Fiberglass Structurals

[miecz]

This issue keeps coming back to me: Fiberglass elements in a structure.

Recently, we were the EOR for a project where the project manager specified a fiberglass roof for two circular tanks. These were constructed of fiberglass trusses and we were asked to review the shop drawings. At that time I could find no stability design criteria for fiberglass. The designer borrowed formulas from the steel code and wood truss plate institute.

The IBC 2003 Code has sections for Stuctural Steel, Aluminum, Masonry, Wood, and Concrete, but none for Fiberglass Structurals. I'm not aware of any Institute like AISC or ACI that publishes a nationally recognized set of design rules for fiberglass structurals.

On a current project, the project manager wants to use fiberglass grating for a 40'x40' floor system over water tanks. The grating supplier has a brochure with extensive span vs. load tables, based on deflection, not strength, and includes the following disclaimer: "This information is provided as a guide to the use and application of our grating and does not represent a specific warranty of the product or its performance. The designer or user must determine the suitability of this product for a specific application."

Right now I see only two courses of action. Select a grating and pray it works, or reject its use outright. I guess my question is: Would you use this stuff? How can I ensure the safety of the floor?

 

[PatBethea]

Several years back, I came across a situation similar to this. Throughout most of the power plant I was working at we had steel grate. At some point, fiberglass grating had been substituted in an area where corrosion had been a problem. Apparently this was done without the maintenance supt.'s knowledge. When he found out he became concerned about its weight bearing capacity, how it performed over time (cyclic loading), and what effects other things such as high temperature would have. Being the junior guy on staff, I had to look into it. I found that the manufacturer was able to provide plenty of technical data when asked and that it was fine in the application for which we were using it.

So, I guess that was a long way of saying that you should call the grating manufacturer. Then call one of their competitors and see what they can offer. The information is out there - you just have to track it down.

And no, I don't think you'll find a standard for fiberglass structures.

 

[csd72]

Treat it like any proprietary product, obtain the necessary code compliance documentation from the manufacturer.

I would not specify something as variable as fiberglas without documentation from the manufacturer to back it up.

regards

csd

 

[UcfSE]

FRP sections tend to be more than strong enough and tend to fail in deflection. This is due to the cracking that takes place in the resin (lowering the stiffness) and also to the lower modulus of elasticity for the given geometric section when compared to traditional materials.

Even if you know this however, that doesn't in itself justify using the product, even if it relieves a little worry about the material.

I would follow csd's advice and request tested (not calculated) values for both load rating to failure and load for different deflections (and the same again after the product has been loaded enough to crack) for the product, and make sure you have the product approval number for the code or state you're in.

 

[Gumpmaster]

Call Fibergrate (

http://www.fibergrate.com).

They have a design guide for their fiberglass structural shapes that lists allowable stresses, section properties, and beam and column tables.

I believe they also have a design service. Their catalog has some pretty impressive structures created solely from fiberglass. I have their full product line binder and it's a great help when evaluating fiberglass grating and structural shapes.

Regards,
Rich

 

[miecz]

So far I've looked at three mfr's product literature. In all three cases, there is plenty of technical data, but the responsibility for structural integrity is disclaimed.

I suppose I can write a code compliance section into the grating specification, but since there is no Fiberglass section in the State Building Code (IBC), I can't very well say, come submittal time, that the product doesn't meet the Building Code. There's nothing there to meet.

Guess Ill specify the live loads, and ask the mfr to certify that the product can "safely transfer the specified loads to the supporting elements shown on the structural plans." Only, I have this gnawing suspicion that no mfr will provide that certification.

Seems that ASCE use to publish a design manual for fiberglass, but it is no longer in print. Anyone know anything about that?

 

[csd72]

I would do a performance spec, even if you dont think it will be certified.

The manufacturer must have done some testing otherwise how do they justify the capacity?

I would also specify UV resistance as this can make it brittle if not protected.

 

[miecz]

Thanks everyone. I'll write a performance spec and ask for a product approval number. The Fibergrate literature lists "ultimate" capacities as well as recommended loads. Not definitive without phi factors, but at least they are talking about strength in addition to deflection.

 

[plasgears]

I had some experience with the extruded FRP structural shapes. At the time I was evaluating reinforced plastics for auto armor applications. One sample was fired upon across the section by as little as a 22cal, and the results were very disappointing. I'm guessing a ball peen hammer would have done serious damage to the section.

This was reported to the supplier, and the response was sobering. He brought his boss to the office, and he went on a heated rant about the product. Managers in surrounding offices were wondering what was going on. I had to quietly usher them out of the office, and I explained as best I could what this was about.

Proceed very carefully with extruded structural shapes. The only reliable claim is that they look structural.

 

[DonPhillips]

If the application is exposed to ultraviolet light, I would be concerned about degradation of the strucutral values over time.

Don Phillips

http://worthingtonengineering.com

 

[youngstructural]

I have had a fair amount of experience with fibre reinforced plasitc sections, having designed a bridge (never built) out of them for my full year design project in the final year of my engineering degree.

Good comments from everyone all round, but you really need to deal directly with the individual manufacturer; And although plasgears has a good point, this would apply only to a particular type of SGPL. The mechanical properties vary based on ALOT of variables, and no two products are alike. Although more pronounced with the earlier products, some of these materials will creep under their own weight and others cannot be used in even the most routine of low temperatures (such as 5 to 10 degrees C) without risking cold snap failure (a brittle fracture with very little to no yielding exibited). I use the term Structural Grade Plastic Lumber (SGPL) because although the term Fibre Reinforced Plastics (FRPs) is common, it's too close to the more well-established Fibre Reinforced Polymer (sheeting) used to upgrade reinforced concrete sections, and now in limited applications to reduce deflection of steel beams.

The US has been working on a SGPL design standard (code) for some years now; I believe it is currently called X-something (a draft code name). I was forwarded a copy for my review and consideration by USACERL near the end of my project. I was quite proud of the fact I had much earlier received the background reasearch and reached nearly the same conclusions as the code committee.

I still have a copy of the draft code, and would be happy to scan a copy and post it. Please bear in mind that today Saturday for me, and that New Zealand has a public holiday on Monday. I will (at the earliest) be able to scan and post on my Tuesday morning, Monday night for most of you.

Two quick points, if I may:

1) Proformance based spec is absolutely right, and I think you're handling the use of SGPL in exactly the right way.

2) It is not fair to point to SGPL manufacturer's tables and data, etc, and take exception to their disclaimers. EVERY manufacturer does that; When was the last time you took a close look at the fine print on your Cold Formed Steel supplier's span and load tables, or for timber joists, or for structural cladding you're using as a diaphragm, or for truss hangers, or concrete anchors, etc, etc, etc. They will all say the same thing, while providing you the information to make a design call. That's why YOU'RE the EOR; You need to check the design against their tables, think through your load paths, etc, and before you ever specify the product, you need to verify that it's going to meet the requirements of the exposure conditions (just like for concrete you have to specify the cover based on exposure).

Let me know if you'd like me to find and post the draft code.

Good luck & regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[miecz]

youngstructural-

You're right, my steel deck manual contains a warranty disclaimer similar to the fiberglass manuals.

The disclaimers wouldn't bother me so much if the fiberglass design was backed by some established entity, or were addressed by the governing building code.

The designs for deck and diaphragms in the deck manual are done in accordance with the American Iron and Steel Institute Specification for the Design of Cold Formed Steel Structural Members, and are certified by the Steel Deck Institute. The theory behind these tables is well established. AISI and SDI have a long track record. This is not the same as a draft code that's never been released.

Steel Decks are covered under Section 2209 of the IBC, the governing building code. Fiberglass grating is not, to my knowledge.

Same goes for timber joists. They are designed according to a National Design Specification approved by the American National Standards Institute, and covered by Chapter 23 of IBC.

Concrete Anchors are covered by ACI, and Section 1913 of IBC.

Thanks for the offer of the draft code, but I'll pass for now. The fact that it's still in draft form tells me that it hasn't been agreed upon yet. Goodness, I have enough trouble keeping up with the codes that have been released.

 

[JKW05]

ASCE has apparently started developing a design standard (I haven't actually ordered it yet). Here's a link:

http://www.pubs.asce.org/

http://WWWdisplay.cgi?9700170

Delta Composites also has a pretty decent Design Manual that can be downloaded:

http://www.deltacomposites.com/lit_library/DelDesMan.pdf

Unless it were a very simple application (e.g. a single, short span beam supporting some grating), I would write a performance spec and require the manufacturer to provide a sealed engineering design.