Cambering of Light Gage Purlins 2

[shei]

I would like to know if:

1.) Pre-cambering of light gage purlins can increase its
bending capacity?

2.) Can it also help reduce deflection?

3.) What is the tolerable ratio for camber over the entire
beam length?

 

[MotorCity]

Shei,

1. No. The the camber in a member has absolutely nothing to do with its bending capacity. However, any adverse affects due to heating the member to achieve the camber should be carefully observed.

2. Yes. This is the primary purpose for camber.

3. Refer to Part 1 of the LRFD manual.

Good Luck.

 

[jetmaker]

MotorCity,

I thought one of the benefits of precambering, if done correctly, was to induce a residual compressive stress in the lower flange/member so that as the part deflected it transfered the compressive stress to a tensile one, thus increasing the capacity.

Thanks for the clarification.

jetmaker

 

[epr]

jetmaker,
You're confusing pre-cambering with prestressing.

 

[shei]

Thanks for all your inputs.

Here's a follow up question.

1) Wouldn't the camber section behave like an arc
transmitting stresses axially instead of by flexure?

2) Or would it only behave that way at a certain camber
height over length ratio? If so, how much is that
ratio?

Thanks....

 

[structuresguy]

Well, just to clear up a few things:

1. Precambering does not reduce deflection. The total deflection will remain the same. Only the relative deflection will change. The purlin will start out at some precambered distance above the reference centerline, and will deflect to some distance below the reference line. The point of deflection below the reference line will be less than the non-cambered purlin. However, the total deflection stays the same. T

he point to cambering the purlin would be to camber it an amount equal to the expected sustained dead load deflection which will occur. THis way the purling will become flat after the dead loads are applied, and then only live load deflections will occur.

2. It will DEFINITELY NOT behave like an arch in any normal range of cambering (1"-3" over 25 ft, say).

Hope this helps.

 

[agrant]

I've been involved with light gage for awhile and I've never seen anyone camber floor or roof joists for the serviceability benefit. Not that it wouldn't help, but I don't think it would be economical and (nothing against drywallers) they may not be installed properly.

 

[jetmaker]

epr,

So to clarify, does precambering mean that the item is fabricated in the deformed shape thereby having zero stress in the precambered shape?

Thanks,

jetmaker

 

[epr]

jetmaker-
Effectively, YES. There may be some residual stresses due to mechanically cambering a member, but these stresses are minimal, practically indeterminate, and have negligible, if any, effect on the design stresses. Also, as structuresguy stated, the precambering has no effect on amount of deflection.

 

[jetmaker]

epr,

Thanks.