Session 2 Foundation of High Rise Buildings PDF

2 3  A raft (mat) foundation to support the entire structure may be feasible for buildings of moderate height. However, for very tall buildings, such a shallow foundation may not be able to develop adequate resistance to horizontal and moment loadings. Raft/mat foundations are relatively...

2 3  A raft (mat) foundation to support the entire structure may be feasible for buildings of moderate height. However, for very tall buildings, such a shallow foundation may not be able to develop adequate resistance to horizontal and moment loadings. Raft/mat foundations are relatively large in size; hence the foundation vertical bearing capacity is generally not the controlling factor in the design. The effects of lateral and moment loading should be incorporated into the assessment of ultimate bearing pressure. Soil stiffness is important in the raft/mat design to understand load distribution in the mat and for evaluating bending moments and shears in the raft. For rafts founded on rock, the bearing capacity is highly dependent on factors such as the intensity and orientation of joints, degree of weathering and other local or general defects. 4  Tall buildings very frequently have one or more basements to cater for car parking and/or commercial and retail space. In such cases, the construction of the raft involves excavation of the soil prior to construction of the foundation and the superstructure. Because of the stress reduction in the underlying ground caused by excavation, the net increase in ground stress due to the structure will be decreased, and hence it may be expected that the settlement and differential settlement of the foundation will also be decreased. The resulting foundation is termed a compensated or buoyancy raft, and can be very beneficial when constructing buildings on soft clay or loose sand, as the settlements that occur can be significantly less than those if the foundation was located at or near the ground surface. 5 Figure 6.7 indicate that the net pressure increase in the soil under a mat foundation can be reduced by increasing the depth of the mat. This approach is generally referred to as compensated foundation design and is extremely useful when structures are to be built on very soft clays. I this design, a deeper basement is made below the higher portion of the superstructure, so that the net pressure increase in soil at any depth is relatively uniform. 6 Pile Foundations- Bored Piles or Drilled Shafts Drill cylindrical hole, install reinforcement cage, and pour concrete 8 Bored Piles- Reinforcement Cage 1  Pile Foundation- Driven Piles Prefabricated members driven into ground 1 Pile Foundation- Jacked Piles 1 Bored Pile Construction- Grab & Chisel 1  Bored Pile Construction- Drilling in Rocks A 2.3 m diameter drill bit 1  Piled Raft Foundation Many high-rise buildings are constructed with thick basement slabs. When piles are used in the foundation it is generally assumed that the basement slab does not carry any of the foundation loads. In some cases, it is possible to utilise the basement slab, in conjunction with the piles, to obtain a foundation that satisfies both bearing capacity and settlement criteria. A piled raft foundation is a composite system in which both the piles and the raft share the applied structural loadings. Within a conventional piled foundation, it may be possible for the number of piles to be reduced significantly by considering the contribution of the raft to the overall foundation capacity. In such cases, the piles provide the majority of the foundation stiffness while the raft provides a reserve of load capacity. 1 Points to be taken in to consideration while designing foundations for high-rise buildings: 1.Ultimate capacity of the foundation under vertical, lateral and moment loading combinations. 2.The influence of the cyclic nature of wind, earthquakes and wave loadings (if appropriate) on foundation capacity and movements. 3.Overall settlements. 4.Differential settlements, both within the high-rise footprint, and between high-rise and low-rise areas. 5.Possible effects of externally-imposed ground movements on the foundation system, for example, movements arising from excavations for pile caps or adjacent facilities. 6. Earthquake effects, including the response of the structure-foundation system to earthquake excitation, and the possibility of liquefaction in the soil surrounding and/or supporting the foundation. 7. Dynamic response of the structure-foundation system to wind-induced 1  Problems Related to High Rise Buildings  The building weight, and thus the vertical load to be supported by the foundation, can be substantial. Moreover, the building weight increases non- linearly with height, and so both ultimate bearing capacity and settlement need to be considered carefully.  High-rise buildings are often surrounded by low-rise podium structures which are subjected to much smaller loadings. Thus, differential settlements between the high- and low-rise portions need to be controlled. 1  Problems Related to High Rise Buildings The lateral forces imposed by wind loading, and the consequent moments on the foundation system, can be very high. These moments can impose increased vertical loads on the foundation, especially on the outer piles within the foundation system. The structural design of the piles needs to take account of these increased loads that act in conjunction with the lateral forces and moments. 2  Problems Related to High Rise Buildings The wind-induced lateral loads and moments are cyclic in nature. Thus, consideration needs to be given to the influence of cyclic vertical and lateral loading on the foundation system, as cyclic loading has the potential to degrade foundation capacity and cause increased foundation movements. 2  Problems Related to High Rise Buildings Seismic action will induce additional lateral forces in the structure and also induce lateral motions in the ground supporting the structure. 2 Tube System 2 Tube System 2 Structural Wall Frame Structure 2 Braced Framed System 2  Case Study Taipei 101 Tallest building in the world from 2004-2010. 2 Case Study-Taipei 101 Overview: Taipei 101 features a 508-meter, 101-story tower A five-story deep basement 61 elevators Most floor plan areas vary between 2000 and 2 500 square meters (21,500 to 27,000 square feet), Building aspect ratio (height/width) to the main roof is about 9 based on its ‘waist’ (and 6.8 coun ting the wider base). Construction began in 1999 Finished in 2004 Cost $1.8 billion 2  Case Study- Taipei 101 Building Central Braced Core Components and Resists Moments and Gravity Loads System Large Perimeter Mega-Columns Concrete Filled Steel Boxes - Reinforced by Moment Frame Outrigger Trusses 8 Segments of 8 Include a Story for Structure Diagonals Through Occupied Space Connections 5 Different Types Typical Floor Frami ng Plan - Lower Sto ries (1st-26th) CORE Typical Floor Framing Plan - Upper Typical Floor Framing Plan - Lower Stories (27th-91st) Stories (1st-26th) 2  Case Study- Taipei 101 Foundation & Soil Type: 660 feet away from a fault line 21m deep basement Groundwater usually 2m below the surface Soft rock usually 40-50m below colluvial soils and clay 2 Slurry Wall System One around both the tower and the podium foundation Second around just the tower foundation Drilled Piers Continuous concrete matt transfers point loads 380 piers driven 262ft into the ground 5ft in diameter and can withstand 1100-1450 tons each 3  Case Study- Taipei 101 Foundation Detail: One of the most stable buildings ever constructed Reinforced by 380 piles driven 262 feet into the ground Each pile is 5 feet in diameter and can withstand a load of 1100- 1450 tons, that is 2,900,000 pounds each. 3  Case Study- Taipei 101 Foundation Depth 80m: 3  Case Study- Taipei 101 Reverse Circulation Pile: 3  Case Study- Taipei 101 FOUNDATION CONSTRUCTION STEEL PILES, REBAR, & CONCRETE: 3  Causes of Foundation Failures 1. Load transfer failures 2. Collapsible soils 3. Lateral loads 4. Construction error 5. Unequal support 6. Earthquake 7. Vibration effect 8. Foundation failure due to landslide/ slope instability 9. Foundation failure due to uplift 3  Case Study- Shanghai, China An unoccupied 13-storeyblock of flat building, still under construction, at Minhang district of Shanghai city toppled over. It ended up lying on its side in a muddy construction field. One worker was killed in this accident. Construction work on the block appeared to have been nearly completed, with windows fitted and a tiled facade. Other identical blocks in the same property development were still standing nearby. 3  Case Study- Shanghai, China Causes and Failure: The cause of the building collapse in Shanghai was due to a pressure difference on two sides of the structure, according to an investigation report. Improper construction methods are believed to be the reason of the building collapse in Shanghai, according to a report from the investigation team. The investigation team’s report said that workers dug an underground garage on one side of the building while on the other side earth was heaped up to 10m high, which was apparently an error in construction. 3  Case Study- Shanghai, China Two views of a toppled 13-storey apartment building that buried one worker in Shanghai on 27th June 09 3 Case Study- Shanghai, China 3  Case Study Preventive measures and remedies There is no remedy for such massive failures but definitely preventive measures in terms of “supported excavation system” for “deep excavation problems” can be adopted to avoid such failures Soil nailing is the latest and most widely used technique for supporting the vertical excavation near an existing building. 4  Case Study Conclusion from case studies: On the basis of an extensive ground investigation and a detailed description of the ground, the foundation of high-rise buildings can be planned in an economic and safe manner. The choice of the adequate system is often depending on the proof of the serviceability of the high-rise building and / or neighboring structures. Sources Case 1 - a.https://www.tripsavvy.com/taipei-101-tower-facts-1458242 b.https://www.phase-trans.msm.cam.ac.uk/2005/t101/t101.html Case 2 - a.Dr. N. Subramanian; Rare Foundation Failure of a Building in Shanghai, China; NBM&CW; AUGUST 2009. 4  Technology Advancement for Foundation The factors that may influence the type of foundation selected to support a tall building include the following: 1.Location and type of structure. 2.Magnitude and distribution of loadings. 3.Ground conditions. 4.Access for construction equipment. 5.Durability requirements. 6.Effects of installation on adjacent foundations, structures, people. 7.Relative costs. 8.Local construction practices. 4  Technology Advancement for Foundation The stability of foundation depends on: 1.The bearing capacity of soil 2.The settlement of soil beneath the foundation. Soil behaves in a complex manner when loaded, so it is important to know about its bearing capacity. Improving Soil Bearing Capacity Increasing depth of foundation Compacting the soil Draining the soil Confining the soil Grouting Chemical Treatment 4  Technology Advancement for Foundation Disadvantage of Piling: 1.Very Noisy 2.Causes Massive Vibrations through the soil For this reason, it is sometimes difficult to use them in sensitive locations. For example, if an operational hospital or science lab is to be extended, driving piles would cause unwanted disturbance. Their use is also restricted in residential areas in many countries. The vibrations could also cause structural damage to older buildings that are close by. In such situations it is possible to use micropiling or helical piling, neither of which rely on hammering. 4  Technology Advancement for Foundation Micropiles or minipiles are small piles that are constructed in the following way: Step 1: a hole a little larger than the pile diameter and the full length of the pile is dug into the ground using an apparatus like a soil boring machine. Step 2: a precast concrete pile is lowered or pushed into the hole. Step 3: a concrete grout is poured into the gap between the pile and the earth. Micropile construction sequence using casing 4  Technology Advancement for Foundation MicroPile Details There are a wide variety of installation methods available and the method of reinforcement can be in the form of: 1.Steel casing 2.Steel casing supplemented by internal reinforcement 3.Heavy reinforcement without casing 4.High capacity threaded hollow bar members installed and grouted during drilling. Micropile maximum axial load capacities of up to 2000kN can be achieved. 4  Technology Advancement for Foundation Positive features Can be Installed in Limited Headroom Positions Create minimum disturbance to adjacent structures Can be installed through existing footings Due to the high capacity steel reinforcing elements, micropiles have high uplift load capacity and can be effectively used for tension structures Due to the wide range of installation methods available and the relevant ease of penetrating boulders or hard rock formations, micropiles can be economically installed in difficult ground conditions, e.g. Karstic formations Micropiles can be utilised as soil reinforcing elements providing significant economies in suitable soil conditions where the applied load is shared between the base and the piles Micropiles can be installed as steeply raking piles providing significant horizontal load capacity for a pile group 4  Technology Advancement for Foundation Helical piles are steel tubes that have helical (spiral) blades attached to them. Also called screw piles. These can be drilled into the ground, meaning that the pile acts as a giant drill bit, and is rotated and pushed into the ground from above, much like a screw drills into wood. Once the steel pile is driven into the ground, a pile cap is poured on top of the pile to prepare it for the construction Uses 1.Provide structural support 2.Underpin foundation 4  Technology Advancement for Foundation Sustainability: Scew piles can be removed, moved and reused. Unlike traditional dug-in basements, screw piles are minimally damaging the landscape. 4  Technology Advancement for Foundation Efficiency: Screw piles are quicker and easier to install than traditional concrete footings or foundations This means lower costs and less downtime waiting for foundations to be poured and inspected. Sturdy and resistant to frost and water damage, screw piles provide excellent building foundations in the long term, even through building expansion or addition. 5  Technology Advancement for Foundation Positive features No tailing, rebar, anchor bolts or bore lining required. No concrete curing time required Do not have to dewater casing Can be installed in all weather conditions Single stage installation Reclaimed with minimum ground disturbance No additional wall thickness required to facilitate installation Usually requires shallower embedment depths Reduce material cost and faster install times. 5