point loads on concrete slabs during...
TRANSCRIPT
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Point Loads on Concrete Slabs
During Construction
Dr Russell Keays
Keays Engineering
IntroductionThis paper arose from continuing requests from steel fabricators and
others to tell them which equipment could be used on concrete floors
without fear of cracking the concrete. I found that estimates by others
were simplistic and most restrictive.
• Loads from typical construction equipment
• Metal Tray Slab Capacity
– Punching Shear
– Sagging Moments
– Hogging Moments
– Crack Control
• Reinforced and Prestressed Slabs
• Code & Legislative requirements
• Conclusions.
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But first – some cracks
3In-service cracking at a Dan Murphy store.(I must be doing something right – I have no evidence of cracking from construction loads.)
Access Equipment
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Telehandlers and Forklifts
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Tyre inflation
pressure is
used to
calculate
footprint.
Spider Lifts
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Mini Cranes with long outriggers
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Bubble Cranes & Frannas
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This is what happens if you don’t do
calculations!
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Incident: Friday Morning 26/09/08 – Castle Hill/Sydney Australia
- 9 Day Old 250t Leibherr Crane (never used before)
- $4mill price tag
- Failure of back propping beneath the 200mm thick concrete deck
- Crane support outrigger punches through slab causing crane to lose
balance and collapse across the site and onto adjoining property
- Crane balanced in the air for approx 1 hour before entire rig & boom
collapse completely across site and rigs falls through to the basement
level.
Amazing that no-one was seriously injured or killed.
Other bits and pieces
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Scaffold stanchions28kN working
42kN ULS
Panel Props – up/down forces
Best located directly over band beam
Loading DocksDown Loads on edge of slab. Up load
internal.Design props to be stronger than primary beam
strength in case Dock is hit by crane
hook/descending load.
Tower Cranes and
Concrete Pumping Units
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Usually require change to concrete design.
So position is typically fixed before concrete in placed.
Calculation of
Wheel/Outrigger Loads• Sometimes provided by supplier
• Otherwise:
– Calculate overturning load from load
– Estimate boom weight & its overturning load
– Assume design for stability is 75% of overturning load to get
c.g. of machine’s weight.
– Thence simple maths
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Summary of Typical Loads
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Machine Working Ultimate Pad Size Spacings Notes
kN kN* mm m x m
Spider Outrigger 16 24 133 x 133 3.0 x 3.0 Adjacent at 50%
Scissorlift 18 27 250 x 120 2.1 x 4.0 All
Formwork Standard 28 42 150 x 150 2.4 x 1.2 All
Boomlift 10m 34 51 300 x 188 1.7 x 1.7 Adjacent at 50%
Forklift 35 53 250 x 235 0.9 Front Axle
Mini Crawler Crane 42 62 200 x 200 4.5 Adjacent at 50%
Panel Prop 41 62 225 x 140 1.5 typ. Maximum
Telehandler 50 75 603 x 414 1.85 Or 80% at 1.5t max
Boomlift 18m 53 79 400 x 219 2.2 x 2.2 Adjacent at 50%
Bubble Crane 115 172 400 x 400 4.5 Adjacent at 50%
Analysis of Metal-Tray Slabs - Punching Shear
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1. Assume load uniform over pad (or tyre footprint).
2. No particular rules for metal-tray slabs in AS3600.
3. EuroCode consider along-rib but not across-rib shear.
4. ANSI/SDI C2011 (USA) adopted, but with AS3600 factors.
5. Along ribs – same as AS3600, but add shear strength of ribs.
6. Across ribs – used section depth above ribs, transverse reo used to calculate
capacity.
Sagging Moments
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Effective width – AS3600 Cl.9.6 gives Pad Width + 30% Span.
Used ½ this following yield line analysis.
Assumed simply supported span of 3m.
Loads are for 18m Boomlift. Bending strength – passes.
Hogging Moments
• If slab is strong enough in sag as a simply supported
beam, then analysis of Hogging Moments is not required.
• Otherwise, it depends on how much fire and crack control
reo is provided.
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Crack Control
• As a general rule, there should be no wheel or equipment loads on the slab until it reaches
design strength. It might be worth considering a lower strength if calculations show a partly
cured slab can take the design loads.
• If a screed topping is to be added to the slab after steel construction work above is
completed, this should seal any cracks developed during construction. Hence, analysis of
cracking during construction should not be required.
• Detailed analysis depends on the amount of negative reo in the slab.
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Conclusions on Metal Tray Slabs
• It looks as though a 130 slab in N32 with 1.0mm Bondeck is tough
enough for the normal range of equipment for the steel erector.
• The author has some doubts about whether the effective width in
AS3600 applies to metal tray slabs. Someone should do some
testing!
• ANSI/SDI C2011 (USA) recommends against using metal tray
slabs where fork-lifts are to be used in service.
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Reinforced & Prestressed Slabs
• Given the wide range of possible configurations of the slabs, it is not so easy
to generalize on their capacity.
• But then there is typically less need for heavy plant for constructing the floor
above. (Except perhaps a forklift for removing formwork.)
• This may not be the case when the Plant Rooms have a steel-framed roof.
However, Plant Room floors are typically designed for 7.5kPa, making them
more likely to be robust enough to take the loads from the steel erection
equipment.
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Code & Legislative Requirements
• AS/NZS 1170.1 “Actions resulting from construction are not covered in this Standard”. The
Commentary warns of consequences, but provides little further assistance.
• AS3600, Clause 1.4(d) requires design documentation to include “Any constraint on construction
assumed in design”. At face value, that would require the designer to nominate the maximum wheel
loads allowed during construction, but I have never seen that!
• “Safety in Design” legislation in some States requires the permanent works designer to consider
construction needs, but all I have seen is delegation to the builder.
• Is this appropriate? Given the current situation where buildings and bridges are more likely to collapse
during construction, isn’t it time that permanent works structural design engineers paid more attention
to providing a safe working environment for the construction crew?
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END
© Keays Engineering 2016
Dr Russell Keays
Keays Engineering
367 North Road, Caulfield South
0412 770 700
Point Loads on Concrete Slabs
During Construction
Other floors
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