Transient Loads

[jcox]

I know that the IBC Code no longer allows a 1/3 stress increase factor for working stress design. However, the code states that if two or more transient loads are used it is permitted to multiply the combined effects of the transient loads by 0.75. I used to only apply the old 1/3 factor to the effects of wind or seismic with combinations. The question may sound odd, but in my mind wind and seismic were transient loads. Is live load and snow now also considered to be transient. The code never really defines the word transient.

 

[dlew]

jcox,

I would not consider live load as a transient load. The factor of 0.75 allowed in ASCE 7 (and I assume UBC follows the same reasoning) when there are two other loads acting on the structure in addition to the dead load, is based on the lesser probability than both loads are at their maximum values at the same time.

Regards

AEF

 

[austim]

jcox,

I support dlew's comment. I would not consider either live or snow loads as transient. But I would also have a seriously good re-read of the Code. I am not required to use your IBC, but I would be astonished if there isn't a definition of such an important term as 'transient' somewhere in there.

 

[DaveAtkins]

I interpret the IBC to mean that no reduction is allowed for DL in combination with WL (or seismic load) alone; however, when DL is combined with WL (or seismic load) AND floor live load and/or roof live load, then the 0.75 factor applies. An editorial comment: I think it is a real bummer that components such as cladding cannot be designed with the 1/3 increase anymore.

 

[dougantholz]

I have to respectfully disagree with austim & dlew about live loads. Design, floor live load is a transient load. It is based on a statistical worst case load in an area - this is why we are allowed to do live load reduction. If you need further clarification look at ASCE - 7, in the appendix. It lists the typical occupied live load for an office building as around 11 psf.
jcox, the way that I read the code, and have talked to other engineers, transient loads are ANY loads that aren't a permenant part of the structure, or of permenantly in-place load (i.e. water in a water retention structure, or soil loads in a retaining wall).
Typical transient loads include, but aren't limited to:

roof live load
snow load
construction loads
floor live load
wind load
seismic load
impact loads (such as horizontal live loads)
rain load
flood load
impulse loads (different from impact loads, these are for designing machinary bases to reduce vibrations)
temperature induced loads*

these loads are permenant:
dead load
10 psf for partitions (where applicable)
self straining force
temperature loads*

*temperature really depends on the application - be very careful with this one.

 

[whyun]

I agree with dougantholz about the definition of transient load. Transient, by definition, means not permanent.

In 1997 UBC, the one-third increase in allowable stresses does not apply to all transient loads but only a few - basically any combination which includes wind or seismic.

For 2000 IBC Sec 1605.3.1.1, it is basically the 1997 UBC equation 12-11 from section 1612.3.1. Get the combined effect from all transient load, multiply 0.75, add the full dead load effect.

Hope this helps.

 

[austim]

dougantolz,

As far as I am concerned, forget your 'respectfully' - your opinion is just as worthy of respect as mine. Somewhat surprisingly, perhaps, I wholeheartedly agree with very nearly all that you wrote. For design of commercial and residential buildings, I would not argue with you at all.

jcox did not tell us what type of structure he is considering. Accordingly my stated position was put forward to give the conservative position to cover the situation of 'semi-permanent loadings', which may arise in industrial and plant structures.

The only point on which we really differ is in regard to a definition of 'transient'. The position that all loads must be either permanent or transient, in my opinion, too simplistic. There is a significant grey area, which includes some of the loads that you define as 'permanent'.

If the IBC does not provide a define of such an significant term as transient (or alternatively, define 'permanent'), and each individual engineer is left to make up his/her own, then there is a good field for the indemnity lawyers to exploit. Fortunately that is not my problem.

You treat the contents of water tanks and retained soil pressures as permanent. Yet many/most water tanks can vary from full to empty (in that they are no less variable than normal floor loads), and earth pressures may change with seasonal changes in ground water level.

What about the weight of material stored in silos and bunkers, or the material burden on conveyors and other bulkhandling equipment? Again, these will often vary from maximum down to zero.

The probable frequency and durations of maximum values for this sort of loading would vary from plant to plant, and would be quite difficult to codify.

You choose to categorize these loads as 'permanent'; I prefer 'semi-permanent'. But I think that we are both agreed that these variable loads should not be treated as 'transient', despite their variability.

 

[dougantholz]

Austim,
I never meant any offense, just offering a different opinion. Here is what I know about transient loads and what to design for. I believe the spirit of the .75 multiplier is based on the low probability of having two design load occurances at the same time. Basically - what is the probability of having a grain silo full at the same time you have a design wind event? EACH ENGINEER SHOULD USE THEIR JUDGEMENT FOR THIS. I never meant to say that the list above was difinative, but that you are staying within the intent of the code if you think that a silo that is full 15 days a year and a wind event with a 40 year return period have a low probability of simultaneous occurance.
Just my thoughts, other opinions are welcome.

 

Wind, earthquake, snow etc. may be called transient but I will be surprised (dumbfounded) if the 'spirit' of the code puts live loads as transient! The fact that live loads vary all the time does not make them transient. If variation was in the code authorities' mind, then we are left with a) Dead Loads
b) Transient Loads.
Farewell to Live Loads!!

 

[haynewp]

It seems like there are still engineers that are not sure about live and snow load being considered as transient loads. I asked one of the engineers at ASCE about this the other day. He agreed that live and snow WERE to be considered as transient loads. Regardless, I am still not going to use the 0.75 factor for live loads in areas designated for storage. The load could be in that one area for the entire life of the structure.

 

[JAE]

Sitting on our local city committee to review and adopt the IBC as our local code, we reviewed much of the early debating documentation for the IBC compilation (consensus reviews and commentary). Live loads were considered transient.

If you only consider seismic and wind as transient, then the code in illogical because nowhere do you combine wind and seismic together. So a code provision indicating that "when two or more transient loads are..." would only make sense if loads OTHER than wind and seismic were considered transient.

That said, I agree that the code writers were quite BIZARRE in not including a definition to the word transient.