Plate Stringer

[SteelPE]

I have a project where the client is using plate stringers to support precast treads on a stair. The precast plank is supported on a system of carrier angles that are welded to the bottom of the stringer. These carrier angles are then welded to an plated that is embedded in the tread. The question I have is in relation to bracing for lateral torsional buckling.

Since the planks are supported by these carrier angles at the bottom of the stringer I am inclined to say that the stringers should be checked against lateral torsional buckling for the full length of the stringer (unbraced). However, since the angles apply their load from the tread slightly eccentric to the stringer one could argue that lateral torsional buckling would take place "into the stair" and that the stringer would bear on the treads and effectively become braced. Problem is, I am not exactly comfortable with this scenario. What does the community thing?

 

[KootK]

I lean towards calling it braced for just the reason that you mentioned. For the case of the top of the plate rotating into the stair, you should have a moment connection with the treads that will provide torsional bracing to the stringer. And, from an energetic perspective, bucking the other way would be pretty unlikely as it would raise the applied load initially, increasing the potential energy of the system. Of course, size the plate to preclude local buckling if the compression edge as well.

It would be good to see a sketch showing the proportions and nature of the connection.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.