Checking ConcreteColumns for Column Hung Shoring (slenderness effects)

[hawkinjc]

Hello all,

I work for a contractor and am on a job using a column mounted shoring system for the elevated decks. When checking the cantilevered columns (with an eccentric load from the column jacks) the typical floor-to-floor height keeps my kl/r value less than 22 and I am allowed to check the columns using the interaction diagrams. I am good there (barely working the columns).

My questions arise on the first couple of levels where my kl/r values are greater than 22. I don't have the programs, experience, etc to be able to run a second-order analysis and whatnot on these taller columns in a timely fashion. (and even if I did to make sure I did it correctly). Here are a few of my questions:

Is there an easier way to check these columns without running a second order analysis (even if it is very conservative)?

Is this something a structural engineer (the EOR) can check quickly without too much trouble?

Can I check the column as a cantilevered beam?

Would it be best to just use pipe bracing to reduce my unbraced length and achieve a kl/r value less than 22?

Everyone likes to tell me that the columns are designed to carry the structure so they should have no problems carrying just one level. (Obviously not from engineers, we are loading the columns differently than designed) However, is there any truth in this?

I have loads on each side of the columns of about 45K about 18" from the columns center. (14x28 typical)

Any advice on previous experiences is greatly appreciated. Thanks!

 

[ishvaaag]

If you can find a controlling buckling mode and the column is typical in some way one should be able to find a elastic buckling factor; you may conservatively choose a lower E than actual to mimick material nonlinearity. You have formulas for the reduction of axial and bending stiffness in the AISC 360-05 code, but if the stresses stay low you may need none. Hence it is essentially the same check you are already doing.

Another question is that the formulations use to be derived for member of up to geometrical slenderness about 30. Most likely both your component members and built-up column (if built-up) would respect such limit, it is unusual to make excessively slender anything.

 

[Lion06]

This is not something the EOR can easily do - at least not any more easily than you. Most would not check it anyway as that's a "means and methods" issue. Do you not have access to ACI? ACI has an approximate method. You should be able to set up a spreadsheet if you have a lot of them to do.

It's hard to tell from your post how much eccentricity you have. If you have 45 k on each face equidistant from the centroid then you have no eccentricity and can use the minimum eccentricity out of the slender column section.

How do your construction loads compare to the actual in-service loads?