CIVL4903/9903 Lateral Stability Systems
Lateral Stability Systems
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CIVL4903/9903 Assignment 3 – Lateral Stability Systems
Assignment 3 to be submitted, by each student by, Friday 19/4/2024 (Week 8) at 11.59pm
Assignment 3 – Tower building (20 Marks total)
This assignment is based on concept design of the lateral stability system for the 32-storey building. It
involves assessing loads and then preliminary design of concrete core and braced steel structures.
Refer to lectures for information on methodology.
Part 1 - Loading (10 marks)
a. Wind load – AS1170.2 (3 marks)
Use AS1170.2 to calculate the net lateral pressure (ie the sum of windward and leeward pressures x
combination factor Kc) on the building at roof level in the ’along wind’ direction, for the purpose of
assessing overall wind forces on the stability system (core and steel brace). Consider the main wind
directions from N, S, E and W. (since building is symmetrical, you can for the assignment, look at the
worst of N or S and the worst of W or E) Use the following data:
• Building height above ground level – see elevation – assume the highest level is roof level at
+130.7m, and that ground is at elevation +6.4m
• Terrain category 3, Sydney CBD
• Wind return period, 1000 years (V1000), regional wind speed for region A2 (see AS1170.2
Table 3.1)
• Direction factor: Md, See AS1170.2 table 3.2. For each of the 4 main wind directions, use the
greatest value of Md within 45°. Eg for wind from the North direction, consider NE, N and NW.
• Dynamic factor : assume Cdyn = 1.0 (from AS1170.2 section 6.2, assumed for this assignment,
based on a period of approximately 2.9 seconds)
(Note we are only looking at roof level pressures in this assignment. A full assessment would require
consideration of pressures varying with height).
b. Wind load - Results from a wind tunnel study (2 marks)
From a preliminary design, assume that the calculated overturning moment, based on a code-based
assessment moment at the base is 360,000 kNm for the East west direction and 650,000 for the
North-south direction,
(i) Based on the preliminary design, a wind-tunnel study has been carried out, which has
returned an envelope of wind forces and moments for use in final design (see wind tunnel
report posted on Canvas). What approximate % difference is there in these results
compared to the preliminary estimates above? How would this information affect the
design?
(ii) Based on the acceleration response figures in the wind tunnel report, does your building
have a serviceability acceleration problem? If it was concluded that the building did have
an acceleration issue, what are two examples of methods you can suggest as a structural
engineer to reduce this problem?
CIVL4903/9903 Assignment 3 – Lateral Stability Systems
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c. Seismic load - AS1170.4 (3 marks)
The seismic load is dependent on the site location, ground conditions, natural period(s) and the
seismic mass above the base level. We will study options for the floors in the next assignment. For
this assignment, assume as follows:
• Number of suspended floors basement 1, ground floor, levels 1 to 32 and roof (consider similar area to
floor) = 35 levels
• Mass of each floor:
• Floor slab 7kPa
• Roof 3kPa (selfweight and SDL)
• Superimposed dead load 1.5kPa on floors
• Live load on floors: based on 3kPa x live load reduction x ψe (see AS1170.0)
• Façade around full perimeter 2.5 kN/m per floor
• Core and columns, allow 1300 kN/storey
Estimate the seismic weight of the building by adding together the seismic weight of all floors and
roof. Assume all floors are equal weight.
Calculate the natural period for seismic design by the simple method in AS1170.4 cl 6.2.3. Comment
briefly on whether this value is more or less than would likely to be calculated from a rigorous
dynamic analysis. What are the main aspects of the building design relevant to dynamics that are not
included in such a calculation?
For the following parts, assume that you have made a dynamic analysis of the building which has
calculated a natural period of 2.9s and that periods for N/S and E/W directions (modes 1 and 2) are
similar
Estimate the total base shear according to AS1170.4 section 6, using the following parameters:
• Site class Ce
• Structural system ‘Limited ductile shear walls’ AS1170.4 Table 6.5 A
Estimate the bending moment at the base due to this force (see lecture slides for a quick
approximation)
Note: The building is founded two storeys below ground, which is the base level.
d. Robustness and load comparison (2 marks)
Robustness
Calculate the notional robustness load and associated moment on the whole building per AS1170.0
Section 6.
Load Comparison
Which load case (wind, seismic or robustness) is most critical for building strength and stability in the
N/S and E/W directions? Comment on whether this is as expected for a building of this type and
height.
CIVL4903/9903 Assignment 3 – Lateral Stability Systems
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(iii) If a similar assessment of lateral loads was carried out for a 3-storey building, how would
you expect this conclusion to change and why? (brief answer - no calculations needed).
Part 2
a. Concrete core (4 marks)
In the E/W direction, the reinforced concrete core carries all lateral forces. In the N/S
direction it shares with the steel brace..
For a preliminary assessment, proceed as follows:
• The concrete core can be considered initially as a rectangular tube. Assume the
core to be 10m x 10m with a wall thickness of 450mm.
• Assume that the core resists an overall bending moment due to ultimate wind
450,000 kNm (based on N/S direction)
• The core carries gravity load due to its own weight and the weight and live load
(with live load reduction) of supported floors. Assume that the core carries 250
sqm of floor at each storey.
(i) Estimate the maximum compression stress due to the above loads, and the
maximum tension stress Use load factors from AS1170.0 for these checks.
b. Steel bracing (4 marks)
The steel frame shares the lateral load with the core for the N/S direction. The distribution
between steel bracing and concrete core depends on the positions relative to the centre of
applied loads, and to some extent the torsional stiffness of the core. We need to chose a
credible preliminary maximum % of lateral load that will be transferred to the steel bracing
system. For this assignment, based on the position of the core and bracing, and the
eccentricity of the force, assume that the brace system may carry 70% of the total wind
force in the N/S direction. (ie 650,000kNm x 70%)
(i) Sketch a conceptual diagram showing the west elevation of the building at a scale
of 1:500 showing how you could arrange the bracing and columns to transfer
lateral loads down to foundation level. At this scale, the columns and bracing can
be drawn as single lines, draw the floor levels and overall building width (see
slides). Add brief notes or description to the sketch to describe how your design
works and its main features.
(ii) Work out an estimate of the ultimate axial forces in steel members, for the lowest
level of the steel bracing and the lowest columns. Base this on the % of total force
noted above.
Take account of the following:
• How many braces do you have at the lowest level to share the shear force? Allow
for the slope of the braces – see lecture slides)
CIVL4903/9903 Assignment 3 – Lateral Stability Systems
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• How will you transfer the load down to the founding level in the basement? (steel
bracing? Or a concrete shear wall below ground? Be clear on your sketch)
• Make sure there is a load path for lateral forces at each level
• Bracing may connect every 4 or 5 levels max – no need to explicitly brace every
level
• Allow for gravity load in the columns, which increases compression and reduces
tension.
Part 3 - Crown Casino example (2 marks).
The last two slides of the case-studies lecture show the Crown Casino tower in Sydney, also
shown below. Describe and explain the significance of the unusual geometrical and structural
features of this building with respect to wind forces and also the structural system. (see how the
outer shape varies with height?, and also columns are not vertical?) Brief thoughtful points on
these issues please! (1/2 page maximum)
page maximum.