Designing a Composite Member
Designing a Composite Member
(OP)
Hi,
I am trying to design a composite floor member that will be subjected to the typical floor forces.
The idea is to create a composite floor element that uses Cross-Laminated-Timber (CLT) and Autoclaved Aerated Concrete (AAC).
The reason for this is to increase the Fire Rating of the floor system.
The AAC is to be fixed on top of the CLT using screws - each screw has a pull-through capacity of over 1kN.
How do I ensure the connection between the two materials are composite? Is there a way to calculate the minimum spacing and number of screws required?
Thanks
I am trying to design a composite floor member that will be subjected to the typical floor forces.
The idea is to create a composite floor element that uses Cross-Laminated-Timber (CLT) and Autoclaved Aerated Concrete (AAC).
The reason for this is to increase the Fire Rating of the floor system.
The AAC is to be fixed on top of the CLT using screws - each screw has a pull-through capacity of over 1kN.
How do I ensure the connection between the two materials are composite? Is there a way to calculate the minimum spacing and number of screws required?
Thanks






RE: Designing a Composite Member
RE: Designing a Composite Member
This seems like more hassle than it's worth.
RE: Designing a Composite Member
I am not sure how to determine the strain capatability of the two elements, however as the AAC element is reinforced, I would have thought the only check required would be the shear flow at the junction of the two materials and ensure the screws can withstand that force.
The shear capacity of the screws in the AAC panels (just before it starts ripping through the AAC) is 0.85kN per screw.
Can I simply use this to ensure I meet the shear flow requirement?
RE: Designing a Composite Member
Some project specific testing has definitely been done and associated design methods developed. I'm not sure as to whether or not provisions have been adopted into any major codes. I'd recommend contacting FPInnovations. They're at the forefront of the research associated with these technologies and often partner with the consulting firms designing the projects. If anybody knows where the state of the art is at, it'll be them.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Designing a Composite Member
Without knowing the strain compatibility between the two materials, you can't determine shear flow. If the Young's Modulus of both materials is known, a transformed section may be found from which the shear flow may be calculated.
BA
RE: Designing a Composite Member
Thank you for the insight, I will start looking into this and will also see what FPInnovations have done. As I am using prefabricated elements (not possible to do with in-situ AAC, since it would all need to be cast in an Autoclave), I am trying to get a screw option to work. Preferably vertically installed screws as I know the shear capacity through the AAC.
@[BAretired]
I see your point made regarding the fire protection from the top of the CLT - not too sure why this is the case, however I will ask the question.
I am slightly confused at this stage as my understanding of shear flow (as of this today once I have started to look deeper into it) is that q = VQ/I
Does the youngs modulus come into effect where I calculate Q[=A'Ybar']?
Would that mean A' would be equal to the transformed area of AAC to CLT? as opposed to A' being the area of AAC not transformed?
Seems to make sense to me now that I have written this - no other areas in the calculation requires the need for youngs modulus from what I can see.
RE: Designing a Composite Member
I believe that is correct. Also, to find 'y', you need the center of gravity of the transformed section.
BA
RE: Designing a Composite Member
Thank you very much! It's all clear as crystal now
RE: Designing a Composite Member
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Designing a Composite Member
There has been independant testing that has determined the shear capacity of screws in reinforced AAC panels. The recorded results included characteristic values at both the initial shear slip as well as ultimate shear failure.
For the purpose of my calculations, I was only going to use the initial slip values to ensure a conservative design.
Is there anything that states I cant do this? Am I looking at this incorrectly?
RE: Designing a Composite Member
BA
RE: Designing a Composite Member
Neat. Can you direct us to this online? Was the testing done on AAC fastened to wood?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Designing a Composite Member
No, the concept was to only use screws. This is to ensure the install time is kept efficient and the costing can be kept to a minimum. However this may be considered should it be found that the screw-only option is not viable based on the calculations.
@[KootK]
Unfortunately as this assessment has been completed using a specific brand of AAC I cannot disclose any documentation at this stage unless approved by the document owners.
Initial testing was completed using AAC screwed into timber, however as it was quickly determined that the AAC panels governed the design; the rig setup was changed to eliminate another variable in the testing.
The testing rig setup was updated to using a steel angle connected to a hydraulic ram which had a screw passing through the angle and into the AAC panels. The ram would then be loaded which produced a shearforce vs deflection curve. Hence we could obtain initial shear-slip and ultimate shear capacity.
RE: Designing a Composite Member
A. Initially, the AAC will have to be drawn down to match the self weight deflection of the CLT. That will lock in some non-composite stresses into both the CLT and the AAC. I doubt it would amount to much but it would be an issue to consider if it cracked the AAC.
B. The concrete and the CLT will both creep under load, at different rates.
C. Only include every other, parallel to stress CLT lamination in your VQiT calcs. That will shift the neutral axis closer to the concrete and increase fastener demand.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Designing a Composite Member
RE: Designing a Composite Member
Similar application from a few years ago here in NZ? I've never seen it used though, no idea if it considered composite action or not.