AS4100 Clause 6.6 Restraints

[ShawThing]

Hi
Currently involved in a project where a commercial building is being converted into residential units.
The roof structure consists of steel roof trusses at 6m centres as the primary roof members which support purlins. The bottom chord of the truss has fly bracing each side at regular centres.
As part of this project the fly bracing is proposed to be removed and a fire rated stud wall constructed immediately adjacent each side of the trusses forming the party wall between the units. The fire rated walls each side of the truss will be back to back 76mm steel studs at 450 centres. This design has been supplied by the fire engineer.
Trying to justify the removal of the existing fly bracing utilising the steel stud walls by providing a connection between the stud wall and the truss bottom chord and I'm not quite following the explanation in clause 6.6.3 of AS4100. Have calculated 2.5% of the compression force in the bottom chord of the truss. Say the fly braces are at 3600mm centres. Am I allowed to divide that load by 7 (referenced in clause 6.6.3) to calculate the load on each double stud?
Hope I have explained this sufficiently.
Cheers.

 

[Agent666]

Each truss delivers a restraint load as noted, only up to 7 trusses need to be considered in accumulating the maximum restraint force.

https://engineervsheep.com

 

[ShawThing]

Hi Agent666
When you say truss are you referring to each pair of back to back wall studs?
Sorry, I'm not quite following.
Thanks for the reply.
Regards

 

[sdz]

When designing bracing for the compression chord of parallel members the load for ONE member is 2.5% of the compression load. If the brace must restrain multiple members you them multiply by the number of members restrained, up to seven. (Obviously this is a simplified design rule). For fly bracing the loads do not accumulate because each fly brace is restraining one truss.

In your case the load you have calculated is the lateral restraint load for each truss. You do NOT divide by seven. You can however calculate a uniform lateral load for a continuous restraint by dividing by the restraint spacing. This can be provided by a diaphragm ceiling if there is one.

The fire engineer is only looking at fire issues and there is no guarantee that his proposal is structurally sound.

 

[ShawThing]

Thanks sdz
So the closer the restraint spacing, the higher the continuous load? My restraint spacing will be 0.45m.

Yes I am aware the fire engineer won’t be considering any structural concerns.

 

[human909]

In your case the load you have calculated is the lateral restraint load for each truss. You do NOT divide by seven.

Agreed.

So the closer the restraint spacing, the higher the continuous load? My restraint spacing will be 0.45m.

The closer the restraint spacing the LOWER stiffness/strength required of the restraint.


Assuming you have designed the wall to be sufficiently stiff and strong for the internal wind serviceability pressure then I have few doubts it will be strong and stiff enough for providing continuous lateral restraint for the bottom cord of the truss. (You should still check this though if you want to be thorough.)

If however this is a 90mm studd wall 10m high or something then it will likely fail wind loads.
(I know this is internal, but you still have pressure differentials on internal walls.)

 

[ShawThing]

I calculated a compression load in the truss bottom chord of 485kN x 2.5% = 12kN. If the restraints are at 0.45m can I just multiply 12kN x 0.45m = 5.4kN.
Yes human909 I should consider internal wind pressures as well.
If the light weight framing can't support this which seems likely may have to incorporate a steel column in the plane of the light weight wall. There is also a mezzanine floor in some of this area also which will include perpendicular walls beside the trusses however given the fire wall situation won't be able to be fixed to them.

 

[human909]

0.45m can I just multiply 12kN x 0.45m = 5.4kN.
If the fly braces are every 3m then it would be 12/(3m/.45m) so below 2kN. (From memory of AS4100)

I looked at a similar project about 10years ago. 150UC columns and 200purlins for the wall.

 

[ShawThing]

Thanks human909
Rondo has designed the wall for everything else, including internal wind pressures etc. They may be willing to include this additional load in their design?
The studs are 5500mm maximum at the ridge and they currently have double studs at 450mm crs. If not steel columns at larger spacings maybe the answer.

 

[sdz]

For a 485kN axial load in the truss chord you look up the maximum effective length, Le. Then for a continuous restraint you require 485/Le kN/m.
As @human909 says you also have to provide adequate lateral stiffness. This is not addressed in the code since most restraint systems have more than enough, but this might not be the case for a stud wall. The attached page gives stiffness requirements for a single restraint. However if the wall has concurrent lateral wind load a lot of the required capacity could be used up.

Posting some diagrams would help. Perhaps you should seriously consider an alternative restraint system.

Can you post on the EA forums?

 

[ShawThing]

Thanks sdz. I think you are right. Probably best to leave the stud wall to Rondo and design a column in the location of the existing fly brace locations to restrain the truss bottom chord. The column will be able to be hidden within the fire wall and will not impact anything aesthetically.

 

[Euler07]

If the wall is continuing up to purlin level then just Tek screw the top plate under the purlins and you have effectively removed the loading from the truss. Is this possible?

 

[ShawThing]

Not a bad idea Euler07 but the new walls are internal non load bearing fire walls plus we'd be changing how the existing suspended slab is loaded. Thanks for your input.