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CIVL4903/9903 ASSIGNMENT  Floor and Roof Systems

Part 1 – Column layout and Prestressing for Typical Office Floors (4 marks)
Structural Layout
The builder (your client) is considering composite steel or post-tensioned (P/T) concrete construction for
the typical office floors, and Mass Timber for upper floors. In this assignment you will develop preliminary
designs using each of these systems to contribute to the tender cost plan.
The brief requires a minimum typical column grid of at least 12m x 8m for the typical floors, and you will
need to think how best to lay out columns to best suit the plan and achieve a good balance between flexible
open space, structural depth, and economy. You are considering two main options for structural layout in
the typical areas of the floor, one in P/T and one in steel composite construction:
Option 1 - Two internal columns per bay spaced at 15m with a 5m cantilever zone at each end and no
perimeter columns – using P/T concrete
Option 2 - One internal column and two perimeter columns per bay to create two 12m spans (allowing
for columns) - using composite steel/concrete
Near the core, column spacing can vary. Try to avoid columns very close to the core (why? – ask a tutor).
Note: Use an E/W grid spacing generally of 8.4m so that at basement levels, columns can align with car
park spaces (not required to design the car park in this assignment).
At the west end, the floor will be supported fully on the braced steel structure
Design Loads for typical floors (for composite steel and P/T options)
• Live Load = 3kPa (with live load reduction per AS1170.1 cl 3.4.2,applies to beams only in this
case, find out why?)
• Super Imposed Dead Load (SDL) = 1.5kPa
• Self weight (concrete 25kN/m3, steel from section tables)
• Dead load of facade 2.5kN/m around full perimeter.
Questions for Part 1
a. Make A4 sketches to 1:200 scale showing the two options for structure on a typical floor. Show
edge of slab, columns (draw as 900mm diameter although they may vary at different levels), beams
(steel beams as a single thick line, concrete beams as two dashed lines) core outline (assume 10m
x10m) and gridlines. No need to show structural sizes yet (typical sizes will be assessed in
following parts).
Prestressed Beam and One Way Slab - Schematic Design of Beam and Slab sizes
Assess the following for one typical band beam and one slab span:
b. Allowing that the band-beams will normally span in the long direction (N/S), work out an initial
depth for your beams based on the layout Option 1 from part 1 above. Adopt a beam width of
1800mm. Use L/D ratios for 'End Span', see Tutorial 2.
CIVL4903/9903 2024 S1 –ASSIGNMENT 4 - Floor and Roof Systems
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c. Work out an initial depth for the slab (round up or down to nearest 10mm).
d. Return to your Option 1 plan in Part 1 and add the typical sizes to it. Include a 300 deep x 500 wide
thickening along each façade to stiffen the edges.
e. What beam and slab depths would you expect for comparable designs with reinforced concrete
band beam and slab?
Conceptual Design of the amount of prestress for typical spans
For structures with normal design live loads (3-5kPa), a rule of thumb that is often used is to calculate the
amount of strand required to balance about 80% of the structure self-weight dead load.
For this exercise we will study the actions in a main 15m span. In real life, pattern live loads will
complicate the design, but we will ignore these for now.
Carry out the following for the main 15m span of layout option 1 above:
f. Calculate the total self-weight dead load (concrete slab + beam) carried by an individual band
beam. Use units of kN/m.
g. Calculate the amount of prestress force ‘T’ required to 80% balance this load on the prestress.
Remember T x e = wpL
2/8
h. Calculate the number of strands required to provide this load balance, and round up to the nearest
multiple of 4 – there are normally 4 or 5 strands per duct.
Parameters:
• Use 12.7mm strands, refer to tutorial (and AS3600 Table 3.3.1)
• The effective prestress in service conditions is approximately 65% of fpb. This can be
taken as 120kN per strand for 12.7mm strands.
• The depth from the top and bottom surfaces of the beam to the centroid of the strands
is 60mm at the support and the mid-span respectively. Hence e = D - 120mm.
Note: Strands are normally stressed to about 85% of fpb, but subsequent losses of stress occur due to
anchor draw in, friction in the ducts, elastic, creep and shrinkage shortening of the concrete, and
relaxation of the strands. Due to these effects the effective stress available in service conditions is
generally around 55 - 65% of fpb.
When a member is loaded to its ultimate strength bending condition, the bending stresses in the strand
increase from this level up to the σpu value, similar to the development = t of yield stress in normal
reinforcing steel as the section bends. For this reason the σpu values used in strength design are
typically in the range of 0.90 – 0.95 of fpb.
CIVL4903/9903 2024 S1 –ASSIGNMENT 4 - Floor and Roof Systems
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Part 2: Composite Steel (3 marks)
a. Sketch a plan of a typical bay layout for layout option 2 using tables of calculated beam results
to assess typical beam sizes for office loading. Secondary beam spacing to be 2.5m to 3m.
Span tables for simply supported Composite beams calculated to AS2327.1 2017 are available on Canvas,
in 'Data', document Design Note 3. (Use Table 1, ‘Standard Office’ using the next higher span length if the
table does not include specific values).
You can alternatively use the software provided by Onesteel, to install on your own Windows computer.
https://www.infrabuild.com/en-au/resource-centre/resources/comppanel-v3/
For Option 2, (two span) assume all members are simply supported.
b. Compare the calculated or tabulated beam sizes from part a. with two x 12m simply supported
spans, to the approximate rule of thumb for span/depth shown in the lecture slides. How well
does this approximate estimate match the calculated results from tables or software?
CIVL4903/9903 2024 S1 –ASSIGNMENT 4 - Floor and Roof Systems
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Part 3: Mass Timber floors (4 marks)
• For reasons of sustainability and aesthetics, the upper levels of the building are proposed to be of
mass timber construction.
• The floor structures at level 31 and 32 will be similar to each other and constructed of mass timber,
using Glulam beams and CLT floor panels.
• Note feasible spans for mass Timber are less than for P/T and composite steel structures
• The required fire rating, is 120 minutes.
• The floor of Level 30 will be conventional steel composite or P/T concrete. It will be strengthened
to accept added loads from columns supporting timber floor above, which are likely to be a closer
spacing than at the typical levels. You do not need to design this floor but consider it when
choosing positions for columns supporting Level 31 and Level 32.
• Design load is as for the typical floors, with SDL 1.5kPa and Live load 3 kPa plus self weight (of
timber)
Questions for Part 3
a. For one of the two similar timber-framed floors at level 31 and 32, sketch a feasible arrangement in
the form of a full floor plan and describe the main features and reasons for your choice. Show the
columns supporting the timber, the beams, and the layout of CLT panels, span direction of CLT
and how lateral stability of the timber floors will be achieved. Consider how building services could
be distributed from the core to each part of the floor. Structural sizes are not required in this part.
b. For the selected layout estimate the main structural loads and provide member sizes with sketches
and approximate calculations of key floor members, for a typical part of the timber framed floors at
Level 31 and 32 (ie one secondary and one primary, or one main beam if there are no
secondaries, and one typical CLT floor panel).