steel rafter laterally restrain - AS4100

[SayGoodDay]

Hi

I have a case which the rafter has webside plate (weld to the web of the rafter, the length of the plate is approximate half of the depth of the web from the bottom of the top rafter) bolt (two bolts) to the purlin at 0.9m spacing. the bolt hole is slot hole (2.5 x the bolt dia) instead of the normal bolt hole (2mm + bolt dia).

My question is, for this case, can I consider the purlin can provide laterally restrain to the rafter so that the segment length is 0.9m?

If to consider the segment length is to be the distance between the column support (which is about 5m), is it little bit conservative?

Thanks for your comments.

Cheers

Saygoodday!!!

 

[hokie66]

No, that doesn't brace the rafter bottom flange. Suggest you use some fly braces to cut down the unbraced length.

In some cases, full depth stiffeners combined with purlin connections can be considered as flange bracing, but the method which you described is not adequate.

 

[SayGoodDay]

the slope of the roof is very flat, and no upward force on the roof. Bottom flage will be always in tension except at the region near the intermedia support.

My question is, does the slot hole arrangement good enough to provide the top flange laterally restrain?

Thanks.

 

[hokie66]

As you are using AS4100, I assume you are in Australia and using the wind loading provisions accordingly. Unless your roof is very heavy, I suggest that you would have net uplift loading conditions.

I assume your purlins are bolted to cleats above the top flange. We do assume the purlins brace the top flange, even with the short slotted holes which are for tolerance. However, these holes are typically not as long as you described. Refer page 17 of the attached reference.

http://www.bluescopesteel.com.au/files/dmfile/LysaghtZedCeesPart1v2Aug2010.pdf

 

[SayGoodDay]

Hi hokie66, yes, I am at Australia. The reservoir roof structure is existing and more than 70 years old. The top of the purlin is at the same level as the top of the rafter. Web side plate welded to the rafter with slot holes for the bolts to the purlin.

It is not a new design, and engineering judgement is need for the consideration of the laterally restain. My feeling is, by using the slot hole (2.5 x dia), it is not ok to be considered to provide full laterally restain to the top flange, not there should have some laterally restain. Instead of considering the rafter segment length is 5m between support, there may be a factor to consider the benefit from the purlins.

 

[hokie66]

You are correct, this is where engineering judgment comes in.

 

[BAretired]

Why were slotted holes used?

BA

 

[apsix]

As long as the bolts are snug tight I'd be happy to assume lateral restraints at 900 crs, especaially with 70 years worth of corrosion.
Restraint force should be significantly less than the 2.5% of flange force due to the close centres.