Jack Beam

[SteelPE]

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.

 

[jike]

I do not how it would be possible to select a beam with the same stiffness (bending stiffness) as a column (axial stiffness). It would most likely be extremely large.

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.

 

[cntw1953]

I suspect a single jack beam can replace the column efficiently. Maybe a few of such spread out uniformly in N-S direction, and bring the loads to the edge beams. If done carefully, you might not need to mess around foundations, but strengthen the existing steel framing.

I agree it is tricky task to alter metal building structural arrangement.

 

[SteelPE]

Jike,

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?”.

 

[cntw1953]

SteelPE:

Been through the similar situation many times, I understand your reasons for sending the client away, I have done that before

 

[Lion06]

I agree that it would be near impossible to get a beam to have a bending stiffness (especially for that span) equal to the axial stiffness of a column. In fact, I think that one could argue thta the beam's flexural stiffness would need to be greater than that of the replaced column by a factor of 2. If you assume that the columns the beam is spanning to have the same axial stiffness as the column being removed, they would shorten by 1/2 the amount of the removed column before you ever even take beam stiffness into consideration.

 

[SteelPE]

I don’t look at it as sending the client away.

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.

 

[SteelPE]

StructuralEIT

The thought about the supporting columns crossed my mind as well.

 

[Lion06]

In my opinion, this is the biggest reason to NOT design to the gnat's ass. It gives you no flexibility whatsoever to make alterations.

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!

 

[cntw1953]

SteelPE:

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.

 

[SteelPE]

StructuralEIT

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.

 

[Ron]

SteelPE...it's either Jim Beam or Jack Daniel's...drink either and you'll solve your problem!!

 

[msquared48]

Jim Beam it is... But I prefer Wild Turkey.

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
Motto: KISS
Motivation: Don't ask