CIVL4903/9903: Civil Engineering Design
Semester 1 - 2024
Structural Floor Systems
Reinforced Concrete, Prestressed Concrete, Composite Steel & Mass Timber
Weeks 9 to 12
This week (Week 9)
• Design brief
• Floor systems
• Reinforced and Post-tensioned concrete,
• Tutorial 2
• 2nd lecture - step through Tutorial 1, view online
Next week (Week 10)
• Composite Steel
• Case studies, view online
Weeks 11 and 12
• Timber and roof design
Assignment 4 – due in Week 13 Friday 24th May at 11.59pm, submitted on Canvas
People
Services
What is a building?
Ground
Level
Soil
Columns
Foundations
Stability
Floors
Walls/cladding
Roof
Design brief: A new Sydney tower
Example: 8 Chifley Sq - 2013
Example: 100 Mount St – 2019
7
5
0
0
1
0
,0
0
0
7
5
0
0
1000 42,500
N
o
rth
Reinforced
concrete core
(10m x10m
preliminary)
Steel
frame
Design brief: Typical floor plan
East/West Section
Roof 130.7m
Level 32 126.7m

Level 31 122.7m
Level 30 118.7m
Level 29 115m
Level 28 111.3m
Level 3 18.8m
Level 2 15.1m
Level 1 11.4m
Ground level 6.4m
Basement 1 3.2m
Basement 2 0m
Column number and
positions indicative TBC
Assignment 3
Lateral stabilty
Assignment 2
Foundations and Geotech
Assignment 1 Bridge
Assignment 4
Floor structures
Design parameters
• What column spacing to use?
– At least 12m x 8m (timber floors less)
– Main columns straight down to foundation. Avoid transfer
structures – needs to work through car park
• What type of structure?
– Reinforced Concrete? Prestressed Concrete? Steel?
Timber?
• Design loads:
– Self weight 1.5kPa Superimposed Deal Load SDL,
– 3kPa Live Load
• Façade load: 2.5kN/m dead load around perimeter
• Design issues
– Cost, depth, weight, constructability, durability
CIVL4903/9903 – Week 9 to 12
• Intro to common structural floor systems
• Concepts for:
– Reinforced concrete
– Post tensioned concrete
– Composite steel
– Timber
• Simple scheme guidance for preliminary design
– Week 9 - RC and P/T concrete
– Week 10 – Composite steel
– Week 11 & 12- Timber and roof
Tutorial 2 and Assignment 1
Assignment
• Part 1 for Week 9 . Work out and draw a column grid
• Preliminary scheme in P/T concrete – Typical bay
• Preliminary scheme in composite steel/concrete
• Preliminary scheme in timber
• Cost estimates
• Comparison
Tutorial 2 – Post tensioned concrete – Week 9
• Explore ‘Rules of Thumb’ and P/T concepts
Assignment 4 - Part 1- Typical Office Floor Assingment will be submitted individually
1. Structural Layout (5 marks out of 20)
• The builder (your client) is considering composite steel or post-tensioned (P/T) concrete construction for the typical office
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, 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
• 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).
• 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
Structural floor systems – some basic concepts
▪ Horizontal spanning
▪ Transfer gravity loads to the vertical
system
▪ A significant % of the structural cost
▪ Highly repetitive, therefore, efficiencies
will be realised throughout the building
>> cost savings
▪ Categorised as one and two way
spanning
Concrete vs Steel Floor Systems
Concrete
▪ Traditionally more cost effective in Australia
▪ ‘Formed’ and adapted into many ‘systems’
▪ Reinforced or Prestressed
Steelwork
▪ Used extensively in the UK and USA
▪ Offers relative speed of construction.
▪ Lighter floor loads.
▪ Additional strength through composite
action
One way systems
▪ There are very few true one way
systems
▪ Mostly in houses and low rise units that
use fixed internal wall systems
▪ Introduction of beams and columns
creates a hybrid system:
➢Individual elements span one way
➢All loads have to be carried two ways to
reach the columns
▪ This is an important principle when
designing two way slabs.
Two way systems
▪ Reinforced & Prestressed concrete:
➢ Ribbed slab and beam
➢ Beam and slab
➢ Band beam and slab
➢ Flat plate
➢ Flat slab
▪ Structural Steel
➢ Beam grid with permanent metal
deck formwork
▪ Span in two directions to bring
loads back to a column grid.
▪ Opens up the floor space - much
greater flexibility of use.
▪ Used widely in all major structures.
▪ offices; car-parks, high rise
apartments; exhibition floors and
so forth.
Concrete vs Steel Floor vs Timber Systems - Continuity
Concrete
▪ Generally designed as ‘continuous’,. Ie
multi-span beams
▪ Benefit of using shorter end spans and
cantilevers
Steelwork and Timber
▪ Generally designed as a series of simply-
supported beams so simple joints
▪ More difficult to achieve cantilevers
▪ But can create continuity, but with more
complex joints (steel loses composite
action in hogging zone)
Concrete Floor Structures
Reinforced and Prestressed Concrete
Ribbed slab and beam Beam and (2 way) slab Band-beam and slab
Flat plate – 2 way Flat slab – two way
Reinforced and Prestressed Concrete
Ribbed slab and beam Beam and (2 way) slab Band-beam and slab
Flat plate – 2 way Flat slab – two way
Common
Office & Car-parks
Prestressed
Historic &
Precast
Historic &
Industrial
Residential
Smaller spans
Large areas:
Hospital &Office
Drawing Floor Structures
Sketching Concept Designs
Concrete layout plan drawing - Band Beam
2400
360 150
Step in
underside
Gridline – for
reference
Column
Thickness of
concrete
A
A
Section
Beam and slab depths
Slab span for L/D ratios in band beam systems
Beam depth slab depth
Section A-A
Concrete layout plan drawing
Flat slab
300
Slab span for L/D
ratios in flat slab
systems
400
400
400400
Step in top
300
400
Step in top and soffit
300
300
300 step
Steelwork marking plan drawing
120
Sketching Concept Designs
P/T design options for assignment: 25m x 8.4m
Reinforced and Prestressed Concrete
Ribbed slab and beam Beam and (2 way) slab Band-beam and slab
Flat slab – two way
Office or
Car-park
Prestressed
Historic
Industrial
Large areas – 70s
& 80s - band
beams are more
popular today
Two way slabs supported on beams and walls
30
One way spans vs two way spans
50%
50%
Two way flat slabs
31
One way spans vs two way spans
100%
100%
-WL/10 -75% -25% -75%
WL/12-16 +55% +45% +55%
-WL/10 75% -25% -75%
Two way flat slabs & flat plates
32
Flat slab, moment distribution – approximate – applies in both directions!
0
Overall bending
in one bay approx
Column strips
Middle strip
-75% 55% -75%
-25% +45% -25%
-75% 55% -75%
Two way flat slabs & flat plates
33
Flat slab, moment
distribution – approximate
0
Column strips
Middle strip
Two way flat slabs – Hospital Floor
34
Note thicker slabs for end spans
Two way flat slabs – Hospital Floor
35
Prestressed option – note reduced thickness of slabs.
Beam and slab (Band-beam system)
36
Beams span long span, slabs span between beams
100% 100%
One way slab and simply supported beam
Office floor

One way slab and two span band beam – office floor

One way slab and band beam – office floor

Floor Systems and Design Creativity
▪ Systems have evolved into current forms
over time
▪ The result of intense design creativity
▪ Systems can always be extended in new
ways.
▪ No 1 Bligh St floor system - many
creative features
No 1 Bligh St Typical Floor
Post-tensioned Prestressed Concrete
• Many applications
• Very common in
Australian building
construction
• Important to
understand some
basic concepts
• Post-tensioned
• Grouted = bonded
tendons
Prestressed Concrete

Post-tensioned Prestressing: – 2 key concepts: Pre-compression and Load balance
1. Beam with normal non-
prestressed reinforcement:
Cracking reduces stiffness,
increases deflection
2. Beam with external Pre-
compression, so less cracking,
less deflection
3. Tendons apply prestress:
Tension in tendons add
compression to concrete
4. ‘Draping’ the tendon:
provides upward load to carry a
% of self-weight
Unloaded Loaded Stresses Effective
section
Concrete in lower part of
beam has cracked in tension,
so ineffective. Only the
reinforcement is in tension
Post-tensioned Prestressing: – 2 key concepts: Pre-compression and Load balance
1. Beam with normal non-
prestressed reinforcement:
Cracking reduces stiffness,
increases deflection
2. Beam with external
Pre-compression: so less
cracking, less deflection
3. Tendons apply prestress:
Tension in tendons add
compression to concrete
4. ‘Draping’ the tendon:
provides upward load to carry a
% of self-weight
Unloaded Loaded Stresses Effective
section
Post-tensioned Prestressing: – 2 key concepts: Pre-compression and Load balance
1. Beam with normal non-
prestressed reinforcement:
Cracking reduces stiffness,
increases deflection
2. Beam with external
Pre-compression: so less
cracking, less deflection
3. Tendons apply prestress:
Tension in tendons add
compression to concrete
4. ‘Draping’ the tendon:
provides upward load to carry a
% of self-weight
Unloaded Loaded Stresses Effective
section
Post-tensioned Prestressing: – 2 key concepts: Pre-compression and Load balance
1. Beam with normal non-
prestressed reinforcement:
Cracking reduces stiffness,
increases deflection
2. Beam with external
Pre-compression: so less
cracking, less deflection
3. Tendons apply prestress:
Tension in tendons add
compression to concrete
4. ‘Draping’ the tendon:
provides upward load to carry a
% of self-weight
= Load balancing: Less bending
stress & less deflection
Unloaded Loaded Stresses Effective
section
Prestressing Concepts– Load Balancing

Prestressing Concepts– Load Balancing
Need to prestress against something –in this case the slab:
A suspension bridge cable is loaded, but not prestressed.
Although parts may be ‘jacked’ for dimensional adjustment

Prestressed Concrete – Load Balancing
Consider the concrete as a free-body
If the prestress uplift ≈ the concrete self weight.
Concept > Load balancing: T x e ≈ wL2/8
What is the equilibrium of the concrete as a free body?
Uniform compression stress σ = C / A - normally called the ‘P / A’
(Remember: the tendons are un-grouted at this stage)
Equilibrium
Prestressing Concepts – Key Developments
• Higher strength concrete – higher load support and high
anchor stresses.
• High tensile strands – up to 1870 MPa
• An amount of the prestress is lost due to concrete creep and
shrinkage.
• For example a shrinkage strain of 500 x 10-6 leads to 100 MPa
loss of the prestress – significant in lower strength bars.
Prestressed Concrete – Improved Serviceability
• Sections uncracked and load balancing helps
support dead load
• Much reduced deflections
• Allows sections to be approximately 70% of
the depth of an equivalent RC section
• Seen in span / depth ratios – explored in
tutorial
P/T and R/C Slabs
54
Post tensioning allows longer spans to be achieved
Rules of Thumb – Simplified Design Aids
• Designers have developed Rules of Thumb through
experience. For example:
➢ Span / Depth (L/D) ratios for initial sizing of members –
some examples are given in the tutorial.
➢ In prestressed concrete design start by calculating the
amount of strand required to balance about 70% -80% of
the DL (This will vary with structure type, loads etc.)
➢ Reinforcement ratios used in scheme design have been
developed through experience of existing designs.
• These are very useful during conceptual design stages.
• Be aware of limitations - they must be adapted when
parameters vary – for example under very heavy loads.
Initial Span/depth ratios for estimating RC and P/T
floor structures
Explore in Tutorial 2: why do these work?
Why is P/T less deep than RC?
Design Assignment
1. Rules of thumb
2. Practice simple scheme design techniques
3. Make sketch drawings of concept designs to communicate
preliminary information.
4. Computer programs will carry out the detailed design
refinements, but:
5. Need to start with a structural scheme that is sensible for the
application!
Guidance for Scheme Design
General
• Arup Scheme Design Guide
› http://www.mediafire.com/download/xygkvrc41caf9uv/STRUCTURAL+SCHEME+DESIGN+GUIDE+BY+ARUP.pdf#!
Concrete Framing
• Post Tensioning in building structures, Ed Cross, PTIA:
POST-TENSIONING IN BUILDING STRUCTURES download / post-tensioning-in-building-
structures.pdf / PDF4PRO
• Guide to Long-span concrete floors, C&CAA:
http://www.ccaa.com.au/imis_prod/documents/Library%20Documents/CCAA%20Technical%2
0Publications/CCAA%20Guides/CCAAGUIDE2003-T36-Long-span%20floors-TBR.pdf