Lateral bracing for beam

[faromic]

I have a situation where I'm designing a transfer beam in a concrete building. The columns are cast in place with brackets picking up precast T beams. The case is that one column near the center of teh building needs to be removed. I designed new TS columns near next to the existing columns that are to remain. The column between these two will bear on the transfer beam. I designed the beam as unbraced but my supervisor still wanted to me to provide a TS kicker from the top of the beam to the underside of the precast T beam. My question is: Since the beam was designed as unbraced, what lateral force should I design the brace for? It's going to be a parking garage so I was thinking of just designing it for the impact of a car in the case that someone drives in with a pickup truck that doesn't clear. Or I could just use a TS4x4x1/4 and call it a day, but I have no calc or justifcation.

 

[271828]

Can you adapt the 13th Ed. AISC Spec. Appendix 6 for this problem?

BTW, I think it's a good idea to provide such a kicker too. FOr one thing, if I'm understanding your problem, the load bears on the top of the beam. Therefore, it'll have a destabilizing effect compared to the case of load applied near hte centroid.

 

[hokie66]

It is difficult to know from your description what the detail will be at the support of the removed column on the transfer beam, but I would have thought that the connection would brace the transfer beam at that point. What is the sequence of construction? How do you get the load of the removed column into the beam? You will have to jack between the beam and the structure to preload the beam, otherwise the deflections will likely cause distress above. In jacking, you have to ensure that the element of the existing structure which you jack against can take the force. If you do use a "kicker", be careful to install it at the right time or with connections designed so that it doesn't try to take the gravity load.

 

[faromic]

The entire building will be shored and then the column will be cut and the transfer beam installed. There will be two angles welded to the top flange and bolted to the column via anchors for column stability. the beam is designed as unbraced so essentially, the brace is being designed to account for the top flange loading. I came up with a TS4x4x1/4 and the brace will be installed last prior to releasing the jacks.

 

[hokie66]

1) Why would not the angles which brace the remainder of the column also provide bracing for the transfer beam?

2) As I indicated above, unless the beam is preloaded, deflection of the structure above could cause problems. Have you considered this?