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 <= 5 x Min (B, D); and
H <= 100m
• No significant resonant
dynamic response
• Equivalent Static Load
Method [Cl. 5, p.3]
II
(i) fnatural <= 1Hz; and
(ii) H > 5 x Min (B, D); or
H > 100m
• Susceptible to along wind
resonant response
• Gust Factor Method
[Cl. 7, p.4]
III
(i)
• Susceptible to dynamic
excitation
• Recommendation from
literature/ Dynamic wind
tunnel test [App. A, p.7]
Open frame with significant
resonant dynamic response, or
(ii) fnatural < 0.2Hz, or
(iii) Significant cross wind /
torsional resonant response
To determine building
height (H) and width
(B,D)
Building least
horizontal dimension
(B,D)
Building
height (H)
B
Building on plan
To define the height
and least dimension
of building
Sec A-A
b
A-A
h
H
B
B-B
Sec B-B
Wind Load Assessment Procedure (2)
Steps 2 - 5
Step
2a
Method 1 – Static Building
Method 2 – Slightly Dynamic Building
• Calculate Design Wind Pressure
(3-sec. gust pressure)
[Table 1, p.3]
• Calculate Design Hourly Mean Wind
Pressure
[Table 2, p.5]
2b
• Calculate Gust Response Factor (G)
[Appendix F, p.19~21]
3
• Calculate Topography Factor
[Appendix C, p.10~13]
• Calculate Topography Factor
[Appendix C, p.10~13]
4
• Calculate Force Coefficients (Cf)
• Calculate Force Coefficients (Cf)
– Height Aspect Factor, Ch
– Shape Factor, Cs
– Reduction Factor, RA
[Appendix D, p.14~16]
5
• Calculate Total Wind Force
F = Cf. Σ qz .Az
[Eqn (1), p. 3]
– Height Aspect Factor, Ch
– Shape Factor, Cs
[Appendix D, p.14~15]
• Calculate Total Along-Wind Force
F = G. Cf .Σ qz .Az
[Eqn (3), p. 4]
Step 2a – Design Wind Pressure/ Design
Hourly Mean Wind Pressure
• Wind Code 2004
– Only One Terrain
• Open Sea Terrain
Wind Profiles Below 200m
Wind Pressure Profile Under 200m
250
1983
1983
(Stepwise)
PNAP150
Height (m)
200
2004
150
100
50
0
0.00
1.00
2.00
3.00
Pressure (KPa)
4.00
5.00
Step 2b - Along Wind Dynamic
Resonant Response by Gust Factor
Method (1)
• The original method was developed by Davenport
(1967) and Vickery (1966 and 1971)
• In Wind Code 2004, the equation is simplified to:
2
G = 1 + 2I h gv B +
2
g f SE
ς
(Refer to Wind Code 2004 Appendix F for
description of the other variables)
Step 2b - Along Wind Dynamic Resonant
Response by Gust Factor Method (2)
• Dynamic resonant response is dependant on the
magnitude of the fluctuating load as well as its size
(or scale) in relation to the size of the structure
• The size reduction factor, S, accounts for the
correlation of pressures over a building and is equal
1
to
h/λ λ represents
the size of the
⎡ 3.5na h ⎤ ⎡ 4na b ⎤
b/λ
⎢1 +
⎥ ⎢1 +
⎥
wind gust
V
V
h
h ⎦
⎣
⎦⎣
• The reduction factor, RA, in Table D3 (p.16) does
not apply to the Gust Factor Method in
Appendix F
Step 3 – Topography Factor (1)
• Wind Code 2004
– Speed up ratio adopted from BS6399-2:1995
except that the altitude factor in BS6399-2 was
excluded
(In BS6399-2, altitude factor is used to adjust
the basic wind speed for the altitude of the site
above seal level.)
Step 3 –Topography Factor (2)
Step 3 –Topography Factor (3)
Step 3 –Topography Factor (4)
Step 3 –Topography Factor (5)
These examples are taken
from British reference book
based on British Code. Due to
the different requirements in
British Code and Hong Kong
Code regarding the idealization
of the hill/slope, the actual
hill/slope shall be differently
idealized under the two Codes.
These examples from British
were for illustration only and
the method of idealizing the
hill/slope should not be copied
for application to Hong Kong
Code.
Step 3 –Topography Factor (6)
Step 3 –Topography Factor (7)
Comment: Idealized slope (a) may be more appropriate for Hong Kong Code.
Topography Factor (App. C of HK Wind Code)
Forces on Buildings
1.
Total Force on a Building
F = Cf Σ qz Az
where Cf = force coefficient
qz = design wind pressure at height z
Az = effective projected area of that part of the
building corresponding to qz
2.
The effective projected area of an enclosed building shall
be the frontal projected area
3.
The effect projected area of an open framework building
shall be the aggregate projected area of all members on a
plane normal to the direction of the wind
4.
Each building shall be designed for the effects of wind
pressures acting along each of the critical directions
Force Coefficeints
A.
For Enclosed Building
a) Cf = Ch x Cs
b) From other international codes accetped by
BA
c) For building with isolated blocks projecting
above a general roof level, individual force
coefficients corresponding to the height
and shape of each block shall be applied.
d) For building composed of similar contiguous
structures separated by expansion joints,
the force coefficients shall be applied to
the entire building.
Height Aspect Ratio Ch
Height
Breadth
Height Aspect Factor Ch
1983
2004
1.0 or less
0.95
0.95
2.0
1.0
1.0
4.0
1.05
1.05
6.0
1.1
1.1
10.0
1.2
1.2
20.0 and over
-
1.4
Remark: Linear Interpolation to obtain intermediate values
Shape Factors Cs for Enclosed Building
b/d
Cs
1.0 or less
1.0
2.0
1.1
3.0 and over
1.3
Plan Shape
Rectangular
d
wind
b
Cs for buildings
with closed
spacing
b
d
Remark: Interpolate linearly
Shape Factors Cs for Enclosed Building
Plan Shape
Cs
Circular
wind
Other Shapes
0.75
Cs for the Respective enclosing
rectangular shape in the direction of
the wind
Note:
When the actual shape of a building renders it to become sensitive
to wind acting not perpendicular to its face, the diagonal wind
effects and torsional wind effects should be considered
Reduction Factor RA
•
Gusts are the results of eddies and vortices
•
The speed of gust is a function of its duration
•
The smaller the size of the gust, the shorter will be its duration and the
higher will be the gust speed
•
The larger the size of gust, the longer will be its duration and the lower the
average gust speed
•
A small gust can only create high wind loading on a small local area of the
structure
•
The whole structure should be designed with the speed of a gust which is
just big enough to affect the whole structure simultaneously
•
A 3 second gust can normally engulf a building with frontal area of 300 to
800m2, a longer duration gust is required to be effective on the whole of
the structure
•
A reduction factor is therefore applied when designing buildings of larger
dimensions
(E.C.C.Choi – Commentary on 1983 wind codes)
•
Not applicable for buildings with significant resonant dynamic response
designed by using hourly mean wind pressure
Reduction Factor RA for Enclosed
Buildings
Frontal Projected Area m2
Reduction Factor RA
2004
500 or less
1.00
800
0.97
1000
0.96
3000
0.92
5000
0.89
8000
0.86
10000
0.84
15000 and over
0.80
Note : Linear Interpolation may be used to obtain intermediate values
• Wind Load Case
– X & Y directions are commonly accepted
– Additional wind direction (e.g. diagonal wind
for Y-shape building) is required
– For large frontal area building (say >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 : akskwan@gmail.com