CLT - Smith and Wallwork | engineers€¦ · How efficient is CLT compared to solid timber [? ......
Transcript of CLT - Smith and Wallwork | engineers€¦ · How efficient is CLT compared to solid timber [? ......
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CLT Cross Laminated Timber
or
Consumes Lots of Timber
…a challenging perspective on whether we should be using our only renewable construction material with more care – focusing on value engineering, design efficiency and lessons to be learnt from the steel and concrete industries.
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Content
• Is solid a problem?
• CLT structural efficiency
• Steel & Concrete
• Options for CLT
• Conclusions
“The best friend of man is the tree. When we use the tree
respectfully and economically, we have one of the greatest
resources on the earth”
Frank Lloyd Wright
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Is solid a problem?
No – our forests are growing faster than we harvest
In the EU we only harvest about two thirds of annual growth. Nearly 200 million m3 of wood is added to our forests each year.
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Is solid a problem?
No – our forests are growing faster than we harvest
Yes – we can build more with less timber using stud A CLT building uses about 0.30m3 of timber for every m2 of floor area provided. A timber stud and cassette building uses about 0.15m3 of timber for every m2 of floor area provided.
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Is solid a problem?
No – our forests are growing faster than we harvest
Yes – we can build more with less timber using stud
No – enhanced robustness, fire and acoustic performance
…but how do we put a value to this?
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Is solid a problem?
No – our forests are growing faster than we harvest
Yes – we can build more with less timber using stud
No – enhanced robustness, fire and acoustic performance
Yes – it is an imported product
Subject to fluctuations in the exchange rate. A CLT contract does not benefit the community that places it.
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Is solid a problem?
No – our forests are growing faster than we harvest
Yes – we can build more with less timber using stud
No – enhanced robustness, fire and acoustic performance
Yes – it is an imported product
No – it locks up significant CO2
For every m2 of building constructed from CLT approximately 250kgCO2 is stored.
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Is solid a problem?
No – our forests are growing faster than we harvest
Yes – we can build more with less timber using stud
No – enhanced robustness, fire and acoustic performance
Yes – it is an imported product
No – it locks up significant CO2
Yes – it costs more than the competition
Steel frame £190/m2
Concrete frame £190/m2 CLT frame £240/m2
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Is solid a problem?
Wood Stocks & Use
• EU27 stocks • 22.5 billion m3 total stocks
• 660 million m3 annual growth
• 420 million m3 annual felling
• EU27 sawnwood use • UK - 0.14 m3 per capita
• Germany - 0.20 m3 per capita
• Finland - 0.80 m3 per capita
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Is solid a problem?
100kg/m2 300kg/m2
£110/m2 £40/m2
CLT floor plank versus pre-stressed hollow core plank
span to depth 30 span to depth 40
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Is solid a problem?
50kg 300kg 50kg
75kgECO2 65kgECO2 20kgECO2
£65 £65 £85
(80kgSCO2)
Beams of equivalent structural performance
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Is solid a problem?
• Doubling of materials demand by 2050 • Due to increase in population and wealth
• Existing industrial processes already efficient • Predicts maximum 50% emissions savings still to
be made (through processes and recycling)
• Culture of ‘..substitution of more material for less labour..’
Ref: ‘Materials and the Environment’ Mike Ashby
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CLT structural efficiency
How efficient is CLT compared to ‘solid timber’?
…how much does cross lamination affect structural performance
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CLT structural efficiency
0.0
5.0
10.0
15.0
20.0
25.0
4 5 6 7 8 9 10
Load (kN/m2)
Span (m)
Brettstapel 240mm
CLT 240 L7s-2
CLT 240 L7s
CLT versus Brettstapel - Floors
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CLT structural efficiency
CLT versus Brettstapel - Walls
0.0
100.0
200.0
300.0
400.0
2 3 4 5 6 7 8
Load (kN/m)
Height (m)
Brettstapel 100mm
CLT 100 L5s
CLT 100 L3s
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Steel & Concrete
What have the competition been doing to increase efficiency?
…and what are they planning
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Steel
• Cellform beams
• Decking
• Cold formed sections
• Fastwall
• Composite action
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Steel
• Re-cycling rates
• Re-using steel section
Ref: WellMet – University of Cambridge
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Steel
• Steel honeycomb • A lattice of interconnected
tubes
• 0.9kg/m3
• Nearly 500x lighter than softwood
Ref: HRL Laboratory, UC Irvine & CalTech
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Concrete
• Rib slab
• Beam and block
• Cobiax
• Pre-stress
Change in concrete compared to solid slab
-30%
-40%
-30%
-50%
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Concrete
• UHPC • Ultra high performance concrete
• High strength fibre reinforced
• Cement, silica fume, fine sand and water reducer
• Compressive strengths up to 200MPa
• Flexural strength up to 50MPa
Ref: Tuborg 15 staircase, Hans Exner
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Concrete
• Carbon negative concrete • Uses magnesium silicates in leau of
cement
• Process could absorb more CO2 than it produces during manufacture
• UK venture Novacem acquired by Calix in 2012
Ref: Novacem & Imperial College London
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CLT options
Can we make CLT more efficient?
…can we break the culture of ‘substitution of more material for less labour’
…or is cost king
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CLT options
0%
20%
40%
60%
80%
100%
Sawnwood CLT manufacture Design CLT fabrication CLT transport CLT installation
% cost
0%
10%
20%
30%
40%Sawnwood
CLT manufacture
Design
CLT fabrication
CLT transport
CLT installation
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CLT options
Develop CLT product to use
20% less timber
• Challenging solid • Use part voided or rib CLT • Increase depth to combat
vibration/deflection
0%
20%
40%
60%
80%
100%
% cost
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CLT options
Develop processes,
reduce waste & standardise
• Pre-form openings and bespoke shapes during pressing to reduce fabrication
• Standard panel thicknesses and performance
0%
20%
40%
60%
80%
100%
% cost
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CLT options
Codify, software,
educate and invest time
• Produces design codes (vibration) • Mainstream multi-product
software (incl. connections) • Efficient CLT designs require
educated designers • Spend time honing designs
0%
20%
40%
60%
80%
100%
% cost
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CLT options
Target 20% reduction in waste during fabrication
• Previous actins should eliminate significant waste during fabrication
• Review jointing methods – standard half-lap joint results minimum 2% waste
0%
20%
40%
60%
80%
100%
% cost
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CLT options
Develop CLT product to use
20% less timber
Target 20% reduction in waste during fabrication
cost saving
10% Codify, software,
educate and invest time
Develop processes,
reduce waste & standardise 30%
wood saving
0%
20%
40%
60%
80%
100%
% cost
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Conclusion
• Design for CLT • A CLT sympathetic building could reduce volume and wastage
• Standardisation required • Of panel sizes and performance
• Design and BIM software required • For both panels and connections
• Research needed • Vibration performance and spread of flame
• CLT product needs to evolve • Concrete and steel is!