column reinforcement 3

[broncosfan]

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.

 

[Dinosaur]

Why are you reinforcing the column? Do you have a problem with kl/r or are you in bigger trouble and need more cross sectional area? If it is the first, you don't necessarily have to run the plates to the ends, but if it is the second then you got bigger problems.

 

[JAE]

Agree with Dinosaur - If you run the plates a partial length of the column, then you have improved the stiffness and kL/r of that section. So in analyzing the column, the second order effects (AISC Chapter C) are reduced since you now have a stiffer column along part of its length.

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?

 

[broncosfan]

It's an older building. Now they are coming back and want to add a mechanical platform with two 15000lb units. The W12x53 columns are no good anymore. I want to add cross sectional area to the columns and improve kl/r by adding 12" x 5/8" cover plates each side of the column.

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?

 

[apsix]

No overall answer, but answering your last question I think.
'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.

 

[Dinosaur]

To analyze a column with varying cross section as you have, I would compute the buckling using stiffness functions. Since the problem can be treated as symetric if there is no concern for sidesway, it becomes a three by three matrix determinant problem, managable even with only a calculator. Assuming you are near the answer with your original W12x53 column, you should be able to solve it with some degree of optimization in about a day. If the coverplates extend to within a few inches of the column's ends, then it seems to me the answer would be approximated by determining the kl/r for the strong section, looking up the Fa in the AISC manual, and multiplying the Fa x original W12x53 cross sectional area. This is the rub. If you could take the cover plates to the very ends, then you get to use their cross sectional area. If not, you just get to use their stiffness, if you get my meaning. Good Luck

 

[IceNine]

Just a thought...You're adding 30,000 pounds, and assuming at least two existing W12x53 columns are sharing this load that's 15k each. Depending on your unbraced length, a W12x53 is good for 250k +/- so the additional 15k is only 6% over the capacity. A rule of thumb I was taught is that 6% over capacity is ok for an existing column.

 

[JStephen]

If the capacity is based on older design, can you use newer designs to uprate it? Just an idea.

 

[J1D]

Found a flexural buckling strength equation of reinforced strut from a manual called "Strength of Aluminum". For a strut/column with a reinforced length of kL, the equivalent slenderness ratio is:

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

 

[IceNine]

In addition to my post above, I would really recommend trying to find a way to spread the load out in such a way that you do not need to reinforce the columns.

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...

 

[IFRs]

You could weld an I-beam or channel to the existing column's web to make a "T" sort of shape. This compound section would be easy to analyze.

 

[Lutfi]

Last year I worked on a huge project where I had to reinforce 7 deep roof trusses while under load. I have done research in text books and found nothing. I called AISC and I also talked with Dr. Omer Blodgett. He was best asset besides some articles in AISC Engineering Journal. Dr. Blodgett was gracious to mail me a stack of resources and publication on strengthening structures under load. My project was completed with out a hitch. I like to add that I proof loaded the roof trusses to 125% of their live load and they performed better than expected.

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

 

[miecz]

An article by John Anderson and James Woodward appeared in the AISC Engineering Journal, October, 1972, "Calculation of Effective Lengths and Effective Slenderness Ratios of Stepped Columns."

 

[mrMikee]

Lufti,

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

 

[mfrad]

Consider taking one or two steel coupons to see if the steel grade has a higher yield point, then recalculate column axial capacity. Many Grade 36 steels constructed in the early 1960's tested typically around 47 ksi. Even earlier steels may test higher

 

[frogit22]

Might it be possible to improve the Kl/r by adding weak axis bracing?