years), the building is permitted to sustain severe structural damage but must not collapse; Obviously such checks gave back a positive reply, allowing at this individual building to rise in the Chinese metropolis.. 20.

(96) Minnesota Department of Transportation (MnDOT). Bridge 93863 by P. Johnson and Abridged J. Burgee Local Historic Bridge Report-. Located in the city ofSummary Minneapolis this bridge (ended in 1969) is the firstBridge of a long series of93863 pedestrian Executive Number: walkways which spread in the metropolis. It’s placed between the 7th and 8th over Marquette Avenue th th Bridge 93863, a skyway located 7 and 8 Streets South with over Marquette Ave South, South, linking IDS Center (made withbetween this skybridge together) Baker Center. Atconnects the same time the IDS Center and the Baker Center in Downtown Minneapolis. The skyway was constructed c.1969 others three bridges have been built (93866, 93864, e 93861). The 93863 is a Vierendeel trussasbridge part of the larger IDS Center construction (1969-1973). This skyway is significant as a contributing type (prevalently it is used in mixed structures where there is an interaction between steel and reinforced feature of the National Register eligible Investors Diversified Services Center (IDS Center). The IDS concrete) with single span, aaslength of 37 meters and width of 6,40 The footbridge presents a Center is significant a premier multiple-use blockadevelopment whichmeters. molded the late-twentieth-century Minneapolis skyline and set the stage for the bi-level downtown Minneapolis business and concrete deck. Thirteen ribbon windows are located on the east and west side faces. retail The district lowerofpart of today and as the work of a master, Philip Johnson. the bridge is covered by uniformly spaced tiles and lights. The interior consists of a carpeted bridge with railings along thethis eastbridge and iswest sides of the Matched modular align with the and ceiling. A Since privately owned, anbridge. engineering assessment for theskylights structure’s current condition heating system runs along the baseandofmaintenance the east and west sides of the bridge. stabilization, preservation, needs was not performed.. FIGURE 10. View of Bridge 93863.. Minneapolis Skyway System At least other 16 cities on North America have skyway systems that link more buildings together, but the Minneapolis system is the most developed. Started in 1969 with the first skyway, it has quickly spread coming to count 70 bridges only after 40 years. After a first initial skepticism, the first skybridge between Northstar Centre and Marquette has been built. The future users almost immediately understood the benefits of that link whip creates a safe pedestrian space from the frantic traffic. The skyway systems of Minneapolis are a pedestrian movement system of second level that link offices, markets, restaurants, residential areas and parking lots. This network is made up of over 70 bridges which link almost every building in the downtown area. Except of connections between public buildings and parking lots, the most part of skyways have private owners. To address the decision there are, a code JANUARY 2014 about the skyway standards (it has been developed by a committee of 19 members) and procedures of skyway systems that helps developers and architects in the design process of such bridges. All of this ensures the correct development of new skybridges. Further at the guide lines subsequently exposed, all of the corridors, tunnels and connections are subjected at the Ordinance Codes of Minneapolis and at all of the pertinent building codes. In the next part, the most important guide lines of the skyway network of Minneapolis are discussed. “... - skyway bridges shall be developed at only the second level of the building; - except where crossing streets and alleys, skyway bridges and corridors shall be located within the property lines and shall not encroach over public right of way; - all skyways should run perpendicular to the sidewalks, streets, and alleys that they cross;. 21.

(97) - where ever possible, all skyways should cross sidewalks, streets, and alleys in the middle portion of the block rather than at the extreme ends of the block; - minimum and maximum height above the Street or Alley: the bottom of skyway bridges must be a minimum of 16 feet - 6 inches (16’ 6”) above the street or alley; - minimum and maximum width: Skyway bridges and corridors shall have a minimum interior clear width of 12 feet (12’ 0”) between hand rails; skyways shall be no wider than 30 feet (30’ 0”); skyway width (interior clearance between handrails) should be carefully considered in relation to each skyway relative location within the system, and the projected intensity of sue for that skyway; wherever possible, skyway ridges and corridors within the Downtown Core should have a minimum interior clear width of 18 feet (18’ 0”) between hand rails; - horizontal and vertical alignment: in elevation, skyway bridges shall be designed to be horizontally parallel to the street surface and vertically perpendicular to adjacent buildings; changes in grade shall be accommodated within the skyway such that the skyway appears visually level from the exterior; in plan, skyway bridges shall be designed to be perpendicular to tech alignment of the street or alley they cross. - transparency: skyway bridges shall be designed to be transparent in order to provide views into and out of the skyway; clear glass shall be used for the purpose of maintaining security within the skyway system; - skyway corridors shall be designed to facilitate clear and easy access between street and skyway levels; elevators, stairs, and escalators linking the street and skyway levels shall be located in such a way as to provide convenient, visible links to the skyway level from the adjacent street and sidewalks; - skyway bridges and corridor (including but not limited to doors, ramps, and signage) must meet the Americans with Disabilities Act (ADA) requirements;. . . . .

(98).

(99) FIGURE 11. Map of the skybridge networks of Minneapolis.. that incorporate interior construction - comprehensive design solutions grade changes through the of

(100) all-purpose ramping are favored over design solutions that include stairs and chair lifts;

(101) - all access doors between skyway bridges and the corridors of new buildings shall be power operated and shall open in a sliding, side-to-side direction; - all access doors between skyway bridges and the corridors of existing buildings shall be power . operated and should open in a sliding, side-to-side direction; in cases where a new skyway connects to. 22. . .

(102).

(103)

(104)

(105)

(106)

(107)

(108)

(109) .

(110) an existing building and side-sliding doors are not possible, power activated doors shall open in the prevailing direction of pedestrian traffic; - all access doors between skyway bridges and building corridors may include hold-open features which are integrated with the fire/emergency systems in the buildings that are adjacent to a given skyway bridge; - all building doors leading from skyway level corridors to street-level building doors and all doors between skyway bridges and building corridors may include hold-open features which are integrated with the fire emergency systems; - heating and ventilation: skyway bridges shall be heated to a minimum of 55 degrees Fahrenheit in winter and ventilated not to exceed outdoor temperatures in summer. - external lighting: if street lights are removed for the construction of a skyway, replacement street lighting must be provided on the bottom of the new skyway; lighting should be replaced so as to provide the same level and character of illumination as street lighting in the surrounding block; - internal lighting: internal lighting should be consistant and seamless between skyway bridges, corridors, and vertical circulation elements (elevators, escalators, and stairs) in order to eliminate sharp contrasts in lighting levels; - directional signs and signs indicating hours of operation (approved by the Minneapolis Skyway Advisory Committee) shall be incorporated in the design of skyway bridges and corridors; - building entrances shall include clear directional signage as per the Skyway Signage Program; - elevators, stairs, and escalators linking the street and skyway levels shall include clear directional signage as per the Skyway Signage Program; - all buildings that incorporate new skyways into the system are strongly encouraged to install the standardized electronic information kiosk for the Skyway System on the skyway level of that building; - skyway bridges shall be used exclusively for pedestrian movement; other uses such as retailing, permanent seating, vending and display shall be confined to spaces off the skyway bridge; - some skyway bridges and corridors may be used for seating and public gathering during special events so designated by the Skyway Advisory Committee (such as the Holidazzle Parade); all uses of skyways for such events will adhere to the public standards outlined by the Minneapolis Fire Marshall; - skyway bridges and corridors shall remain open during the following hours: Monday to Friday.......6:30 a.m. - 10:00 p.m. Saturday....................9:30 a.m. - 8:00 p.m. Sunday......................Noon - 6:00 p.m. - property owners are encouraged to keep their skyway bridges, corridors, and vertical circulation elements open beyond standard hours of operation, particularly during the holiday season ...” [Minneapolis Skyway Standards - Standards and Procedures Manual]. Bellevue Skybridge by Sclater Partners With the completion of the new Lincoln Square Mixed-Use Facility in downtown Bellevue, Washington, the owner requested permission from the city to install the first elevated pedestrian bridge in the downtown core area. Located at one of the busiest intersections in the city, the new steelframed and cable-stayed structure has a span of 107 feet. This covered bridge provides easy and safe access between the two major retail and parking areas. Because of the bridge high visibility, the owner specifically requested that the architecture and engineering firms provide a design that was thoroughly modern, transparent and comfortable to its users, without vibration or movement most common to cablestayed structures. In addition, the design had to address the extremely busy, 5-lane arterial below, the existing occupied buildings at each bridge end, and the associated construction challenges. The Sclater Partners architectural design team worked closely with ABKJ structural engineers as they developed a lightweight and open cable-stayed bridge, suspended by two 4 column trunks to support the splayed bridge cables. The “Y”-shaped curved side columns, spaced at 14 feet along the bridge sides, provide unusual and interesting “tree” supports to the upper roof structure. The entire roof area is covered with curved, transparent polycarbonate panels supported by curved and sloped diagonal roof frame pipe. 23.

(111) members. In order to prevent the “Y” columns from creating a longitudinal truss and still maintain their delicate profiles, the entire roof framing is connected to the Y-columns with bolted “pin” connections. Slotted bolt holes allow for both thermal and bridge bending displacement at each column connection. The cables were designed without a pretensioning requirement and were tightened to support partial bridge dead loads plus full 100 psf (4,8 kN/m2) live loads and 25 psf (1,2 kN/m2) snow loads, to suit human comfort and for the bridge dynamic response. Resistance to transverse seismic and wind forces is provided by the “Y” column and bridge diagonal roof pipe frames to deliver transverse forces to the horizontal bridge deck diaphragm, with final distribution to the single “tree trunk” cantilevered columns located near each end of the bridge. Because of the complex geometry, with 3 dimensionally curved members throughout, 3-D AutoCAD computer modeling was employed to develop precise structural steel shop drawings to ensure accurate fit-up. Almost 90% of the bridge members are curved, sloped or skewed in one or more planes.. FIGURE 12. View of Bellevue Skybridge in Lincoln Square. The erection of the 65 tons bridge skeleton is particularly interesting because it was accomplished after midnight on a weekend, using two mobile cranes. Because of the great fit-up between bridge and the trunk columns, the entire erection procedure was finished well before daylight allowing for early car traffic below. Placement of the bridge deck concrete topping slab, roof panels and glass railings followed quickly afterwards.. 24.

(112) Copper Buildings by SHoP Architects Located on the East side of Manhattan, American Copper Buildings, designed by SHoP Architects of New York, tests the edges of engineering. The skybridge, which is 91 meters above from the ground level, is the first important of New York in about 80 years. However, it is not just a link between the two towers; the bridge even transfers electricity and water among the buildings. The complex has been ended in 2016. With a length of 30 meters, the skybridge has three stories, a swimming pool and one bar. The exterior cladding is made of copper. Starting in April 2016, such exterior cladding has begun to pat and at the end of the process the structures will change color. The two towers are designed in such a way that they seem to “dance” with each other.. FIGURE 13. View of Copper Buildings.. Marina Bay Sands by Moshe Safdie One of the most important projects of Singapore is Marina Bay Sands, a multifunctional resort 38 hectares large, with three towers 200 meters high (ended in 2010). The two skyscrapers are slightly skewed, but they stand on straight supports which give at the complex a unique appearance. Though they are independent each other, the three towers are connected at the 23 level by a skyways. This link is 200 meters high above the city. The bridge is 1,2 hectares large, enough to hold more than four A380 airplanes, and it has one of the most cantilever in the world (67 meters). The space is probably better for restaurants, café and a swimming pool 150 meters long. Therefore, it configures as a panoramic bridge that offers a 360 grades view of the Singapore skyline and of the surrounded waters. Furthermore, the sloping geometry of the towers required the use of a system of cables with uprights and tie rods to support the walls during construction. These were removed after the installation of large trusses that connected the towers to the top floor, forming the structure of the skybridge. In addition to these visible architectural qualities, the complex has many other important design features. For instance, because of the extreme height of the buildings, they are prone to wind. In other words, strong winds from the Singapore seas cause motions of the skyscrapers. That’s why there are four movement joints right under the main pools, to counteract these forces. These joints move an average of almost 20 inches to keep the structure stable. And since the buildings are of such weight, the whole complex is inclined to settle on the earth over time. This could lead to some serious tilt problems, so the complex has been equipped with jacks to allow adjustments when required.. 25.

(113) FIGURE 14. View of Marina Bay Sands. Other amazing architectural features are the systems which use the rainwater for cooling systems and the kinetic elevators that recycle parts of their movement to save energy. Rainwater is collected at the ArtScience Museum and is recycled throughout the complex. The design of the resort allows abundant natural light to illuminate the interior areas, reducing the need for additional lighting.. 26.

(114) Redevelopment del Grand Central by SOM The Municipal Art Society (MAS) asked SOM to re-think the urban spaces in and around Grand Central Terminal. SOM replied giving three solutions, all of which provide 2 kinds of improvements, (quantitative and qualitative) to the quality of public space around the station. The first solution alleviates pedestrian congestion at street level to create pedestrian corridors through multiple city blocks, connecting Grand Central to nearby urban attractors. The second is a condensing of the public realm through the creation of additional levels of public space which exist both above and below the existing spaces. To conclude, the third proposal creates an active, 24-hour precinct around Grand Central Terminal in the form of an iconic circular pedestrian observation deck, suspended above Grand Central, which reveals a full, overview of the skyline. This grand urban space moves vertically, bringing people from the cornice of Grand Central to the top of New York City skyline. It is a gesture at the scale of the city that acts both as an amazing experience as well as an iconic landmark and a symbol of a XXI century New York City.. FIGURE 15. View of the two towers and of the circular skybridge.. 27.

(115) -. Dancing Copper Apartments, New York. By SHoP Architects. -. Umeda Sky Building, Osaka. By Hiroshi Hara. -. Marina Bay Sands, Singapore. By Safdie Architects. -. Golden Dream Bay, Qinhuangdao. By Safdie Architects. -. Raffles City Chongqing (under construction (CTBUH, 2017a)), Chongqing. By Safdie. Further examples. In addition to the skybridges showed so far, there are other examples of pedestrian footbridges. Architects between buildings. Because of the little available amount of information, just the Pictures are illustrated.. 2. Tall buildings – development and structure. 2.2.3 Sky Habitat, Singapore. FIGURE 16. Umeda Sky Building, Osaka by Hiroshi Haraands.. 2.2.2 Pinnacle @ Duxton, Singapore. Sky Safdie Figure 2.14 Dancing Copper Apartments (top left),FIGURE Umeda Sky17. Building (topHabitat right), andby Marina BayArchitects. Sands (bottom). Photos from [p, 21], [p, 22] and [p, 23] Figure 2.13 Sky Habitat, Singapore. Photo from [p, 20].. Sky Habitat is another public housing project in Singapore, a 130m high, 38 floor complex designed by Safdie Architects. The complex consists of two towers connected by skybridges. 16. at three levels. The project was finished in 2015 (CTBUH, 2017b). The structure appears to be made of rigid reinforced concrete frames with the support of shear walls at the ends, as well as belt trusses at the link heights. The skybridges are made of steel trusses. Both the belt trusses and truss links are clearly visible on Figure 2.13.. 15. FIGURE 18. Figure Pinnacle @ @Duxton by ARC Architecture. 2.12 Pinnacle Duxton, Singapore. PhotoStudio from [p, 19].. 28. The Pinnacle @ Duxton is a large public housing project in Singapore, completed in 2009. It consists of seven 51 storey apartment blocks connected continuously by skybridges and skyparks at top and mid-height of the towers. It was designed by ARC Studio Architecture, and is 156m high (Wikipedia, 2017)..

(116) FIGURE 19. Portland Tallest Building by William Kaven Architecture.. FIGURE 20. Two examples of skyways in Las Vegas City.. 29.

(117) 30.

(118) 31.

(119) 32.

(120) GUIDE LINES FOR THE SKYSCRAPER DESIGNS. 33.

(121) 34.

(122) Skyscrapers work, for skybridges, as piles for a normal bridge located at the ground level. Thus, it has primary evidence to study the parts which make up them, their shapes, the surrounded environments, etc. In fact, a different arrangement of those can help designer during the skybridge design, for instance, promoting one bridge structural type or vice versa discarding another one. In this chapter, a first description of tall buildings is illustrated, but to better understand the problem, once the buildings have been defined, structural analyses on the considered structures shall be carried out. It is organized in two parts: in the first the superstructure has been described, in particular the main plan solution and their benefits, the main lateral load resisting systems, the seismic and wind effect considerations. In the second part the different types of substructure, their advantages and disadvantages have been shown.. Superstructure Tall buildings, also known as skyscrapers, are originally born in the city of Chicago after the fire of 1871. These huge buildings were important to the born of the steel or iron bearing structures. Furthermore, they improved and overstep the old idea of engineering, existing on that time where structures have been made by bearing walls to transfer loads and to ensure the later stability. The skyscrapers’ thin frames have made possible the achievement of several benefits, among which: further useful space; a reduction of their self-weight and so a decrease of loads that have to be spread out by the foundations; better structural stability. The use of tall buildings has gradually increased with the years. This allowed a notably development of the technologies connected to their accomplishments. One of the first was Mondnock Building (17 stories) followed numerous years after by Empire State Building (102 stories). Such vertical development has early become a competition for the moment resistance frames. In fact the slender columns on the lower stories were not able to absorb the corresponding amount of shear stress (deriving by the wind and seismic action) and to satisfy the deck maximum displacement request. Therefore, the use of reinforced concrete cores was made necessary because they are greater to carry such forces. Another solution was to place all columns on the perimeter of the building in order to decrease the distance between one column and another one. In this way the vertical supports can be The Hancock Building in Chicago is a prime example of this structural system, with its 100 considered more as a wallstories than a single TheTrade useCenter, of the diagonals in atJohn Hancock Building or almost reaching theelement. height of the World world’s tallest as building the blocks time. Figure as 2.6 shows elevation and plan views of some of the different tube systems the decomposition in small in Willis Tower are another solution of such issue. mentioned.. Figure 2.6 Different “tube” structural systems for tall buildings. The World Trade Center is an example of a framed tube, the. John Hancock center a trussed tube, and the Willis Tower is made up of a bundled tube. Figure from [p, 12]. FIGURE 21. Different structural system adopted for skyscrapers.. These structural systems became the primary ways of supporting high-rise buildings, and. With the growing increaseremained of the height, skyscrapers have not only changed their structural system in mostly the same for the rest of the century, but as buildings grew taller just for the order to make them moresakesuitable, even ifwere they primarily just called “Tall Buildings”, now of being tall, but other innovations popping up to improve were the effectiveness of tall another category has beenstructures. created, “Super Tall Buildings. These last are defined by the constructions that normally go over 300Up until meters. buildings one time were but nowadays they always are this point,Such while concrete cores with their tube-like behaviour wererare very structurally efficient, the tallest Burj buildingsKhalifa in the world had been meters built in steel.high). The World Trade Center more frequently instead(for instance isstill830 Buildings as Nakheel Tower start the Hancock Building were both steel structures, however, the emergence of supertall to assume different shapeandfrom the square shape. In fact, a considerable factor in the design process buildings made primarily from reinforced concrete would soon begin. In 1998 the Petronas is the distance between the reinforced concrete core and the exterior facade because it determines the Twin Towers, one of the most famous examples of skyscrapers connected by a skybridge, was number of rooms which can be directly lighted by the solar light and so the light comfort. 8. 35.