column reinforcement
column reinforcement
(OP)
I should probably know this, but let's say you have to reinforce some steel wide flange columns because you are placing a large mechanical unit on the roof. Do the cover plates need to extend from the top of steel to the bottom of steel? Or can you specify cutoff points like when you reinforce a steel beam with cover plates. The only reason I can't extend the plates completely to the bottom of the column is that the column is partially embedded in a narrow 3'-0" high curb that the client doesn't want to bust apart.






RE: column reinforcement
RE: column reinforcement
In the analysis then, the second order moments would be diminished. And the kL/r is reduced. There have been posts on this site in the past asking then, how do you design/check a column with two different stiffnesses along its length. This is a more complex design and I would only be able to refer you to AISC's Design Guide 7 where in its Appendix B they have information on calculating the effects of stepped columns.
There may be other design techniques out there but I'm not sure where. Anyone else know?
RE: column reinforcement
Without going into all of the specifics, this is a problem that seems to come up every now and then and I'm looking for a general design procedure.
How do analyze a column pinned at top and bottom with different segments of kl/r and cross sectional area in between. Let's say the lower segment is 10 feet high and has a certain cross sectional area and a radius of gyration. What length do you use? 10ft?
RE: column reinforcement
'kl' must be based on the distance between lateral restaints to the column, not on the length of differing cross-sections. To determine 'r' you need a method to obtain an 'average' value for the column between the restraints.
If you had access to design software you could input the varying cross-section properties. Whether the software would give the correct answer is a question worth asking.
RE: column reinforcement
RE: column reinforcement
RE: column reinforcement
RE: column reinforcement
lambda=L/r1*sqrt(X+(1-X)*(1-k)^2)
where X=I1/I2, subscription 1 is for original section, 2 is for the reinforced section.
You can check if it makes sense with k=0 and k=1
RE: column reinforcement
If you are exceeding the capacity of the columns enough to warrant the addition of 15 in^2 of area, then you are probably exceeding the capacity of the base plate as well as the footing. It may end up being cheaper to build a larger frame on the roof to spread the load out to more columns than having to reinforce the footings and base plates in addition to the columns...
RE: column reinforcement
RE: column reinforcement
I like to share what I learned and I hope anyone who reads this gets the benefit. The following summarizes our findings and recommendations, from my particular project, that I found in the various references:
1.It is normal practice to reinforce steel members while carrying load.
2.It is desirable to reduce or relief the load on the member being reinforced if practical.
3.If proper procedure is established and followed, the heat effects should be minimized.
4.The anticipated allowable stress can be based on the properties of the new built up section.
5.The stress to be used in calculating the area of the new steel will be based on the new allowable stress of the new section less the actual stress at time of reinforcement.
6.Member connections should be evaluated (at splices and ends).
7.The use of intermittent welds is professionally recognized practice and is incorporated by AISC’s specifications.
8.Based on AISC specifications, for properly designed compression member reinforcement, it is not required to consider residual stresses for overall member.
9.It is preferable that welding is done under static loading conditions.
10.It is important to evaluate the condition of the existing steel (corrosion, loss of section, damage etc.)
11.Evaluate local buckling.
12.It is not recommended to weld transversely on loaded tension members.
13.It is recommended that all welds be done parallel to stress lines.
14.It is recommended that welding start from the middle then towards the ends and weld on both sides simultaneously.
Regards,
Lutfi
RE: column reinforcement
RE: column reinforcement
That's an excellent post. I worked in a Plant Engineering for ten years and did a lot of reinforcing to existing steel. You bring up several good points, in particular the consideration of existing stress when reinforcing.
Regards,
-Mike
RE: column reinforcement
RE: column reinforcement