Building Big: Civil Engineering Behind Skyscrapers

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High Rise Building Technology Building Big: Civil Engineering Behind Skyscrapers MB Bureau Report Skyscrapers are gigantic projects demanding incredible logistics, management and strong nerves among

1 High Rise Building Technology Building Big: Civil Engineering Behind Skyscrapers MB Bureau Report Skyscrapers are gigantic projects demanding incredible logistics, management and strong nerves among all concerned in their planning and construction. As long ago as 1928, the American Colonel WilliamA. Starrett wrote that no peacetime activity bore greater resemblance to a military strategy than the construction of a skyscraper. P lanning a high-rise building would be in conceivable today without the help of experts and technical consultants. Extensive soil analyses are required to determine the strength of the subsoil before deciding on the location for a high-rise building. In the majority of cases, cores are drilled into the load-bearing subsoil to obtain soil samples. The drilling profile of the geological strata making up the subsoil and laboratory analyses of the soil samples provide the basic data for the soil report which is in turn used as the basis for planning the supporting structures and choosing a suitable foundation structure with due regard for the 180 The Masterbuilder - August loads exerted by the high-rise building. The forces acting on the high-rise structure in the event of an earthquake must be taken into account when erecting high-rise buildings in areas prone to seismic activity. The same applies to wind loads and particularly to the dynamic effects of windstorm or earthquake loads. The additional vibration loads can result in overall loads of the same order of magnitude as the load exerted by the deadweight of the structure. The situation is particularly critical if the vibrations reach the resonant frequency of the building: in such a case, the vibrations can intensify until the entire building collapses.

During construction state, top-down and bottom-up method were adopted in construction. Top-down construction method become more popular because it shorter the construction period.

2 Technologies Used Many technologies are adopted in high-rise building construction to overcome problems. The following are most commonly used:- Top-down construction method In centre business area (CBA), the buildings general have a basement and connect other building and mass transit railway (MTR). During construction state, top-down and bottom-up method were adopted in construction. Top-down construction method become more popular because it shorter the construction period. Excavation will carry out without the need for strutting to support the excavation because the slabs act as the horizontal support. Therefore, it is another advantage for top-down construction method. The difficulties are the limited headroom for excavation. Therefore, a special machine maybe need during construction state. Prestressed Concrete As the height of building increase, the material and construction will be change to fit the actual need. For example, the reinforced concrete beam will change to prestressed reinforced concrete beam in order to decrease the depth of beam and/ or slab. Prestressing means the intentional creation of permanent internal forces and stress in a structure or assembly, for improving its behaviour and strength under service conditions. Since concrete is strong in compression and weak in tension, prestreeing the steel against the concrete would put the concrete under compressive stress that could be utilized to counterbalance tensile stresses produced by external loads. Pre-tensioning High-tensile tendons are tensioned before the concrete casting. When the strength of concrete reaches the designed level, the wires are released to produce compressive stress. Suitable curing can accelerate the strength establishment process of concrete. For example, steam curing. Post-tensioning Prestress tendons are checked before concrete casting. The prestressing steel can be introduced after concrete has set by casting in duct-tubes at the appropriate positions that are extracted before the steel is inserted. The tendons are anchored at one end of concrete unit and stressed by jacking against the other end. The steel is subsequently grouted under pressure through holes at the ends of the unit to protect it from corrosion and to provide bond as an additional safeguard. Advantages of Prestressed Concrete - Better appearance and durability - More efficiently than RC - Allow for shrinkage creep at the design stage - Longer span - Greater rigidity under working loads than RC Disadvantages of Prestressed Concrete - Higher construction cost due to material and supervision - Maintenance is need after superstructure completed - Broken tendon may kill the worker during prestressing. Figure 2: Concreting works for Pile Cap Figure 1: Prestress tendon is post-tensioning slab, Condition of tendon before concreting Figure 3: Classification of Foundation The Masterbuilder - August

Figure 4: Driven H-pile works in Progress Figure 5: Drilling rig is used in drilling works Figure 6: Mini-pile was used because of headroom limit Transfer Structure Mini-pile In order to have a large

g. H-pile, bored pile, mini-pile, etc. Refer Figure 2 for a typical transfer structure construction. Deep Foundation Sometime, the soil condition is not suitable for high-rise building.

3 Figure 4: Driven H-pile works in Progress Figure 5: Drilling rig is used in drilling works Figure 6: Mini-pile was used because of headroom limit Transfer Structure Mini-pile In order to have a large space for shopping mall, transfer structure will be adopted to transfer the loading to column. Pile caps also a transfer structure in Hong Kong. Pile Caps transfer the loading from column to pile (e.g. H-pile, bored pile, mini-pile, etc. Refer Figure 2 for a typical transfer structure construction. Deep Foundation Sometime, the soil condition is not suitable for high-rise building. A deep foundation (e.g. large diameter bored pile (LDBP), driven H-pile) will be developed to solve this problem. Ground Investigation (G.I.) Before foundation works, designer use the G.I. to found out the data for foundation design. Before actual foundation works (mini-pile, bored pile) commenced, second G.I. (pre-drill)is needed to check the design assumption After all foundation works (mini-pile, bored pile) completed, third G.I. (post-drill) is need to check the design. Interface coring and proof-drill are needed to check the quality. Sometime, the location is not suitable for large machine for foundation works. Other construction method will be produced, such as, mini-pile. Odex method is used in minipile construction works. Refer Figure 6 for the mini piling process. Large Diameter Bored Pile (Refer Figure 7 to 12 for bored piling process) Advantages of large diameter bored pile a) length can readily be varied to suit the level of bearing stratum b) Soil or rock removed during boring c) Less noise and vibration is produced compare with driven H-pile Disadvantages of large diameter bored pile f) expensive compare with footing g) soil erosion may be occur if the intersection of soil and rock is not horizontal h) Large machine is needed compare with mini-pile. Figure 7: Steel casting is used in bored pile works Figure 8: Equipment is used to ensure the drilling rig vertical Figure 9: Vibro hammer is used to insert the steel casting 182 The Masterbuilder - August

Figure 10: Machine used to construct the bell out Figure 11: Reinforcement fixing for bored pile in progress Figure 12: Excavation works for pile cap and cut-off Driven H-pile (Refer Figure 13 and 14

Check the verticality 5. Connect the steel H-pile by welding 6. Painting the pile in order to rust-proofing 7.

4 Figure 10: Machine used to construct the bell out Figure 11: Reinforcement fixing for bored pile in progress Figure 12: Excavation works for pile cap and cut-off Driven H-pile (Refer Figure 13 and 14 for H-Piling process) The construction sequence of H-pile 1. set out the location of H-pile 2. engineer check the setting out, dimension of pile and etc 3. Driven the steel pile 4. Check the verticality 5. Connect the steel H-pile by welding 6. Painting the pile in order to rust-proofing 7. Final-set test is adopted to check the design assumption Advantages of large diameter bored pile a) length can readily be varied to suit the level of bearing stratum b) Compare with bored pile, H-pile is cheaper c) No soil or rock were excavated; therefore, transportation of debris is not needed. d) Settlement of adjacent is usually small compare with large diameter bored pile. Disadvantages of large diameter bored pile a) No Soil or rock removed during driving b) expensive compare with footing c) Large machine is need compare with mini-pile d) Large noise and vibration is produced e) Longer construction time is needed because only 3 hours per day for divining works f) Adjacent buildings may be affected due to large vibration Shoring Shoring is the means to provide temporary support to structures that are in an unsafe condition till such time as they have been made more stable. Sheet Piling and Slope stabilization (Geotechnical Works) Normally in urban areas, the buildings are mainly surrounded by other building and slope. Therefore, a lot of technology was used to solve such problem. For example, sheet piling, grout column, grout curtain and pipe pile wall for excavation and lateral support. Soil nail and rock dowel for slope stabilization works. Pre-cast facade Pre-cast façade is very common in high-rise building. It is because government encourage the developer to use such construction method. Sometime, contractor will use precast concrete to reduce the amount of construction waste generated on construction sites, reduce adverse environmental impact on sites, enhance quality control of concreting work and reduce the amount of site labour. Refer Figure 19 and 20 for precast façade installation process. Figure 13: H-piling works in progress Figure 14: Verticality was checked during H-piling works Figure 15: Shoring was provided to support adjacent building 184 The Masterbuilder - August

- a careful design and - install a water-stop during concreting on site Different Structural Types used in High-Rise Buildings Construction Figure 16: Sheet piling was installing by vibrating hammer

Less debris produced: It is because all form formwork was the same. It can be recycled. However, in construction site, the formwork may be placed in a wrong position.

5 - a careful design and - install a water-stop during concreting on site Different Structural Types used in High-Rise Buildings Construction Figure 16: Sheet piling was installing by vibrating hammer Advantage of pre-cast façade is below - Better quality control: Better quality control is achieved because the precast façade was produced in a factory, where all procedure were closely monitoring - Less debris produced: It is because all form formwork was the same. It can be recycled. However, in construction site, the formwork may be placed in a wrong position. - Less noise produced: All the sequences were conduct in factory; therefore, less noise will be produced. For example, hammer hit the formwork. - Faster construction period: The strength of the pre-cast façade can reach the design strength earlier because steam curing can be used. Disadvantages of Pre-cast facade - Damaged during transportation, lifting operation- It can be solve by careful design to ensure the pre-cest faced suitable for lifting up by tower crane. - Water leakage- Water leakage between the construction joints is the major problem in high-rise buildings. However, it can be solved by the following: Throughout time, there are many structural systems developed. It started for building the structure as a rigid frame to building as a long cantilever. Moment resistant frames can be effective options for buildings upto 20 to 30 stpries; tubular frames and trusses can reach a lot higher. Other systems have characteristics taken from both. There are many factors determining a structural type for a building, this include the general economic considerations, soil conditions, fabrication and erection considerations, mechanical systems considerations, fire rating considerations, community factors, legal factors and availability and cost of main structural materials. Bearing Wall System This is the traditional structural system in the erection of tall buildings. The vertical structural elements carry the loads directly to the foundations. The common building materials would be stone, brick and reinforced concrete. The height of the structure is limited by the strength of the bearing materials. Buildings in this type are not going to be too high because of the accumulated weight of the walls plus the other dead and live loads. Too high a structure will result in wall becoming so thick that lower floors can no longer function. Bearing Walls with Core In this system, one or more cores are added to the parallel alignment of the bearing walls in order to create a lateral load resistance in a direction perpendicular to the bearing walls. The core is formed by grouping two or four walls perpendicular to each other to create a closed geometry. Typical shape would be tube, round or square that is stiff and can resist torsion. The core is often placed in a central location for the convenience of distributing building services and for an increase in structural integrity. If placing the core off center, it can create additional torsion and rotation, which might require extra resistance mechanisms. This structural system allows a greater free floor area and this is a common system in Hong Kong for building 20 to 30 storeys reinforced concrete office building. Self Supporting Boxes Figure 17: Sheet piling were installed to protect the existing road in drainage improve works In 1970s, this system was developed when the prefabrication of reinforced concrete structures was at the peak popularity. Prefabricated concrete floor are place on top of each other in a way that each later oriented perpendicular to the one directly below it. The Masterbuilder - August

Figure 18: Pipe Pile Wall is used to support existing buildings in shaft excavation Core with Cantilevers In this system, floor slabs are cantilevered out from the solid core supports in the middle.

Rigid Frame Rigid frame system is developed by structural designers from the bearing wall system. However, it is not very efficient.

In order to reduce the lateral sway of the structure, a stiffening core would be added to the standard rigid frame.

6 Figure 18: Pipe Pile Wall is used to support existing buildings in shaft excavation Core with Cantilevers In this system, floor slabs are cantilevered out from the solid core supports in the middle. The advantage will be the absence of interior columns and freeing the façade of the structure. The disadvantage is the additional thickness needed for larger cantilever. Rigid Frame Rigid frame system is developed by structural designers from the bearing wall system. However, it is not very efficient. Each member in the system must help in the transfer of lateral loads to the foundations through rigid connections. Buildings in this type are often very regular and not very tall. In order to reduce the lateral sway of the structure, a stiffening core would be added to the standard rigid frame. Tube in Tube The development of this system is very important in modern skyscraper of great height. The exterior and interior columns of the structure are placed so close together that they almost form a solid surface. The entire building acts as a huge hollow tube with a smaller tube in the middle. This resists a Figure 19: Pre-cast facade installation works in progress Figure 20: Pre-cast facade before installation great amount of torsional loading and the lateral loads are supported between the inner and outer tubes. A number of tallest buildings in Asian countries used this system. Some Special construction methods BMW Headquarters, Munich The headquarters of BMW A.G. differs from conventional buildings to create an impressive corporate symbol in the form of a 100-m-high four-cylinder structure. The requirements for appropriate office organization yielded a basicout line in the shape of a clover leaf. Stairways, elevators and sanitary areas are accommodated in the central core. In this way, all the offices can be reached by the shortest possible route. Trendsetting methods were also used for the construction work. A reinforced concrete version was chosen as the most economical solution. According to the design concept, the entire building with 18 office floors and a technical floor was to be suspended from a girder cross at the top of the roughly 100-m-high core via four central king posts. This is a modification of the outrigger truss. The entire load of the building is transmitted to the foundations via the core as the central element; it also absorbs all wind forces. A mighty girder cross with a projection of16 m is mounted at the top of the core. The four king posts are secured to this central girder cross, each king post comprising 105 threaded steel bars with a load-bearing capacity equal to a suspended weight of 4,600 Mp. Small 188 The Masterbuilder - August

outer columns are additionally located between the floors. These outer columns are designed as compression columns above the technical floor (12thfloor) and as king posts below.

7 outer columns are additionally located between the floors. These outer columns are designed as compression columns above the technical floor (12thfloor) and as king posts below. Time and costs were the decisive reasons for choosing this innovative construction method. All 19 floors were successively produced at the foot of the shell and core; the first floors were even produced complete with facade and glazing during construction of the supporting cross. The finished floors were then connected to the supporting cross via the king posts and raised one floor at a time every week with the aid of hoisting gear so that another floor could be produced in the space vacated at the foot of the core and then connected to the floor above (lift-slab method). Completion of the facade, glazing, installation and interior finishing proceeded on the suspended floors, unimpeded by the structural works and lifting operations. In addition to reducing the construction time required, this method also eliminated the need for expensive tooling and assembly work. LA Grande Arche, Paris This building, which has already been mentioned, takes the form of a giant cube open on two sides with edge lengths of 110 m. It was completed at the end of 1989 on the 200th anniversary of the French Revolution and took 5 years to build (Refer Figure 21).The building has a weight of more than 300,000 Mp and is mounted on neoprene bearings, the loads being transmitted 30 m into the subsoil via twelve concrete pillars. The cube s main support is in the form of four prestressed upright reinforced concrete frames 21 m apart. They are complemented by horizontal members measuring roughly70 m at ground and roof level. Each of these members is 9 m high, the equivalent of a 3-storey building. Since the two vertical sides of the cube would be without roof-level transverse bracing during construction, the required stability for that phase of the work was produced by means of horizontal steel truss reinforcements. A total of 37 office floors are accommodated in the two 18-m-wide wings of the cube (each with an area of 42,000 m 2 ). Conclusion High-rise buildings are still the essential form of building structure constructed extensively in urban are as, in particular, in the hearth of the commercial zones of metropolitan cities. On the other hand scarcity of land supply encourages the construction of high-rise buildings. For the construction of high-rise buildings, site planning including activity scheduling and site production layout has to be reviewed and re-plan from time to time in practice as site conditions and resources are dynamic and uncertain. Every building is special of its kind and has to be given special consideration. Because every building has its own surrounding condition which is different from the other. The techniques mentioned above are more general, it need to be applied with modifications to suit for every individual case. Reference Figure 21: LA Grande Arche, Paris - CHEW (2001), Construction Technology for Tall Buildings, Singapore University Press, Singapore - R.C. Smith and C.K. Andres (1986), Principles and Practices of Heavy Construction, Prentice-Hall - C.W. Griffin(1986), Manual of Low-Slope Roof Systems, 3rd Edition, McGrawHill, New York - W. McElroy (1993), Roof Builder s Handbook, PTR Prentice Hall, New Jersey - C.K. Andres (1998), Principles and Practices of Heavy Construction, 5th Edition, Prentice Hall, New York - D. T. Coates (1993), Roofs and Roofing Design and Specification Handbook, Whittles, UK - S. Hardy (1997), Time-Saver Details for Roof Design, McGraw- Hill, New York - H.W. Harrision (1998), Roofs and Roofing: Performance, Diagnosis, Maintenance, Repair and the Avoidance of Defects, Building Research Establishment, Watford, Hert - W. Schuller (1990), The Vertical Building Structure, Van Nostrand Reinhold - High-rise buildings in the course of history, Technology of highrise buildings, Risk potential & Insurance by Munich Re Group. The Masterbuilder - August

Civil Engineering Construction I (CBE5031)

Civil Engineering Construction I (CBE5031) Precast Concrete 1 Definition of Precast Concrete Precasting means casting a concrete member at a place other than where it will be used and then moving it to