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Photovoltaic Panels & Manufactured Metal Plated Roof Truss - Addendum

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To those who are following this discussion on the SEAINT List, I wanted to side track and add a couple of additional comments that I observed or thought of in retrospect. You may wish to respond separately to this one.

 

As I noted previously, the contract for this client comes from a client of their competition. The client supplied the company with a sample of the calculation done by a local engineer about 40 or 50 miles from me. I am not familiar with him, but the submittals he signed off and the calculations he produced were accepted by the local building departments as adequate. I do not agree with this, but need to preface my statement that my disagreement is because he is not verifying the capacity of the wood members as conventional stick framing for roof joists on a sloped or flat (shed) roof, but rather that he is applying his professional assumptions to a metal plate manufactured wood roof truss spanning approximately 40-feet or more.

 

The point I failed to make is that when providing an analysis of an existing truss using a common 2D or 3D truss program (such as RISA2D which is what he did), the results may be accurate but are limited to the wood member in axial load conditions (method of joints). The convention for truss design is to take the uniform loads and apply them to the nodes rather than in uniformly along the chords. While the results verify if the wood is within allowable stress limits, the engineer is failing to check the allowable capacity of the joint connection. This is the rub. This is not possible on a proprietary plate and pressure manufacturing system for wood trusses – the one reason why we don’t have a license to run MiiTek’s software or other competitors software that checks the plates is that the plates themselves are proprietary and are assumed to be the competitive edge that one truss manufacturer has over another. If all plates were equal, I assume we would be able to know if the load modeled at the node is within the safe allowable stress of the manufactured pressure connection.

 

This is the weak link in the work that had by done by the other engineer and I would not feel comfortable guessing at this result. If the issue were a simply supported beam – no problem. But a truss is a system and the number of panels producing concentrated  loads on this system at the nodes needs to be very carefully considered or at the least, the original truss manufacturer and truss calculations package needs to be available to retrofit in new panels. This is rare – I have yet to find a home older than 10-years that is not part of a large development (tract) where the structural drawings and the truss package is maintained by the owner or by the local building department that issued the original permit. I have also found that in most cases, the developer of the housing projects when not specific custom homes, have had the trusses and lateral load resisting system designed for the minimum compliance with the materials used at the time the home was permitted. Rarely is there residual capacity left in the system that I would want to gamble a professional opinion on.

 

I would be pleased to hear from anyone who disagrees or has addressed the retrofit of these new energy devices mounted on existing homes (including Solar hot water systems).

 

Dennis S. Wish, PE

 

Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

La Quinta, CA 92253

760.564.0884 (Phone, Fax and Answering Machine)

dennis.wish(--nospam--at)verizon.net

http://structuralist.wordpress.com

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