Pre-engineered metal building (c 1992) with steel bar joist roof + girders? 4

Ben29 said:

Basically I am trying to figure out if I should bid on a job that is associated with this building.

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Associated in what way?

- New RTU's?

- Adjacent addition?

- Column removal?

Different scenarios would tend to flavor my anticipation of risk vs reward.

PEMB's absolutely can be a nightmare to evaluate for retrofit. So you're right to approach this with caution in my opinion.

Unless it's with a longstanding or potential whale of a client, I'd stay away.
PEMB Stages (of grief):
[ol 1]
[li]First of all, no drawings.[/li]
[li]Tedious measuring of as built conditions.[/li]
[li]Trying to figure out what code it was built to, if any.[/li]
[li]Having to make large leaps of faith on the analysis.[/li]
[li]Realizing that there's a chance the building never worked, for any code.[/li]
[li]The ethical and legal dilemma of telling a client that their perfectly good building doesn't meet code.[/li]
[/ol]
Lastly, wishing one of your competitors took this on.

KootK - solar panels!

JedClampett... I 100% agree. Unfortunately this is a potential whale of a client (solar panel company), and this is the first roof they asked me to assess.

In fact, with my first interview with them I told them that I wasn't going to certify solar panels on a PEMB, yet here I am.

So are you suggesting that there is no way that those bar joist are "real" bar joist per my Vulcraft catalog?

I mean, Nucor does bar joist as well as PEMB designs, so maybe there is a chance?

Ben29 said:

KootK - solar panels!

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I'd be game so long as I managed my risk properly and made that clear in my proposal terms. I'd want it one of two ways:

1) Low profile panels are used that don't appreciably affect snow drift or wind load and/or;

2) Baring more information, you get to skip joist evaluation and go straight to joist reinforcement with the reinforcing intended to add the required capacity regardless of the original capacity.

The situation that I want to avoid is the one where:

1) I expend a bunch of effort trying to evaluate the joists favorably.

2) The joists fail.

3) I expend a bunch of effort trying to reinforce economically.

4) Everybody hates me because nobody wanted to reinforce the joist.

A job where you lose money and piss off your client is a job best left for the competition.

Ben29 said:

...so maybe there is a chance?

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Of course there's a chance. But, as usual, it's prudent to hope for the best and plan for the worst. If you go into this assuming that you'll find nice SJI tags on all of your joist seats, you'll likely be inviting some pain that your future self may not appreciate.

It's definitely possible and also probable that the joists are not special PEMB joists like Butler's system. I'm a PEMB guy and I've worked for one of the largest companies in the US and a smaller regional player. For roofs we have only ever used joists provided by big SJI companies like Vulcraft and New Millenium.

That being said, we'd use some other type of system if we could find one that does everything we need for faster or less expensive. That company in 1992 probably bought those joists out rather than using an in house system but what was readily available back then?

Also, just because the joists are from a large SJI company doesn't mean they're actually compliant with the SJI code. One of the big players that I won't expose here tried to sell us on a type of joist that wasn't SJI compliant but that supposedly would be more economical due to reduced bridging. But when we started really asking questions their new system could actually result in more bridging under certain circumstances while still not being SJI compliant. No idea if they still offer that system.

Ben29 said:

Unfortunately this is a potential whale of a client (solar panel company)

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Also be clear minded in recognizing that the reason that this whale of an opportunity exists is precisely because most structural engineers shy away from this work because they view it as unfavorable from a risk / reward perspective.

Charge by the hour, not a fixed fee.

Ok, so lets say I can rationalize the joist and joist girders. But I won't be able to rationalize the additional horizontal thrust at the column base due to the additional dead load. I can't rationalize the DL takes the place of the RLL because RLL = 20 psf (min) and Snow Load is also 20 psf (min). You can swap out the RLL, but you can't swap out the snow load.

Also, this is a ballasted system, so the actual dead load of the solar panel system comes out to be 12.5 psf when distributed over the whole array.

Clearly I have done alot of homework on this job I don't actually have yet. But that is what you gotta do.

Ben29 said:

But I won't be able to rationalize the additional horizontal thrust at the column base due to the additional dead load.

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If the proposed setup is such that you are truly concerned about load increases that far downstream from the source, let that be your litmus test for whether or not you ought to pass on the assignment.

Ben29 said:

Unfortunately this is a potential whale of a client (solar panel company), and this is the first roof they asked me to assess.

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As others have said, the reason you see these guys as a "whale of a client" is because others don't want to take on this work. I spent the first 5 or so years of engineering doing racking designs, ballast designs, new system designs, roof checks including with wind tunnel testing with an average of 20 to 30 hours a week spend just on solar items, at the time it was profitable(ish) but now days, the companies are trying to shave pennies out of these and drive the engineering cost to a minimum to the point that it was no longer feasible to even consider doing this kind of work at that company. If I were to price this, I would price it accounting for all the unknowns and build a safety factor into my fee and make it clear it's possible it doesn't work. When they come back to ask you to reduce your fee, I would stand my ground or walk, but honestly I wouldn't even take this on due to the headaches involved with PEMB's.

Ben29 said:

In fact, with my first interview with them I told them that I wasn't going to certify solar panels on a PEMB, yet here I am.

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Notice here, that your very first project you are willing to bend to their will, not a good sign as they may see it as they can push you around. Keep in mind, solar companies are sales people, many of whom are spineless.

Ben29 said:

Also, this is a ballasted system, so the actual dead load of the solar panel system comes out to be 12.5 psf when distributed over the whole array.

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Good luck, I don't need to check this to know this isn't going to work. An attached system is really the only option you will ever come close to getting to work on a PEMB, unless the PEMB was designed for this loading up front.

Have them build a separate new PEMB just to hold up the solar panels.
They can use the money they saved in 1992 building that contraption.

Be honest with the client.

Tell them PEMBs are built to ultra fine margins and have effectively zero capacity for any additional load. If they want to install solar panels then they will need in effect a new separate structure or heavily reinforced one which you can design for them but there is no chance that the existing structure will stand the extra weight that would pass any inspection or review. And it won't be cheap.

If they want to pay you to prove a negative then fine, but telling clients bad news is the easiest way to never get any more work from them ever again. All IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Would a PEMB manufacturer have their own proprietary add-load joists? Those look like add-load joists, straight from a joist factory.

Having done similar projects to this, we often found that there was very little reserve capacity in the structure to support additional loads. Even adding solar panels weighing only 5 psf was often a problem. And yes, it wasn't uncommon to discover areas of the structure which seemingly didn't meet code even without the additional load.

These projects often started out as a grand opportunity presented by the solar panel company (sales people) telling us how we'll be able to provide engineering for hundreds of these every year. It never really worked out that well though. After a few projects, our involvement was often seen as an obstacle. In some cases, we would end up telling them that the job couldn't be done without strengthening the entire roof. Meanwhile, there's usually another local "engineer" who's cool with it because the roof has been standing for x years and we're only adding a little weight. These projects tend to gravitate towards them.

I've got a similar project coming up soon that I'm dreading. I've already done a walk thru and the joists are like nothing I've ever seen. Some kind of mutant Macomber. And ALL the tin joist tags are illegible. The building name plate says "Sonoco", as in the Sonotube manufacturers... So no chance of obtaining any information whatsoever. And the foundation is yet another question mark, since who knows if there are tie rods or hairpins etc.

So yea, renovating these types of buildings are just setting us up for failure one way or another.

I am fine doing foundations for PEMB structures, but modifying or checking an existing structure is something I avoid.

Those things are shaved down to the ounce; if a bird lands on the roof when it's snowing it's overloaded. Half the time they fail without any load on it, let alone trying to get 12.5 psf more.

There are very few jobs that I will say no to, but we all know going in that PEMB's don't work on a good day so I'm in the "stay away" group on this one.