Old Metal Building KL/r >300

[structuralengr89]

I looked at an old metal building today where the owner wants to remove one of the gable end columns. The gable end wall paneling was removed and a new metal building was added next to the existing metal building. We discussed adding a new structural member and posting down with some new columns/footings.

I can come up with something that will work, but my question is about the existing construction. The end wall columns have a kl/r of over 300. I looked at the column capacity and the girder capacity and they both can handle about 19-20 psf of TOTAL gravity load for the roof. IBC Roof Live load reduction is 18.5psf. So that leaves the building with about 1 psf for all of the dead load.

I also noticed that the new metal building next to the old one had much larger member sizes for columns/beams that had fairly similar bay spacings.

I'm concerned about the liability and thinking about telling him to look elsewhere for advice. Thoughts?
Thanks!

 

[JedClampett]

My thought is to run away. You're never going to know if the existing structure was inadequately designed (probably not, but possible) or they used some approach that optimized every ounce of capacity out of that frame. And after you finish that frame, how about the next one? And the one after that? Oh, and by the way, anything you touch, you own.
Owners need to be educated that metal buildings are fine until you need to change them. As long as there's never any chance that they'll be remodeled, they're the way to go.
I just attended a webinar by Alexander Newman of ASCE on Remodeling Metal Buildings. I advise you check it out.

 

[BAretired]

You should advise your client to beef up anything you believe to be undersized. If he doesn't want to take your advice, you may have to blow the whistle on him.

Telling him to find another engineer does not relieve you of your responsibility to report a condition which you know to be unsafe.

BA

 

[woodman88]

What code/year was the building design under? Assuming a flat roof, per the UBC's (for a 201 to 600 sq. ft. area) the 20 psf live load could be reduced to 16 psf. giving you 5 psf allowable dead load. In the IBC 2009, Chapter 34 section 3404.3.1 would allow keeping this reduction of the live load.

Garth Dreger PE
AZ Phoenix area

 

[JedClampett]

Before you go to the owner, I'd consider the assumptions that might of been made to qualify the building. Let's face it, if the kl/r was 300+, the building is unlikely to make it through a good breeze, much less the winds it's actually seen. I don't have any building details, so these are strictly guesses.
Where's the "l" ends? At the connection? At the top?
Could the girts be assumed as column bracing?
Might the top be considered fixed as far as axial spans?
How about the base plate? Could it be assumed fixed or partially fixed?
Just because you or I wouldn't stretch things that far doesn't mean it hasn't been done. Just don't go to the owner unless you're sure there's an issue. You don't want to cry wolf. If you're sure, than BAretired is right, you need to tell them.

 

[structuralengr89]

Thanks for the replys. The end wall column looked like an old rolled section. It measured equivalent to a W10x12. It was 20' tall. The thing that bothers me is that at the end wall the frame line has very small members. In fact the beams and columss were all W10x12...20' spacing and about 20' tall. The adjacent frame had somewhat larger members. Now I'm wondering if when the owner removed the girts and paneling at the endwall and added another metal building to butt the existing, he removed the x-bracing cables from the end walls. There are two column lines at the end wall- old metal building and new. The new column line is obviously a frame- much larger members than the old building. Could the engineer who did the new metal building have tied his roof diaphragm into the existing metal building to provide lateral support and then delete the x-bracing cables?

I'm going to refer the owner to an engineer experienced in metal buildings...

Thanks!

 

[ajh1]

A couple of observations:
1) Yes, there were probably diagonal brace rods or cables in the old endwall prior to the expansion. There is some possibility that the endwall sheeting was being considered as a diaphragm in lieu of rods, but since that is no longer there either, it doesn't make much difference either way. While this omission affects the lateral stability of the frame, it doesn't really resolve your original concern with l/r > 300 other than in the determination of an appropriate K.
2)The combination of girts and paneling is almost always used to provide lateral support points for the weak axis of the columns, particularly on light endwalls. Note that it is the combination that does the job, not just the girts. It is not unusual for someone to remove the paneling in a case like this and leave the girts and think they still have the same resistance that they did when the building was complete.

 

[ToadJones]

Are we talking about a main column in the MB moment frame or just a wind column?

 

[ToadJones]

I can see there being a reason for the existing end wall to be of smaller construction since it likely only sees half of the gravity load of the adjacent bay.
Did the owner load the existing end wall roof framing with new roof purlins?