No net load increase with building modification - review necessary? 1

[OHEcoEng]

Hi, everyone, hoping for a little enlightenment...

I am holding up a permit application for a solar array installation because I won't sign off on the structural section without doing a full review. This is residential structure with a flat roof. I have the original building plans and truss design, but there was never a PE stamp applied to either. I have been insistent on performing an analysis of the trusses before signing off.

The thing is, the building was designed/constructed with 10 psf of ballast stone. This would be removed prior to start of construction, and the solar array would be installed with a net increase in loading of 0 psf.

Am I unnecessarily holding up the permit? I am wondering if code allows this since there is 0 increase in loading? That's what I am interpreting from the Ohio Building Code, but wanted other opinions? This is my first experience with this.

If this can move forward, I am still going to make sure that the original builder/designer didn't cut any corners...

Mike

 

[frv]

Uplift would be about the only thing I'd worry about. It may be substantially higher than without the panels. Especially if the ballast is removed. Are you in a high wind area?

 

[OHEcoEng]

I guess I should clarify that this is a rather small array, and ballast is only removed within the array footprint...

 

[SkiisAndBikes]

The solar panels, being a projection above the surface of the roof, may result in aerodynamic shading which could result in additional snow loads due to drift accumulation.

The wind and/or snow loading on the panels themselves will result in higher point loads where the legs/rails of the frame connect to the roof structure. i.e. even though the psf resolves itself as a zero increase in loading, be careful that the distributed loads from the solar panels, which appear as concentrated point loads at the leg locations, are adequately supported by the structure.

 

[MiketheEngineer]

If I remember, if there is less than a 5% increase in ANY stress, you should be good to go.

Please verify w/ local building codes.

 

[JAE]

If you are a building official (which it sounds like you are) and you have a concern over a structural issue, it is not uncommon or improper to require the owner to hire a structural engineer to investigate, calculate, etc. and provide an opinion (signed/sealed letter for example).

As per the posts above - there are considerations beyond just comparing psf loads over set areas.

 

[JennyNakamura]

I don't work in areas with snow so I can't comment on that. But I do know that for PV systems where the panels follow the slope of the roof, there is negligible effect on wind load (if the panels are within 12 inches or so of the roof). So uplift and downlift remains the same. Also, the roof framing must have been designed for 16 or 20 psf of live load. Once panels are installed, the live load is no longer counted because PV panels are not designed to support live loads. PV arrays weigh anywhere from 3.5 to 5 psf. So with ballast you have about 15psf gravity load replacing 16 or 20 psf of live load. It is a wash and a full blown gravity and lateral building analysis (wind) is a waste of time and unreasonable to ask. However, while the ballasted PV system is essential replacing the roof live load in the gravity check, there is an effect on seismic since the ballasted PV system weight will be included in the calculation of seismic weight whereas the live load was not. A stamped certification letter from a local civil or structural engineer should be sufficient. The letter should also say that the increase in stresses in the lateral system is less than 5% or whatever the local requirement is (for seismic, wind is not affected). This is all that is required where I live, and there are tons of residential and commercial PV systems in this area.

 

[JAE]

JennyN... - the OP handle had OH (Ohio) in it so I'm presuming there is snow load, which still applies....but of course it depends on the location of the project.

 

[SAIL3]

Modification of existing structures has always been a gray area in my own experience.
While I agree for the most part on all the points made, I am still left wth the following questions or decisions to make:
1. Application of new verses old codes...the 5% increase in stress
is the cut-off point....does this imply that the engineer
should run the current loads thru the structure. If so, it is
difficult to avoid not checking the existing members for
adequacy even if it is as per old code.
2. In the OP's case there is no apprecable net change in the
new applied loads so one could easily conclude that it is
OK. This,in my opinion, is implying that the original design
was adequate in the first place without confirming this by
checking the structure.
The main point I am trying to make is that as a Professional Engineer I am required to ensure that the existing structure
was correctly designed in the first place.

 

[frv]

Sail.. If I understand correctly, it seems that you are suggesting that when one adds solar panels and finds that the added loads are essentially a wash, it is still the responsibility of the engineer to ensure that the original structure was properly designed.

I think you are reaching. How much do you expect the homeowner to pay? At what point are you satisfied that the original design is adequate- roof framing only or do you follow it all the way down to the foundation? What if the home has metal plate roof wood trusses? how are you going to verify that the node capacity is adequate?

I don't think it is unreasonable to assume that the original design was adequate. This definitely falls under the cost/benefit consideration. Think about how long it would take you to check everything in that home. And there'd almost certainly be no drawings to reference.

 

[SkiisAndBikes]

While there may very well be no net change in the applied loads, one cannot easily conclude it is 'ok'. We have to be careful in how the loads are applied to the existing roof structure. The assumption that the new loads (dead load of solar panels plus snow/wind load on panels) often can no longer be considered a uniformly distributed load anymore due to the method of support for the frame which supports the solar panels.

In the OP's case, it is a distinct possibility that either the legs or rails which ultimately transfer the loads to the structure, which in this case I am going to assume could be wood trusses, may not fall on panel points thus inducing bending and shear due to point loads on individual members of the truss.

I am of the opinion that for most structures where I can find either a copy of the original truss shop drawings or the structure has a history of satisfactory performance, it is reasonable to assume the original design was adequate for the design loads at the time of construction.

 

[SAIL3]

frv...I understand your point.
Suppose there was a serious flaw in the original design?
Suppose a portion of the original structure has deteriorated over the years?
If there is evidence of a PE involved in the original design, then
I would take that into consideration and modify my analysis accordingly.
If there is no evidence of a PE being involved in the original design, then all bets are off.
My primary responsibility is for the safety of the public. Often this may come in conflict with "cost/benefit" considerations.
I have been in these battles many times......and have heard the typical arguements for limiting the extent of the consequences of
modifying existing strucures or current designs...
.."well, it has functioned ok for 30yrs, so the original design must be ok".
...."you are too-conservative....so-and-so engineer says it's ok"
..."this will blow the budget"
etc.
Even if the proposed line of action is legally ok, I still have
an overall responsibility above and beyond this and should evaluate a course of action in that light.

 

[frv]

Sail.. I fully understand and embrace my responsibility as an engineer. But I'm also a firm believer in common sense.

Engineer involved in the design of a home> I'm not sure where you are located, but in the US. the VAST majority of all homes are designed using the prescriptive requirements of the IRC.

Again, I ask you, where does it end? Are you going to have somebody dig under the home to take measurements of the foundation? Are you going to have a geotech come out and perform a soil analysis?

I'm certainly not suggesting that you ignore, for example, signs of deterioration. That is perfectly within the purview of the structural engineer. However, in the absence of any information suggesting there is a problem, I simply see no justification whatsoever to force a home owner to pay for us to go out there and review the entire design of their home (again- assuming the loads are essentially a wash). If you (the inspector) require an engineer to do so, you have essentially priced a lot of consumers out of the possibility of adding something to their home. Seems way over the top to me.

 

[OHEcoEng]

Well, I have been away from computer access all weekend, but the points made here are exactly what I am struggling with.

-Regarding snow loading: these are at 10 degree tilt in landscape; by the time drifting becomes a controlling factor (20 psf flat roof load), the array is completely covered with snow, hence, no drifting issues.
-live load is not a consideration here, even if modules are in place. A decrease in live load does not equate to an allowable increase in either snow or dead loading, as we all already know...
-I have run through the calcs for wind loading for this type of array many times, and have yet to see wind control.
-The structure was constructed in 2001. There are plans. I have obtained truss design documents...but there is no PE stamp (only architect).

And Mike(TheEngineer), Ohio Building Code Section 3403.2 is the "5% increase" part I am talking about and has me wondering if this is an unnecessary exercise - other than verifying that everything "looks sound".

My concern has been as already stated: if there is no net increase in loading, should I spend the time going through each of the calculations (there are a total of 6 individual truss designs here) for each truss? It will be incredibly time consuming, but I do feel that I have a responsibility to verify that the structure was designed correctly in the first place.

 

[Forensic74]

Call me crazy, but if the OP is making the decision that it's not over 5% increase, isn't he practicing engineering without a license?

 

[hokie66]

Who said he is not licensed?

 

[keyPitsimplE]

frv is exactly right, with respect to the building code, which is what we use to keep ourselves defensible, yet work in grey areas. I work with remodels including PV installations all the time and that is the only reasonable way to approach them. Of course, if you as engr see something that is grossly under-designed, I believe it is your responsibility to bring it up, but it is the Owners decision whether or not to spend money correcting something that has been existing for years. This is exactly why the code gives us that 5% allowable increase without having to upgrade.

 

[TXStructural]

I think the only thing the OP can do is simply provide a letter stating that the small revision (remove ballast and install PV array in the same area) will not change the loads or fundamental nature of the structure. S/he should not get into evaluating the original design nor sign or seal anything related to the project except the single statement mentioned above.

If you start down the path of evaluating the original design, you risk requiring major remediation, or worse. Also, you cannot simply evaluate an existing structure based on drawings, it will require an thorough inspection to be sure the evaluation is for the building as it currently exists - damage, bad construction, and all.

If it was never engineered, look at the code under which it was designed, and decide if it is still within the load envelope, including any load factors. Where there is a question, check the Intl Existing Building Code. (606.2 says OK to exceed original design loads up to 5%)