Eccentric Roof Bracing (Purlin to bracing offset)
Eccentric Roof Bracing (Purlin to bracing offset)
(OP)
Hello All;
I have a bit of a problem and I wonder if I can get a few opinions. Please consider:
- Structure is a 21mx15m steel portal-frame building (360UB45) with CFS light gauge metal purlins (Dimond DHS 200/18).
- Because of client's specfic requirements, the purlins are being set inside (but not fully inside) of the steel flanges (see image) so I am having to brace at the bottom chord. Centre line of purlin to bracing distance is 260mm; Beam is 352mm deep. Peak load in bracing is ~100kN and causes an ~65kN compression in the purlin.
See: www.engsoc.org/~leito/PurlinCleatDetail.jpg
- Roof bracing consists of a proprietary steel rod system, attaching to the BOTTOM flanges of the portal frame rafters.
I have some concern regarding the eccentricity between the bracing line and, what is in effect, the compression strut (purlins) for the latteral load resisting system. My gut says the effect is negligible, but I am hoping to get a couple of opinions.
Should I be worried about this eccentricity?
Thanks in advance,
YS
I have a bit of a problem and I wonder if I can get a few opinions. Please consider:
- Structure is a 21mx15m steel portal-frame building (360UB45) with CFS light gauge metal purlins (Dimond DHS 200/18).
- Because of client's specfic requirements, the purlins are being set inside (but not fully inside) of the steel flanges (see image) so I am having to brace at the bottom chord. Centre line of purlin to bracing distance is 260mm; Beam is 352mm deep. Peak load in bracing is ~100kN and causes an ~65kN compression in the purlin.
See: www.engsoc.org/~leito/PurlinCleatDetail.jpg
- Roof bracing consists of a proprietary steel rod system, attaching to the BOTTOM flanges of the portal frame rafters.
I have some concern regarding the eccentricity between the bracing line and, what is in effect, the compression strut (purlins) for the latteral load resisting system. My gut says the effect is negligible, but I am hoping to get a couple of opinions.
Should I be worried about this eccentricity?
Thanks in advance,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...






RE: Eccentric Roof Bracing (Purlin to bracing offset)
RE: Eccentric Roof Bracing (Purlin to bracing offset)
RE: Eccentric Roof Bracing (Purlin to bracing offset)
Several of the NZ companies have this info in their purlin manuals. Compression capacity is presented in tables. Instructions for bending/axial interaction is also given.
Purlins are pretty good compression members because they are deep and they are laterally restrained by the roof sheeting.
RE: Eccentric Roof Bracing (Purlin to bracing offset)
If relying on purlins as bracing struts I would normally use a double purlin, even if a single one is shown to be OK.
RE: Eccentric Roof Bracing (Purlin to bracing offset)
- If you're considering loading a beam in torsion, you need not consider the resulting moment and torsion effect if you are within a zone of torsional stiffness defined by:
a= SQRT((EIw)/GJ)
This would apply up to any position of full or partial lateral restraint. In my case, I am well within this distance from full depth stiffeners, and as such the eccentricity is not of concern.
Thanks again everyone...
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...