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Modified PEMB Column
2

Modified PEMB Column

Modified PEMB Column

(OP)
I have a situation that has me a little bit stumped. On a project, we are modifying an existing PEMB for additional rooftop loads. Significant retrofit plate work has already been designed to accommodate the new loading. However, along one sidewall, a building addition will also be constructed. To maximize the efficiency of the two buildings, we are removing girts and x-bracing along the length of the wall. In x-braced bays new portals have been designed to take lateral loads, as well as to provide weak axis/compression flange bracing to the existing columns.

At bays where there are no adjacent portals we have been asked to eliminate any bracing full height. This increases my unbraced length from 7.25' to 16.5' which is quite a large increase to deal with. I have looked at three methods and have eliminated 1 based upon AISC requirements.

The three methods are:

1) Cover Plate Method
1) Build up the member using traditional cover plates.
2) Need 5/8" plates to work for an unbraced length of 16.5'. This is 3 times the thickness of the original column flange (3/16") and thus violates AISC F13.3 limiting cover plates to 70% of the total flange area.

2) WT + Cover Plate Method
1) Add WT sections to the center of the web to increase capacity. I know this increases capacity from a compression standpoint, but I am not sure how to quantify the level of LTB resistance (if any) it will provide.
2) Cover plates would then be added to increase capacities for yielding, local buckling, etc.

3) Cover Plate Method + Side Plates
1) Add cover plates to the flanges to increase capacities for yielding, local buckling, etc.
2) Add side plates at the flange edges to create a "box" type section to help reduce the "twist" of the member. Doesn't help with translation though.


I have attached a rough sketch of what I am thinking for methods 1 and 2. If anyone has any guidance on this, I could sure use the help. Maybe there is a better way that I am missing. At this point though, adding braced points to the column is out of the question. If I have exhausted all other options, then maybe I will approach the client with the possibility of adding some bracing to the member. I just want to do my due diligence before having that conversation.

Thanks in advance!

RE: Modified PEMB Column

Ah, welcome to the briar patch of PEMB remodel. It's easy if you remember some simple rules:
*The building probably didn't work when you got the project.
*The more you do to fix it, the worse things get.
Feel better?

RE: Modified PEMB Column

A third item for Jed's list... if you modify it, you 'bought it'...

Dik

RE: Modified PEMB Column

(OP)
Totally agree with you both! We seem to do a fair amount of them though, so it is getting a little easier and quicker for me to accomplish.

I was simply hoping someone had a thought as to how I could increase the LTB buckling resistance. Let's, for the sake of an easier discussion, disregard the fact that it is a PEMB column. If you had a straight column that the unbraced length more doubled and you couldn't get the required section using cover plates, how would you retrofit rigid frame column such that LTB was not a controlling limit state?

RE: Modified PEMB Column

We usually try to add on channels to form a sort of box section. Box sections rule, baby!

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