Determining Moment at the Top of a Steel Column

[SoFloJoe]

Hi All,

Newbie here! I am working on a structural steel beam system to support a cooling tower. Right now I'm on the column loads, more specifically the flexure design using the AISC equation H1-1b.

I know this was probably covered before (tried searching and did not find what I needed), I need to determine the moment at the top of the column (Mrx & Mry) without using software, where the beam connects. This feels so basic, but cannot find what I need. I used a simple statics (Horizontal Wind Load P x Length) but the moment is too high so I need a more specific method. Can the Euler's formula be reworked to determine moment or deflection? Or rework the Moment diagrams on a beam (Table 3-23 AISC)?

The wind load was already calculated and I used the resultant divided by the tributary area of the worst case support.

Any help here is much appreciated. Thank you!

 

[Mjkkb2]

Simple statics usually don't lie. Force times the arm will be your moment. Obviously you need to take the appropriate force (depending on the tributary and number of columns participating in resisting the wind) and multiply it by the appropriate arm (wind resultant to the top of column(s)), but that's the right approach.
If you're willing to provide a sketch, we will have better picture of your problem and likely better answers.

 

[SoFloJoe]

Thank you. My traditional statics is very conservative, where since this is a framed member (see diagram below) the top is more rigid than the calc. I was very close, only off by about 10% of the value I needed. I ended up bumping up the HSS tube steel.

Thanks!

 

[MotorCity]

If you have a braced frame as shown above, you should not have a moment at the top of the column

 

[KootK]

In the usual case where the member joints are assumed pinned, I agree with MotorCity. Are you considering your member joints to be fixed? If so, that's a pretty determinate thing and not easily estimated by hand other than say that the braces ought to be the stiffer load path by far.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Mjkkb2]

Agreed with the above comments. Unless you provide a stiff enough connection between col and beam you will not get a fixed condition. And even if, your brace will be stiffer and will take more of the lateral load which will reduce the moment in your joint.
I would model it as pinned at corners and let all the lateral be taken by the bracing. Otherwise you may be under sizing the brace and its connections assuming the joint takes more load than it actually does.

 

[SoFloJoe]

Thank you all for the suggestions. Yes I took it as pinned.