pipe bridge and supports

[jmy61]

I am designing an exterior pipe bridge and trying to decide the best way to provide lateral support in the 70 ft. direction while allowing for thermal expansion of the bar joists.
The pipe bridge consists of two bar joists located approximately 30 feet above grade and clear spanning approximately 70 feet. The bar joists will be supported by braced steel frames at each end. The frames are oriented perpendicular to the 70 ft. direction.
To help carry the wind loads in the 70 ft. direction and minimize lateral displacement, I am considering the use of fixed end moment connections between the bar joists and braced frames.

My concerns are as follows:
If I fix the ends of the joists to the frames, I prevent thermal expansion of the joists; therefore, the joists must be capable of taking the axial load that will build-up from the thermal stresses. Also, the thermal force in the joists will exert an overturning force at the top of the frames.
On the other hand, if I provide slotted connections at one end of the joists, it seems that I will have one frame in a cantilevered condition which will be problematic given some existing site and structural conditions I must deal with.

I would appreciate any suggestions on how best to approach this issue.

 

[paddingtongreen]

My first concern is relative stiffness of the column and bar joist (it sounds as though that should be a truss). If I needed to use a moment connection, I would be tempted to fix one end only to a stiff column and the other end pinned to a flexible column. Failing that, I would use your scheme but make the legs flexible enough to absorb the expansion while being sufficiently stiff to resist the wind load. I think that the expansion of a seventy feet length is not going to be significant at thirty feet high.

You didn't mention pipe loads, They can put some big loads into a bridge

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

 

[ishvaaag]

It seems that you have the right structure and two members don't comply. Change them. I mean, then dismiss the weak joists and put competent members able to take what you need. Whith more competent members you will have more possiblities to develop at ends if wanted, or simply, strength enough to stand the thermal effects.

 

[jmy61]

Thanks for your responses.

My pipe loads are very light due to there being only a few small diameter pipes and conduits and due to the transported material being very light weight.

Even with moment connections between the joists and frames and fixing the column bases, I am still getting large deflections due to wind loads in the longitudinal (70 ft.) direction. In lieu of using moment connections, I am going to use standard joist connections where the joists bear on the rack braced frame column cap plates. I will control longitudinal displacement of the system via two braced frames, one each installed perpendicular to the rack columns at one end of the system. While not the most desirable solution, given schedule and site constraints, it will suffice.

Thanks again.