Built up Columns

[jray2900]

I'm working on a project right now where part of our scope of work requires us to come up with a design to reinforce a W14X550 (Grade A36). The column is a bit overloaded due to some retrofits so I've been trying to come up with a couple of different options to boost the capacity. A couple of questions I was hoping to get some insight on are:

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1. Will any preheating be required to weld to this shape? And if so how would this be done?.

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2. Does the reinforcement have to be continuous through connection points?

W14X550 Properties:

Ax = 162 in^2
tf = 3-13/16"
tw = 2-3/8"
lb = 22 ft
Governing Force = 3300 kip
Allowable per AISC 7th ed. = 2842 kip


Any insight would be helpful. Thank you in advance.

 

[ztengguy]

not to go off subject, but how can your footing handle an additional 15-20%?

 

[jray2900]

This building was constructed in the 70's. The 3300 kip is the DL + LL reactions at the baseplates from some retrofits that were done in the 80's. So the extra load has been there for 30 or so years. The only reason the columns are being reinforced now is because we refused to add anything to the building until the columns were reinforced. But to answer your question I don't really know that it can however, it is sitting on a 10' thick mat on top of pilings.

 

[KootK]

That.. is a whole lot of column. I suspect that you would need preheat but I'll leave that question to other forum members. Several are much better versed in such things.

The reinforcement does not necessarily need to be continuous through the connection points. In fact, for the sake of constructibility, it's probably best that the reinforcement is not continuous. Of course, your design checks will need to reflect the constructed reality regarding continuity of the reinforcement.

What are you planning to use as reinforcement anyhow?

While not related to your specific questions, these documents may be useful to you if you don't already have them:

Link
Link
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https://www.dropbox.com/s/jcxb5iv5u...Welding to Existing Steel Structures.pdf?dl=0

]Link[/url]

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jray2900]

Thanks for commenting. I thought this might be the case it would be really difficult to do this there are 4 36" beams framing in at the first floor so re-framing those would be really difficult.


I was thinking of plating the web from flange to flange with 1" plate. Or perhaps welding some W8x18's in the web on both sides. I haven't run any calcs. on it yet but i think the plates might be the best option. Do you have and advice?

 

[structSU10]

Any way you can just reduce the unbraced length of the column? You also may be able to get away with partial reinforcement to improve the stiffness of the weak axis enough to improve buckling (if that is what controls)

 

[KootK]

Much depends on what your current mode of column failure is. Unless you're very close to your column squash load (phi x As x Fy), you're usually better off reinforcing in a way that increases your moment of inertia and improves buckling capacity. That usually means:

1) adding cover plates or channels to your flanges.
2) welding WT's to your webs to create a cruciform section.
3) Adding plating to effectively turn your column in to an HSS.

You need to pay homage to the fact that much of your column stresses will be locked into the exiting section and no amount of reinforcing will change that. Given the size of your load, I'm guessing that stress relieving isn't a viable option.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[canwesteng]

CSA W59 has a table outlining when/what preheat to use for various metals/electrodes. In any case your flange and webs would require significant preheat.

 

[dhengr]

Jray2900:
In a column that large, have you taken full advantage of any LL reductions from above, prior to adding the new loads and checking the column cap’y? Joints on columns that size are usually primarily bearing joints and the two column ends should have been milled or saw cut square and true for bearing. How do you know what bending or lateral loads are on the col. at your critical location? Those are taken by the joint. And, if you have col. cap’y. problems, are you sure they don’t continue through any col. joints? How you would reinforce that col. would be a real dog around those W36 beams, but I can’t see it from here, and don’t know where the new loads are applied. Does the base pl. check for the new loads and what about conc. bearing stresses? If the base pl. checks out o.k. with the added load, then the 10' mat on piles is probably o.k. for a 458k added load. But, you should still do some min. calcs. there too, just to rationalize this redistribution of new loads, in your own mind. Those heavy sections do require some preheating because they are such large heat sinks in terms of cooling the welds too quickly. The req’rd. preheat temps should not be so high as to cause shortening (or softening) problems during the welding process, but I’d still do some shoring around the col., under the W36's, during the welding. Remember, your reinf’g. pls. only pick up new loads, in proportion to their area vs. the existing W14, unless you partially unload the existing col. prior to applying the new reinf’g. pls. With or without this partial unloading of the existing col. you will add some load to its already over stressed condition before the new reinf’g. pls. even start to pickup their share of the new loads. Also, the welds at each end of the reinf’g. pls. must be quite large to start to transfer loads into it, in a fairly short distance. The mid-height welds really only act to stitch/stabilize the new pls. to the W14. Remember, you only have about 12" btwn. the insides of the flgs. in which to weld (access for welding) and get added web pls. in place. You might be better off to use pls. from flg. tip to flg. tip, say about 14-16" wide by some thickness. This leads to simpler welding, and a much more stable col. cross section.