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Part II Re: Wind Loads on Roof Mounted Solar Panels

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I have had similar questions on similar projects.

One project I was involved with, the City required testing of the assembly. The test was on a mock-up of an S-5 clip on standing seam panels attached to a substrate with the roofing manufacturer's panel anchor clips. The assembly failed as predicted, at the roofing anchor clip (not the S-5 clip). This didn't fully address your question #1, but it did imply that the point load applied on the standing seam did not fail at a dramatically lower load than predicted. In predicting this failure, I used the section properties of the panel, and calculated the stresses and deflections under point load, just like metal decking.

As to your question #2, I had the same concern, and to the best of my knowledge, there has not been testing of this condition. I estimated the panel attachment loads using the full C&C loads on the panels, including edge effects of the roof. I would guess that this would be conservative, as Thor suggested, but I can't point to testing that has substantiated this.

Dmitri Wright, PE
Cascade Engineering, Inc.
245 SE 4th Ave, Suite B
Hillsboro, OR  97123-4033
dmitri(--nospam--at)cascade-structural.com
503-846-1131



Subject: Part II Re: Wind Loads on Roof Mounted Solar Panels
From: Paul Ransom <ad026(--nospam--at)hwcn.org>
To: <seaint(--nospam--at)seaint.org>

Subject: Wind Loads on Roof Mounted Solar Panels

I have been asked to check the existing framing on a roof for the
installation of a bank of solar panels.

I am trying to put together the wind loads on the panels to be used in
conjunction with the gravity loading to check the existing framing.
I think the air flow is "unobstructed".

This seems to be an increasingly popular topic. I have 2 current projects:
a) residential 5:12 asphalt shingle, plywood, 2x wood trusses 800 sq.ft
b) pre-eng building 1:12 structural standing seam roof, 50,000 sq.ft
There are future opportunities starting to form a queue.

The problem is, these retrofit panels sit about 4" above the roof surface
and I am not aware of any good authority, research or testing to validate
wind loads for this condition.

Additionally, the solar panel sub-framing converts the UDL load, for which
the building load paths were originally designed, into point loads. This is
especially concerning on the pre-eng building standing seam roof (e.g.
Butler MR-24, Star Starshield, etc.) using the S-5 clip or similar attached
at the top of the rib. I am only aware of manufacturer confirmatory testing
using UDL loads applied to the flat of the panel, primarily.

The installers are quite happy to install whatever sub-framing beneath the
solar panels that the solar manufacturer claims is adequate (that's fine,
it's not my design above the roof). However, I have to qualify the fastening
to the roof and the load capacity of the supporting structure (no issue
there). It's that small point where all the loads accumulate and transfer,
where both sides want to write disclaimers.

My approach at the moment: Unless there is some good research out there,
just say "NO" to load systems that transfer point loads to a pre-eng
field-seamed structural standing seam roof panel. The clip is useful (load
capacities documented), the panel is good (UDL capacities documented), the
combination may be disastrous under significant load conditions (not just
solar panels).

I would appreciate it if anybody can point me to any commentary or research
regarding:
1) point loads applied at the top of the ribs of pre-eng field-seamed
structural standing seam roofs and equivalence to UDL in roof panel
performance, probably by individual manufacturers, and;
2) wind loads on the solar panel system considering the cavity beneath the
solar panel, for the purposes of determining adequate anchorage
requirements.

I am preparing my call list to MBMA, CSSBI, researchers and the individual
manufacturers.

Regards
Paul
--
Paul Ransom, P.Eng.
ph 905 639-9628
fax 905 639-3866
ad026(--nospam--at)hwcn.org




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