Analyzing and code checking a pre-engineered metal building for new rooftop RTU loading 8

[JMASE]

Does anyone have any insight on proper methods to analyze and code check a 1970's era pre-engineered metal building (tapered columns, moment haunch, tapered beams, etc.)? The client wants to add 4 -6 new rooftop RTUs. Will likely need to load the PEMB purlins too. I'v attached a typical section to get the gist.
Thanks

 

[Once20036]

If you keep renovating, do you get to keep adding 5% every time?

This is an "engineering judgement to me I guess

I believe more recent versions of the code specify that the 5% is measured from the original construction, and you do not get 5% per project

 

[bones206]

Engineering judgment is all well and good, but legal judgment is a different ballgame. If an engineer can testify with a straight face that what I did was outside of standard practice, a jury of laymen is not going to see any nuance in a design that was > 100% utilization. They are going to hear "overstressed" or "underdesigned" and it will seem very black and white from the laymen perspective. So personally my engineering judgment takes a backseat to my legal risk tolerance in this type of situation.

 

[WinelandV]

If I design a critical component of a structure at 103% and then the structure fails, even if the failure is for an unrelated reason, how can I justify that my design was code-compliant?

While we don't always hold to it, we have a saying around the office that "red members in your design results are bad look in front of a jury."

(Red members are those that are above 100% in our analysis program)

Please note that is a "v" (as in Violin) not a "y".

 

[phamENG]

It's also worth noting the IEBC 5% thing is 5% over the current (design) loading, not 5% over the maximum calculated capacity.

 

[JMASE]

Thanks to all for responses

 

[RareBugTX]

Like others say, I would not put extra load on the existing structure. Design an independent frame to take care of the superimposed RTUs. I use this approach not only with PEMBs, but any existing structure unless it is clearly overdesigned.

 

[Pmtottawa]

When ia was in Pemb I was told toe design to 1.03 ratio
Reason being code specifies to design to 1.0 so the 1.03 becomes 1.0 when rounding.
That is rhe tRick they used.
Best

 

[XR250]

1.49 can become 1.00 when rounding
1.03 can also be rounded to 1.05

 

[Pmtottawa]

Their rationale is that the code has 2 significant figures (1.0) so 1.03 round to 1.0
1.04 is 1.1
Never liked it but that is industry standard...

A few times against my strong opinion they asked for mill test report and used higher steel grade....

It's a wonder how more don't fail!

 

[phamENG]

Where does the code dictate 2 sig figs? AISC, at least, has safety factors and load and resistance factors to 3 significant figures. Makes me think any result would be taken to the same precision.

Also, what's the logic that 1.04 rounds up to 1.1? I learned that 5 and above rounded up, 4 and below rounded down.

 

[bones206]

Not sure I’d want to use the rounding argument in court. The judge might decide to use the same logic to round up during sentencing

 

[WesternJeb]

I am going to play devil's advocate here. When is the last time anyone has heard of a PEMB failing? I haven't and a quick google search did not show anything outside of erection collapses, and if this seems to be the industry standard then I don't see an issue with it. The rounding down from 1.03 likely wouldn't hold up in court, but it hasn't had to because it works.

 

[canwesteng]

I think they do fail here and there, but I don't think a credible expert witness could point to the 1.03 utilization as the reason for failure, so I'd be surprised if it's ever brought up.

 

[bones206]

I was mostly joking about the judge rounding up, but it does reflect my disdain for these companies pushing the envelope with greed vs safety.

There was a winter about 15 years ago when I was working construction for a company that did a lot of Butler buildings. After a snow storm followed by an ice storm, our crew was pulled off of our construction project to do a bunch of emergency snow clearing on roofs. It was all PEMB’s that were in various states of distress with panicking owners. One of them actually collapsed while a shoveling crew was on the roof. Luckily nobody was hurt because the two bays that collapsed were on the far end of the building.

 

[bones206]

I’m sure 1.03 does work - a certain percentage of the time. And that risk has been carefully quantified and factored into a cost-benefit analysis and determined to be an acceptable failure rate given the long term legal costs vs profit.

 

[WesternJeb]

Well I stand corrected. I do remember about 15 years ago though where we had enough snow it was double our design loading and everyone was in a panic shoveling snow off of every roof, I guess we got to it soon enough that nothing happened around here.

 

[JedClampett]

When is the last time anyone has heard of a PEMB failing?

Here's a picture of a metal building in Panama City, Florida after Hurricane Michael.
So the PEMB manufacturer, if you can track them down, will say "the winds exceeded code..." And I respond, Look at the two residences adjacent. They saw the exact same wind, and their shingle roofs blew off. What would you rather repair, replacing the shingle roofs, or replacing the mangled building?
It's true, they don't fail every day. if they did, it would be unsustainable. But almost anytime there is an unusual event (snow, wind, earthquake, etc.), slighly exceeding code, and there is a failure, it's likely a PEMB. They have limited or no redundancy.

 

[dik]

If the number is 1.03, and it's not acceptable... something is wrong... my calculations are never that precise, even on a good day.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[canwesteng]

The lack of redundancy is separate issue, likewise the lack of any reserve capacity. Those could all occur in any building though. PEMB have their faults, but you can't tell me 3% caused that failure. You're saying if the wind speed was 2 mph lower, the building would have stood up?

 

[bones206]

I think it's more of a statistical/legal/ethical issue than an engineering judgment issue. Codes are set up with certain statistical parameters to protect public safety. PEMB companies are trying to systematically degrade that safety buffer just to make incrementally more money. I realize it's a race to the bottom competitive-bid industry, but there has to be a line drawn somewhere and that's what the codes are supposed to be.