FrancisEng
Structural
- May 10, 2018
- 1
Background:
Working on a project that is an existing metal building. The architect is proposing a new dormer and the addition of a few roof top units. The frames are spanning 72' and are 20' O.C..
Due to the new snow drift loading on the side of the equipment platforms and dormer, I have determined we will need to modify the beam and column sections, and the existing moment connections to support the additional load as it is creating stresses in the existing members much higher than 5% greater than the existing load.
The girder seems to check out for positive moment with the continuous bracing but falls short near the end connections under negative moment. I increased the various section capacities to support the additional load but have a question on the bracing for the compression flanges of the existing girder and beam. I am planning to weld continuous angles with the leg up to the top of the bottom flange and the bottom of the top flange.
Correct me if I am wrong but I think I am headed in the right direction with everything stated above. The last step is providing compression flange bracing at less than the 'Lp' that I calculated for the built-up section.
Here is the real question: Can tension cables at interval 'Lp' be placed between these angles that I am welding to the bottom flanges to meet the lateral bracing requirements? Normally I would put a diagonal angle brace meeting the AISC stiffness requirements back up to the roof joists but they are wanting to attach their new ceiling to the bottom chords of the existing roof purlins and expose the large steel frames which means the diagonal bracing would have to punch through finished ceiling. Simply I am looking for other ways to brace the section while reducing penetrations through the new ceiling.
Any feedback would be greatly appreciated.
Thanks,
Working on a project that is an existing metal building. The architect is proposing a new dormer and the addition of a few roof top units. The frames are spanning 72' and are 20' O.C..
Due to the new snow drift loading on the side of the equipment platforms and dormer, I have determined we will need to modify the beam and column sections, and the existing moment connections to support the additional load as it is creating stresses in the existing members much higher than 5% greater than the existing load.
The girder seems to check out for positive moment with the continuous bracing but falls short near the end connections under negative moment. I increased the various section capacities to support the additional load but have a question on the bracing for the compression flanges of the existing girder and beam. I am planning to weld continuous angles with the leg up to the top of the bottom flange and the bottom of the top flange.
Correct me if I am wrong but I think I am headed in the right direction with everything stated above. The last step is providing compression flange bracing at less than the 'Lp' that I calculated for the built-up section.
Here is the real question: Can tension cables at interval 'Lp' be placed between these angles that I am welding to the bottom flanges to meet the lateral bracing requirements? Normally I would put a diagonal angle brace meeting the AISC stiffness requirements back up to the roof joists but they are wanting to attach their new ceiling to the bottom chords of the existing roof purlins and expose the large steel frames which means the diagonal bracing would have to punch through finished ceiling. Simply I am looking for other ways to brace the section while reducing penetrations through the new ceiling.
Any feedback would be greatly appreciated.
Thanks,
