Bowstring truss frame-reactions at base of columns

[357mag]

I am looking at a bowstring,steel,roof truss about 100 feet span supported on W8 columns at each end. The columns are about 15 feet long. The truss is about 2 feet deep where it connects to the columns and about 9 ft. deep at midspan. The truss connects to the inside flanges of the columns with a heavily bolted moment connection.
How would you determine the horizontal thrust at each column from uniform vertical load on the truss? The columns are pin connected with 2 bolts at the bottom.
The moment of inertia of the truss isHUGE compared to the columns. None of the text articles from rigid frame formulas seem to work with such a huge difference in "I" between the cols. and truss. I'm thinking the horizontal reactions at the column bases would be relatively small because of this but would like your opinions. Thanks much.

 

[paddingtongreen]

As a conservative estimate, take the rotation of the end of the truss, assume that the column rotates by the same amount, at the truss, and find the force required to bring it back to it's foundation and calculate the bending moment. You should probably apply those moments to the truss to see if they make any material difference. My guess is that it will reduce the member forces but because it is an over estimate, you cannot reduce the member sizes. If it increases member forces, that should be in your design.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[slickdeals]

I am going to ask a stupid question here.

The thrust from the compression chord should be balanced by the tension tie in the bottom chord, right? As a result, there should be not net thrust on the columns.

The only moment that the column will develop is a compatibility moment due to the rotation of truss. That moment will produce a shear in the column. Considering the base of the column is pinned, this compatibility moment will have to be carried by the strong axis bending of the W8 column (I am not sure how strong this column is in bending and what the magnitude of the rotation is).

 

[JAE]

I would model the truss and column together - but that's just me.

paddingtongreen suggests de-linking the truss from the column, calculating a rotation in the truss (neglecting the possible resistance to that rotation by the column) then applying that rotation to the column (neglecting the resistance to that rotation from the truss) and designing each accordingly.

This is conservative and probably OK due to the fact that the column probably isn't very stiff compared with the truss...but you might end up neglecting some possible compression in the bottom part of the truss at the column.

 

[paddingtongreen]

I agree that modeling the whole structure makes most sense, so much sense in fact, that I assumed that facility wasn't available to 357mag.

I did suggest that he feed the resulting moment back into the truss though, to check for problems.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[graybeach]

I agree with slickdeals. The tension tie is probably 100s of times stiffer axially than the columns are in bending so negligible thrust will be transferred to the columns.

 

[slickdeals]

I was reading about "smart-columns" developed by Walter P. Moore which lets them not account for compatibility moments by letting the column develop a plastic hinge at that location. The key is that the column be in its weak axis and not be part of a lateral force resisting system.

It seems like that when designing for long-span structures, they only design their end columns for gravity reactions and not design for compatibility due to rotation.

See 2 related articles

http://www.modernsteel.com/Uploads/Issues/March_2010/032010_Mar10_Louisville_Arena_web.pdf

http://www.gostructural.com/magazin..._prove_brains_are_better_than_brawn-6199.html

 

[357mag]

Thank youall for great responses. I used to have some computer software that I could use to model in this "frame" using node, member and load data.I wish I had it now.
Back in the late 1970's I did some related analysis and moment distribution on indeterminate frames but haven't run into much since.
I'm going to study your responses more thoroughly and see what I can come up with.