Wind Beam for Metal Stud Framed Building

[asixth]

Hi guys

I have a metal stud framed building which I cannot rely on the ceiling diaphragm to transfer the lateral forces because the span dimension between bracing walls is too great. I have since decided to detail a hot-rolled channel to act as a header for the metal wall frame and transfer the loads back to a roof framing system which has been designed and can accommodate the loads. The wall studs are prefabricated and are a proprietary item so I will not be provided the design for them, I will only need to provide design for the lateral force resisting system.

1. Is there a typical design practice when the dimensions of the ceiling diaphragm become too large and it cannot be used to resist bracing forces?

2. What is the maximum span dimension of a ceiling diaphragm?

All help will be appreciated.

 

[Ron]

How is the wall shear handled? Are you using the header to supplement or replace the bearing wall consideration of the metal studs? Are there columns at the corners and if so, are they fixed or pinned at the foundation?

 

[asixth]

The header is transferring the lateral forces from the left wall to I-beam which then struts the force to the braced roof on the right hand side. The roof on the right hand side has wall bracing to take the loads to the foundation. All columns will be pinned to the foundations.

 

[Ron]

Should work. The connection between the stud wall and the header has to pick up the lateral load as well...make sure they know that in their design.

The dimensions you queried are obviously dependent on your design constraints. It can be anything, as long as the members are adequately designed and braced. As a practical matter, when your primary spans get over 20 feet, the x-bracing slenderness and dead load deflection start to creep up to the point that you have to have beefy, but relatively light sections (HSS for instance).

 

[hokie66]

asixth,
Left and right are reversed in your last post.

Ron's advice relates to the bracing system as you are designing it, but no one has commented on your queries about the limitations of ceiling diaphragms. As sheet diaphragm usage is not commnon in Australia except in houses, I think your approach is best.

 

[OzEng80]

Asixth

AS1684 and AS3700 has maximum bracing wall spacing (for various wind categories) from which you can infer diaphragm capacities (as hokie stated it is limited to residential – check limitations). Other approaches to your situation would be to provide an element in the ceiling line that can span between your supports. I have seen this done with a ‘horizontal wind truss’ (fixed to the bottom chord – makes the end trusses non-standard), blocking between the last couple of trusses and/or adding plywood to act as a beam – basically anything that can utilize the significant ‘depth’ available. It may also be possible to provide diagonal struts from your top plate to the top chords of the trusses (if that is your bracing plane).

An RHS on its side may be a cleaner detail than the PFC– I am sure you could include that as a requirement for the stud manufacturer (ie 90mm stud wall with a 150x50 RHS top plate). By having the member in line with the gable truss you can provide vertical cleats to enable a web fixing rather than a flange fixing to the bottom chord.

 

[apsix]

As suggested by OzEng80 forming a horizontal wind truss by bracing etc at the ceiling plane could be the best option.
Using a wind beam spanning 8m with a sensible deflection limit would result in a big beam. If the deflection is too large the load will go into the ceiling diaphragm anyway.