wind bracing in steelwork 1

[186]

I am working on a renovation project of an existing steel structure. The structure is of portal frames spaced at about 4.5 metres and the column clear height is about 6 metres. The architect wanted to have a mezanine floor inside this existing one storey structure. The problem is the architect wanted to remove the diagonal wind bracing (side) because it is in the way.The bracing at the roof will remain. What shall I do?

 

[flamby]

What you can probably do is to replace the existing diagonal sway bracings with equivalent horizontal bracings with secondary cross reinforement(verticals).

 

[ishvaaag]

Devise a way to keep the whole structure stable whilst once the obstructing brace is removed. It should be feasible in a number of ways, but if not, don't remove the brace.

You could for example add bracing elsewhere that ensures the general stability. If for such thing the roof needs be collaborating as a diaphragm, ensure diaphragm, brace and collector strength is enough there.

You can also modify the frame panel to be rigid by diverse tricks, such adding truss to the column or supplements to the beams, or knee struts.

 

[valleyboy]

186

You could do one of three things:-

1. Relocate the existing bracing at a position to suit the Architect - ie another bay.
2. Provide portal bracing to one bay of the existing portal. ie create another portal perpendicular to your main frames. This portal will need to be designed to resist point load along eaves due to wind. This will usually involve stiffening the existing columns in your portalised bay with additional rolled I sections placed at 90 degrees to the main portal columns and welded as necesary.
3. In lieu of the bracing, construct a masonry diaphragm wall to transfer wind loads to foundation level.
Remember if you adopt either 1 or 2 to check the existing foundation for uplift under the dead + wind conditions.

Hope this assists!

 

[186]

1.to modify the frame panel to be rigid, do i need moment connection and

2.to relocate the bracing , could it be just the side bracing

3. ishvaag, can u elaborate more on adding truss to the column, please

thanks

 

[valleyboy]

186

In answer to your queries:

1 Yes, if you 'portalise' one of the side bays, you would need moment conections. The structure can be idealised as a flat roofed portal subject to a horizontal point load at eaves.
2. Yes, you can re-locate just the side bracing, providing that the eaves members along the building are able to resist compressive loads, to effectively transfer wind along the eaves line to your new braced bay.

 

[steve1]

If the architect wants to remove the brace to allow a pass through then perhaps a "K" brace with the apex at the top, or a couple of knee braces might work.

 

[ishvaaag]

Well, to add trusses to the column maybe

1) to make the column cantilevers able to take the whole brunt of the lateral force, or

2) as part of a scheme that makes a rigid or semirigid frame of the bay with the brace removed. Even the beam could be reinforced from its bottom, by truss or whatever steel addition.

A FEM model should quite speedily portrait if the situation you will go in is safe enough.

If it shows so be the case, then don't forget to temporarily brace the structure in suitable way before you extract the to be removed brace.