NDS 2001 Section 3.5.2

[cedent]

I need a bit of help with a code interpretation of the following NDS equation for long-term deflection:

Total deflection = (creep factor)(immediate deflection due to long term loads) + (immediate deflection due to short term or normal loads).

The second term is what I am having trouble with. I have one textbook that interprets the 'short term or normal loads' as the live & variable loads.

I have another reference that interprets this term as the dead + live loads. Because the creep factor is typically 1.5, this would result in an ultimate factor on the dead load deflection of 2.5... and that does not make much sense to me.

I would call a 'short term load' a live load. I guess the question is: What is the 'normal load'?

 

[JAE]

The way I see it, the creep in wood is due only to sustained loads over time (i.e. Dead loads). Any other loads are short term loads and do not participate in the long term additional deflections.

 

[csd72]

I havent got a copy of the NDS with me, but comparing it to my experience with non-us timber codes.

The 2.5 factor seems reasonable for the creep, in Australia they multiply the long term load deflection by 2 (if the timber is seasoned (kiln dried) or laminated) and 3 if it is not.

Not all your live load is necessarily short term, kitchen tiles, sinks, benches e.t.c. will be included as part of the live load but will probably be there for the whole life of the structure. Australian codes specify 40% of live load as permanent for domestic structures (which actually seems a bit high to me).

If the area is used for storage, then all of your live load should be considered permanent or long term.

I think the NDS considers both dead and live as 'normal loads' (ie that is what it is normally supporting) and the short term loads are actually things like wind, earthquake, flood e.t.c.

 

[cedent]

Thank you both for your responses. I thought I should add that this question is a general question... not for a specific project.

 

[WillisV]

I agree with csd73 - The intent of the code would read at Delta Long Term = 1.5(Delta D) + 1.0(Delta D + Delta L). So overall its the deflections due to 2.5D+L.

 

[RARWOOD]

Section 3.5.2 is intended to address the increase in deflection that occurs as a result of creep. In most case the creep factor is only applied to the dead load. Generally glulam beam camber is based on 1.5 X DL Deflection.
The 1.5 multiplier is for seasoned lumber.

In applying section 3.5.2 th 1.5 factor only applies to deflection due to dead load in addition to any substained live loads. The second factor applies to live loads of short duration but does not include dead load.

Generally code define roof, floor, snow and wind loads are of low duration. As a result the allowable stress can be increased by load duration factors.

In cases where you have substained live loads which in combination with the dead load, represent a large portion of the total load creep becomes a factor.

See the NDS Commentary. The determination of what is a substained live load is not address by the NDS. This is probably partly due to the fact that the definition of live and dead loads often varies from one code to another. Are storage loads live load or dead loads? While storage of salt might be considered a substained live load snow would not be.

 

[miecz]

NDS F3.5.2 and Appendix F state that deflections due to long term loads need to be increased by apply a deflection factor (for seasoned lumber) of 1.5 to the "long term component of the design load". To get total deflection, add that to the deflection due to the short term component of the design load. I think the key word here is "component". We have long term "component" of the design load and short term "component" of the design load. To me, the use of the word "component" means these loads are mutually exclusive.

Total deflection is then LL+1.5*DL.