Jack Beam
Jack Beam
(OP)
I have a client who came to me with a problem regarding a metal-building. I have since referred the client to another engineer (who is very familiar with these framing systems) but was wondering it other people would approach the problem the same way I would.
The client has a metal building with a steel column located at the ridge (see attached). The client would like to remove the center post from the frame and replace it with a jack beam.
I am not really familiar with the way metal buildings are designed but I do know they like pride themselves on efficiently designing their structures. Since I know load is attracted to stiffness I would conclude that in order to properly remove the column the jack beam should have the same stiffness as the column you are removing. This is to avoid overstressing the frame and foundation systems.
This approach seems very conservative. Is this the correct way to look at this problem?
FYI the column is not part of the LFRS.
The client has a metal building with a steel column located at the ridge (see attached). The client would like to remove the center post from the frame and replace it with a jack beam.
I am not really familiar with the way metal buildings are designed but I do know they like pride themselves on efficiently designing their structures. Since I know load is attracted to stiffness I would conclude that in order to properly remove the column the jack beam should have the same stiffness as the column you are removing. This is to avoid overstressing the frame and foundation systems.
This approach seems very conservative. Is this the correct way to look at this problem?
FYI the column is not part of the LFRS.






RE: Jack Beam
I suggest sizing the beam for the estimated tributory load and then look at what happens to the re-distribution of moments in the frame. Reinforce the frame if necessary.
RE: Jack Beam
I agree it is tricky task to alter metal building structural arrangement.
RE: Jack Beam
I agree with the fact that it would be impossible to replace the stiffness of the column with a beam (which is why I turned down the job).
cntw
The problem that I have is that I don't really want to run an analysis on the existing structure for two reasons.
1) It would take some time to measure up the existing frame, model the frame properly and then check to see if reinforcing is required.
2) I don't think the client would want to pay for that time. After all, they just want a jack beam... "why do you have to analyze the existing structure?".
RE: Jack Beam
Been through the similar situation many times, I understand your reasons for sending the client away, I have done that before :)
RE: Jack Beam
RE: Jack Beam
I recommended someone who I knew, someone who was a specialist. This particular engineer worked in a lead role for a metal building manufacturer. His skills with these systems are superior to mine.
This way I don't feel bad.
RE: Jack Beam
The thought about the supporting columns crossed my mind as well.
RE: Jack Beam
Just to talk about it, even if you get the stiffness to work for the new beam and that particular fram works out ok, I would still want to check the 2 frames that the jack beam is framing to. I mean those columns now have 50% more load on them, plus that extra axial shortening is robbing positive moment from the ridge and dumping more negative moment into the beam/column connection.
Man........ and this is a purely gravity analysis!
RE: Jack Beam
No offense, sorry for the poor choice of word.
However, in my cases, usually I would turn a client down/away after presenting a few quick thoughts and associated costs. I know my limits.
RE: Jack Beam
I agree about the gnat's ass, this is something I preach to all of my clients. From my understanding this is the exact way a metal building is designed. Not much I can do about it.
cntw
No offense was taken.
RE: Jack Beam
RE: Jack Beam
How about placing a new steel truss on each side of the existing column. Much more stiffness than a straight beam, depending on the ridge height.
Mike McCann
MMC Engineering
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