Twist due to Torsion

[LivingTheBeam]

Hi all,

for carrying out torsional deflection checks in some softwares they ask for the angular deflection limit (in degrees). I was wondering is there any code/textbook that gives any guidance on what this limit should be?

For an open sided member such as a PFC I know it would be wise to keep it to a minimum but I was just wondering what is the acceptable/reasonable limit.

Many thanks

 

[human909]

How long is a piece of string? Are we talking strength or serviceability?

I must admit my own oversight in a recent construction. Hand rail attached to the PFC introduced fare too much torsion despite easily strong enough for the DL and LLs. The structure is strong and servicable, but the handrail like doesn't meet code. Furthermore it just doesn't 'feel' solid. So in this case an easy fix will be needed.

For your case. It depends on the circumstances. If strength and serviceability limits aren't a concern then why worry. But it is a whole different concern if you have 1m handrail attached that can apply significant torsion and deflect a significant amount!

 

[BridgeSmith]

Deflection limits are typically based on either physical restraints or serviceability criteria. Some serviceability criteria are given in codes for specific components/situations, but physical restraints are usually project specific.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[GeorgeTheCivilEngineer]

Furthermore it just doesn't 'feel' solid

It that genuinely due to the PFC twisting? Although it'd twist excessively at 0.74kN/m (or equivalent code requirement), actually imparting 75kg of force laterally isn't that easy.

But yes, I've been caught out on this too. Everyone loves PFCs for ease of fixing but just not up to serviceability requirements.

 

[human909]

It that genuinely due to the PFC twisting? Although it'd twist excessively at 0.74kN/m (or equivalent code requirement), actually imparting 75kg of force laterally isn't that easy.

You needed far less force than that to exhibit twist. Hit it with your palm and it would vibrate back and forth for 15! When you have a 200PFC for ~5.5m beam with cleat fixing only at the ends then there really isn't a whole lot preventing twist. Frequent spacing means than strength and serviceability for walkway loads are fine. All you need is a intermediate cross piece and it will completely stiffen up the twist.

FYI for AS1657. It is 350N of horizontal force per linear metre. And 100mmm deflection. That is alot of force! I bet quite a few rails would fail to meet this criteria.

To might slight defence this was a detailer choice and I didn't catch it. But that is hardly a defense. The fact is that I hadn't come across the need to stiffen beams for handrail torsional requirements. This is a design and build job that is still unfinished so it is an easy fix.

But yes, I've been caught out on this too. Everyone loves PFCs for ease of fixing but just not up to serviceability requirements.

Even some of the 200UB of similar length had flex! Probably borderline on meeting the code. But not so bad that it was to the point of flagging it fix, but definately worth a follow up.

Fact is with a 1m long lever arm attached it is pretty easy to twist unrestrained members. More firmly attached grating might help but bottom twist restraint is likely need to do the job properly.