PERIMETER STEEL BEAM

[vincentpa]

Most of my design has been with concrete but when designing steel buildings, there is one thing that bothers me. When designing and detailing a steel beam at the roof level along the exterior perimeter of the building with joists parallel to the beam, we show an angle kicker from the bottom flange of the beam to the top chord of the joist. Many older details use cross bracing for the joists in the end bay. SJI wants us to get away from cross bracing because it doesn't allow the joists to deflect. SJI likes horizontal bracing in the end bays (each horizontal brace provides a minimum of 750 lbs. of lateral resistence). Even if we follow SJI and use horizontal bridging in the end bay, we are still putting in the kicker at the steel beam. Therefore, the beam is not allowing the joists to deflect contradicting SJI's desires. Another problem I have with the kicker is this. We put the kicker to brace the bottom flange from lateral forces coming from wind but when the joists are loaded, the bottom of the beam is just kicked out the other way (outwards from the building).

The same is also true with joists perpendicular to the beam, we have an angle kicker from the bottom flange of the beam to the bottom chord of the joist at the first panel point. Same result as above.

Can anyone share how they treat and detail perimeter steel beams at the roof?

 

[JAE]

We provide kickers as you suggest. However, we also show the vertical reaction of the kicker on the joist so the joist manufacturer can design for it.

If the joist is inhibited in its deflection....doh. Isn't that a good thing?

If you limit the joist deflection you are also implying that the joist won't be stressed to the same degree. The kicker would limit the vertical joist deflection only if the joist is loaded and the beam is not - or if both are loaded and the beam is significantly stiffer than the joist.

In this case, you'd have resulting vertical forces on the joist from the different amounts of load taken by the beam and joist. Again - just design for it.