IEBC Allowable 5% Gravity Increase Example

[HDStructural]

Hello All,

I know this has been discussed a lot lately, see link below, but I am working on a project with a unique situation.
The building is a PEMB and they want to add solar panels to the roof which will weigh just under 3 psf. The building was constructed in 1996 under the BOCA code and was designed for a non-reducible roof live load of 30 psf. The building has since changed owners and the 30 psf is not a state or local requirement.

Per IEBC, the 3 psf exceeds the 5% rule so an analysis has to be done to confirm that it meets current codes (both wind uplift and the controlling downward cases). This building would be designed today for a 20 psf roof live load that is reducible. For the main frames, it would reduce down to 12 psf.

Is it reasonable to have a few sentences in the calcs explaining why the gravity load is okay based on the original design criteria but not actually run any numbers. I believe the roof snow load will be around 22 psf which is still less than the original 30 psf roof live load design even with the extra solar panel weight.

An analysis would still need to be done for the frames and purlins in uplift.

https://www.eng-tips.com/threads/ie...y-increase-on-total-load-or-load-type.529768/

 

[KootK]

Is it reasonable to have a few sentences in the calcs explaining why the gravity load is okay based on the original design criteria but not actually run any numbers.

I certainly feel that would be a reasonable and prudent approach.

 

[dianium500]

You don't always have to run a calculation as long as you document the rationale. Part of being a professional is using our judgment in situations like this.

 

[JAE]

I understand the concept of 22 psf being less than 30 psf so "by observation" it's OK.

But solar panels "stick up" above the original roof line and certainly would cause additional drag effects via wind (that weren't in the original design) and there may be seismic issues I'm not thinking about (depending on where the project is located).

Just worried that wind might be more an effect than thought based on the PEMB tin foil designs.

 

[lexpatrie]

Is there any full-on legit documentation on construction drawings of the 30 psf being actually designed for back in the day? Versus "well the code said 30 psf"?

My main concern here is that PEMB is well-known to be designed right up to the verge of failure with every possible reduction and footnote incorporated. There tends to be little "room" for anything extra, beyond, say, relying on plastification of the section/local buckling, bolt elongation to keep it from collapsing. And all manner of unintentional load sharing of various pieces and parts that aren't strictly on the load path or structural.

If the supplier / manufacturer is still around, or the base manufacturer (i.e. Metallic) can be located that's perhaps ideal as you can shuffle the job to them. 1% chance of that?

BOCA had a 30 psf roof live load that wasn't reducible? I guess I'd forgotten that.

 

[JMASE]

I recently had a similar situation of adding new mechanical units to an older PEMB. The existing structural drawings indicated a 30 psi design LL, and as you note, current codes would point to snow loads below 20 psf. I was able to "borrow" the excess capacity and justify that the new loading, minus removal of old units and added to the current snow requirements was globally less than the 30 psf. My approach to these types of buildings is to minimize loading the Z purlins, and rather try to group the new rooftop equipment along the tapered steel girders and keep close to the existing columns. The net loading on the primary structure (a quick check of tributary loading on the columns and footings) checked out to be within the global loading. It takes some negotiating with the MEP engineers to locate the equipment, but ultimately did not require significant structural analysis of the PEMB frames, which are very difficult to model correctly.

 

[HDStructural]

Thank you all. It is clear on the drawings that the building was designed for a 30 psf nonreducible live load. I'll do the math and show that the new load on the roof is less than the design load. Regarding wind load, the wind pressures it was designed for are greater than those from today's code with solar panels, so no issue with wind uplift.