NDS Load Duration for Rain

[SpiderM]

Could someone direct me to the NDS Load Duration Factor for RAIN? I am analyzing an old building with a flat roof for a client and I must provide, verify and/or justify all data. I need the information for the evaluation the roof structure for the Rain Load and a stability analysis for ponding.

Thanks in advance;

JD

 

[MiketheEngineer]

1.15 - right in the NDS book - live loads

 

[RHTPE]

I would think that the duration of the ponding load is more pertinent than the 'label' of Live Load. A load duration of 24 hours permits a CD of 1.33. A duration of 7 days permits a CD of 1.25.

The 2005 NDS, Appendix B, does not explicitly assign a CD of 1.15 for Live Load (though it does for Snow Load - 2 month duration).

A freely draining roof might then be evaluated by an estimation of the depth of water at any point in time (based on the intensity of the rainfall) over the duration of a rainstorm.

JMHO.

Ralph
Structures Consulting
Northeast USA

 

[focuseng]

I believe basis of load duration factors are considered max event total load durations over the LIFETIME of a structure and not on the basis of measuring one single event. I don't have immediate reference for this. Anyone else confirm this?

YES I do agree with RHTPE that some consideration needs to be taken for your individual environmental conditions and building configurations which may modify these values. Some good information in the NDS commentary about this.

 

[MiketheEngineer]

Focuseng is right - that's why I use 1.15 and MOST everyone in the industry does also.

Sure you could argue for 1.25 or 1.33 or even 1.6 BUT WHY??

Most building officials will accept 1.15 for just about any load situation - because they have seen that for like 50 years or so. Anything else will catch their attention and then you got some 'splaining to do - Lucy

 

[woodman88]

focuseng - 2001 NDS section 2.3.2 "...Normal load duration represents a load that fully stresses a member to its allowable design value by the application nof the full design load for a cumulative duration of approximately ten years..."

I would be hard pressed to see a owner that would allow the rain to pond for a cumulative of seven days over the life of a structure. But it is possible so I would use the 1.15 factor if it works to be safe, unless the Building Department had a writen policy of a 1.25 factor to be used.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[focuseng]

Thanks woodman. I don't think my post was worded 100%. I meant only to agree that in some situation the EOR needs to think rather than copy which is why I directed to the commentary rather than give a value. I can see situations where the Cd factor might rightly be modified better or worse. Only the EOR, by looking at all the variables can make that call (not even us backseat drivers.
For example, I could think the ponding load duration risk could be considered different in Portland, OR when compared to Tuscan AZ. Thankfully I work in a completely normal region where every thing is just right.