Sandwich Roof Panel 1

[asixth]

Hi

Does anyone have experience designing sandwich roof panels? I am designing a structure with this form of roof framing in-lieu of conventional framing (light gauge purlins and roof sheeting).

The manufacturer has provided me with load tables, working backwards from these loads tables I am able to determine a typical moment capacity (i.e using the design load and span length to give and ultimate moment/stress). However, I am concerned about whether the roof panels themselves will deflect too much under serviceability conditions.

In one of the product brochures they wrote about deflections and EI values, typically the EI value increased with depth in an logarithmic fashion but also increased significantly with span length which indicates that the load-deflection response is non-linear.

Also, would it be ethical to assume that the sandwich panel can take in-plane loads. For example, I want to use the panel to restrain the top flange of a rafter for lateral torsional buckling. To do this, I must prove that the panel can resist 2.5% of the flange force in the lateral direction, this equates to 10kN (2.2kips) in-plane force for the panel (essentially 2 layers of roof sheeting in-filled with polysytrene and adhesive for bond).

Has anyone had any experience with this type of material and have any advice, or does anyone know of a good manufacturer of this product or technical papers that have been published?

I have been looking at a product called Ritek.

All help will be appreciated.

 

[ishvaaag]

These materials are addressed in the book

LIGHTWEIGHT SANDWICH CONSTRUCTION

J.M. Davies

Published on behalf of CIB Working Commission
W056 Sandwich Panels (joint CIB-ECCS commission)

Perusing the book seems not to reveal use of the panels for bracing of structural members. This is not to say it can't be done but it says it is not typical. You may analyze the thing if you have will, patience and time enough to delve in both FEM and composite structures. There are manuals dealing with these things like the

HANDBOOK OF COMPOSITES
SECOND EDITION

Edited by S. T. Peters

CHAPMAN & HALL

 

[rowingengineer]

asixth,
If you’re after another manufacture Bondor is in Aust and NZ. I assume you already know that the Panels are 0.6- 1.2m wide with lengths of up to 15m, but difficult to get a connection between them that can take load, but can be done. As for published data, I don’t know of anything other than the bondor technical manual, However I am sceptical and don’t use the product that often because last time I checked they had no roof screw hold downs tests.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[hokie66]

Ritek has concentrated on selling its products to architects rather than to structural engineers. I would not depend on the panels for bracing.

 

[BAretired]

I agree with hokie66.

BA

 

[rowingengineer]

Kingspan has some good data too, a lot more about fixings, as I have just found out. mind you they use a normal roof screw, it is typical for most installations to install roof screw with cyclone washers.

As for the top flange bracing, I don't agree with Hokie and BA, for bracing of a complete building it isn't much chop (Maybe Region A), but for bracing of a top flange, I can see it working. If a C300-34 Purlin can be braced by roof sheeting, why can't a rafter be braced by panel?. You just have to do the calcs or get some test data. However for some reason 10kn per fixing sounds high, is this for fising at each rib totaling to 10kn?

I have been in contact with bondor tech today on another project, if i get anything good i will post.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[asixth]

Thanks ishvaaag, I found that resource to be very helpful and has noticed that the author has plenty of published papers in the ASCE Journal of Struct Eng.

To products in Australia (Ritek and Trideck) both have published data where they show E*I (bending stiffness) to reduce with span length. However, the Davies textbook explains how shear deformations are significant for this building material and contribute more to the total deflections at smaller spans hence the reason why product data would have and increase in bending stiffness with length.

While speaking about shear deformations, I actually found it to be one of the best resources I have read to explain shear deformations. Shear deformations is something that I have found best explained in Timber textbooks (and now this one) and is something that isn't often covered in elementary mechanics books.

I have been caught out with increase wind pressures near the edges and corners of the building which means the sandwich panel does not have the capacity to span the distance which I need it to.

rowingengineer,

I don't need the composite panel to take 10kN per fixing, but I need it to take 10kN at the peak moment region to provide adequate lateral restraint, similar to a fly brace to the bottom flange. I would be prepared to use one panel and say 5-6 fixings to resist this load, and then I would be prepared to say that the sandwich panel adequately restrains the top flange.

 

[OzEng80]

The Ritek marketing men are wizards - i have spent a lot of time in the past convincing architects that the product is not suitable for most of their applications (I work in a high wind speed area with predominantly 'architectural' roofs).

We had a recent application in the office with a similar product where an independent bracing system (for LTB and global) was provided below the sandwich panel. It was determined that the lengths of screws etc. precluded its suitability as a bracing element - that and the manufacturer not having the foggiest idea what we were talking about.

I believe in this situation your design restraint force is probably smaller anyway. Assuming that your beam only requires the single, midspan (downward load, ritek on top) restraint for strength and it is continuously supported along its length - then you can spread out the restraint force and take it over more fixings (ie it isn't a concentrated action like a fly brace).

 

[asixth]

I just want to quickly re-post. I have looked over several different products now which all have span tables. I want to quickly do some of my own numbers to try and verify the capacity of these members and relate them back to the published load test.

Does anyone know the shear modulus of low density expanded polystyrene core. The Davies textbook mentioned above goes through a deflection calculation of sandwich panels using a shear modulus of 3MPa (0.435pcf). Some data published from a US company suggests that the shear modulus is 0.226ksi (1.55MPa) for a 2.0pcf material (320kg/m^3 which I assume would be low density).

Any guidance would be welcomed.