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RE: about load duration factor

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Marcia

 

When modification factors for resistance or loading do not apply they usually have a neutral value of 1. (eg. 1*LL = LL).

 

To Australian codes load duration factor is called up in the materials codes, such as the timber structures code where it is used to modify section resistance to allow for long term creep deformation. It is therefore not used in load combinations, but in checking the member resistance after the load analysis is complete, and generally would only be a concern if designing timber.

 

We do however have ‘chi’ values which are partial load factors which modify live loading combinations for serviceability conditions. These ‘chi’ values could be input through a load combination tab (eg. 1*G+0.6*LL where chi=0.6).

 

Since different codes vary where they place the factors of influence, suggest check whether your 1.25 load duration factor applies to the member resistance or to the load.

 

(eg. I would check load case 0.9G+Wu and magnify timber resistance using short term load duration factor for wind loading, but check 1.2G+1.5LL and reduce timber resistance for medium term load duration, and check G+0.6LL  against resistance of timber using long term load duration factor. Alternatively I could adjust all the partial load factors by the appropriate load duration factor, and check each load case against the nominal timber resistance. The latter would be faster, but would hinder changing from timber to steel.)

Regards

Conrad Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust

mailto:sch.tectonic(--nospam--at)bigpond.com

Adelaide

South Australia


From: marcia mendez [mailto:mendezsole01(--nospam--at)yahoo.com]
Sent: Tuesday, 15 January 2008 06:57
To: seaint(--nospam--at)seaint.org
Subject: about load duration factor

 

Hello, I am just starting in structural design.

 

I ask for load duration factor.

Enercalc 6.0 has the load combination tab. What I have to do if I want use the load duration factor (1.25) designing a roof rafter i.e? Or if I need to design a floor joist where the load duration factor doesn't apply?

 

Thanks for your valuable information

 



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