Integrated Civil Engineering Design Project (Building Structure Design)
CIVL 395
HKUST By : Ir. K.S. Kwan Date: 3/07
Content 1. Building Control in Hong Kong 2. Design Criteria 3. Structural Form (Residential Building) 4. Hong Kong Wind Loading 5. Computer Modeling 6. Design Example
STRUCTURAL FORM for Residential Building •Tower •Podium Structure •Building adjacent to slope
Lintel beam
To identify the wall as structural element and link them together by lintel beam to provide sufficient lateral stiffness
Wall
Slab
Slab Design – Concrete grade Grade 30 to 35 (too high concrete grade may lead to thermal crack during large pour of concrete)
– Steel reinforcement percentage Design as HK CoP 2004 for structural use of concrete Average steel ratio is around 120~140 Kg/m3
– Preliminary slab size estimation About 100mm~400mm depending on the span of slab ( to minimize the number of different slab thickness, say 2 ~3 types, at typical floor for buildability consideration To consider the following loading – Self weight – Finishes (domestic area/toilet/kitchen) (25mm to 80mm thick) – Partition
Slab is designed as one-way or two ways slab
Wall Design – Concrete grade Grade 30, 40, 60 or more is commonly used. By using high strength concrete, it can optimize the wall thickness and increase the lateral stiffness of wall. The concrete grade will also be changed along the height of building e.g. from Grade 60 at lower floor to Grade 30 at top roof. The thickness will be trimmed down along the height of building e.g. from 400 at 1/F and gradually changed to 200 at top floor. The thickness will be changed every 10 ~20 storey to minimize the disturbance on construction.
– Steel reinforcement percentage Design as HK CoP 2004 Average steel ratio is around 100~150Kg/m3
– Preliminary wall size estimation Gravity Load – by tributary method Wind Load – by simple computer model
Vertical Element Gravity Load Estimation by Tributary Area Method 250
2625
200
2625
250
W2
200
W3 W1
W1
3900
C1
Plan
3-D
TRIBUTARY AREA METHOD
Assumption No. of storey = 20 Storey height = 2800 Slab thickness = 150 Beam size = 400x200 (ext.) Beam size = 450x250 (int.) Dead Load = 10KPa Live Load = 3KPa
TRIBUTARY AREA METHOD
(KN) 250
2625
200
2625
250
W2
200
C1
1686
W1
2264
W2
2568
W3
1266
W3 W1
W1
C1
Plan
3900
Lintel Beam Design (where linking shear
Lintel Beam
wall together to transmit wind shear force)
– Size Width as wall thickness Depth controlled by headroom (min. under side of beam i.e. 2100 at door and 2300 under beam Concrete grade same as floor slab for easy concrete pour with slab or more if required
– Steel reinforcement percentage Design as HK CoP 2004 Average steel ratio is around 120 ~160 Kg/m3
– Preliminary lintel size estimation Wind Load – by simple computer model; the size is always controlled by wind shear transmission (in some critical case, steel plate will be used to replace r.c. design to enhance the wind shear transmission) Gravity Load – by tributary method (not the controlled case)
Steel plate at lintel beam
Transfer Structure Tower (Shear Wall system)
Podium (Plate Structure)
Supporting Column (Rigid Frame)
Transfer Girder Structure
The behavior is similar to deep beam when the wall extending to columns such as case a, b & c.
Transfer Plate Structure Shear Wall Structure at Tower above Transfer Plate
Thick plate structure to support all wall structures above
Column Structure below Transfer Plate
Transfer Plate
Transfer Structure Design (Plate or Girder) – Design similar to pilecap or beam – Closed column spacing under the transfer structure to allow truss effect at transfer structure to minimize the deformation of transfer structure (Prestressed transfer structure is required for large span )
– Steel reinforcement percentage Design as HK CoP 2004 Average steel ratio is around 240~280 Kg/m3
– Preliminary size estimation (1.5m ~5m) Depend on the spacing of columns and tower loading Gravity load – as the wall load transmitted tower load to plate level Wind load – the plate behaviour as frame structure integrated with columns below Normally, the thickness is controlled by shear stress
Podium Structure Behavior
Loading from tower including: (P) Axial Load (M) Moment (V) Shear
Transfer Plate Design To cater for gravity load and wind load from tower structure including axial load, moment and shear The transfer plate with column below to form a rigid frame structure All loadings are transmitted to foundation by shear, moment and axial force.
Transfer Plate with Prestressed Tendon
Building Development Adjacent to Slope
Retaining structure is required for building near the slope The extent of excavation will depend on the subsoil condition of slope i.e. Rock / Soil ?? ??
? ?? ? ??
?
Building Development near Slope Walls at Tower
Transfer Plate
Column under transfer structure
Large Diameter Bored Pile
Pile Cap
Retaining Wall Structure
Pile Cap
Retaining structure for semi-basement construction
Retaining Wall Structure with deep excavation required
Two levels basement to reduce the deep excavation
HONG KONG WIND LOAD Wind Load Assessment Procedure
Wind Responses of a Building • Static No movement
• Equivalent Static Load
Wind direction
• WC 2004
• Dynamic
- Along wind response • Gust Factor Method • WC 2004
- Cross wind response
- Torsional wind response
• Literature/ Wind Tunnel Test • WC 2004
Wind Load Assessment Procedure (1)
Step 1 – Determine Method of Calculation •
Determine method of calculation according to the signpost in Cl. 3.3 (p.2) and Cl. 7.6 (p.5).
Method Signpost in Wind Code 2004
Characteristic
I
(i) fnatural > 1Hz; or (ii) H 50m), additional torsional wind load (10% of long face dimension) is required
Wind Load Distribution at Building
Wind Load Calculation as HK CoP (Building is considered as significant resonant dynamic structure)
Wind load calculation at each floor for a building with 40 storey (with 3 floors above domestic floor) and the building width is 40.23m Building structure as significant resonant dynamic structure \ Sa=topography factor
Wind Load Calculation as HK CoP (Building is not considered as significant resonant dynamic structure)
Wind load calculation at each floor for a building with 40 storey (with 3 floors above domestic floor) and the building width is 40.23m Building structure not considered as significant resonant dynamic structure (Note: Total wind shear is larger based on static wind load approach for building aspect ratio just greater than 5) Sa = topography factor
COMPUTER MODELING
Common Structural Analysis Software used in Hong Kong GSA STARIII GTSTRUDL PAFEC STAN
ETABS SAP2000 SAFE SADS
Tall Building Modelling Assumptions 1.
Material – All structural components behave linearly elastically.
2.
Participating Components – only the primary structural components participate in the overall behaviour
3.
Floor slabs – Floor slab are assumed to be rigid in plane unless they contain large openings or are long and narrow in plan
Only the primary structural components are put in model
Rigid in plane
Tall Building Modelling Assumptions 4.
Negligible stiffness – component stiffness of relatively small magnitude are assumed negligible
5.
Negligible deformations – deformations that are relatively small and of little influence are neglected.
6.
Cracking – the effects of cracking in reinforced concrete members to flexural tensile stresses may be represented by a reduced stiffness
This line should be a straight line in assumption due to the small deformation
V
How to apply wind loading in computer model? In common building shape with the rigid diaphragm assumption, the wind load should be applied at the geometry centre of each floor
Wind load applied at floor
Wind load applied at centre of frontal area
What can you find in computer modeling? – Seismic, wind and gravity analysis – Deformation of building under different loading conditions – Member force under different loading conditions
Deflection of building at top floor including the X & Y displacement and Z direction rotation
Q&A If you have any questions about the structural design, please forward email (with your Name and Student ID no.) to :
[email protected]
