Timber Load Duration Factor 1

[Buzzbromp]

I have a question regarding the timber load duration factor, Cd. I have asked people and have received different answers. The way i interpreted the description in the NDS code, is that the amount of time applied is the total time over the normal load distribution (10 years). It states that the normal load duration represents a load that fully stresses a member to its allowable design value by the application of the full design load for a cumulative duration of approximately 10 years.

My question is, how do you determine which factor to use, do you approximate how long the member will be loaded to its capacity over a 10 year life, or do you approximate how long the member will be continuously loaded?

For instance, code staes for a load duration of 7 days, use a factor of 1.25. Can i load the member for 7 days off and on, and use this factor, or can i only load it for 7 days total over a 10 year span?

 

[Lion06]

My understanding is that it is the TOTAL time that load is applied over the life of the member. 7 days means 7 days, whether it is 7 days in a row or 1 day a year for 7 years.

 

[Lion06]

Just to talk about it...... If you considered it as the time duration, your snow load time duration would exceed your live load time duration (which doesn't make sense) because you will certainly have snow on the roof (can be upwards of a month) for a longer period of time than you will have full live load (possibly a few days at most). This example is considering just the single duration, not cumulative total.
Also, I believe Cd accounts for creep. If you have a load on a beam for 7 days and remove it, the creep associated with that load doesn't just disappear, you must account for it.

 

[UcfSE]

You should apply a different duration factor for each load combination you have. Dead plus live load will have a different factor than dead plus snow, and different for dead plus wind, dead only, and so on. The time associated with the duration factor represents the total amount of time that particular load will act on the member for the life of the member. In your example, the 7 days may come all at once or be spread across 50 years, but the total amount of time that load acts cannot exceed 7 days.

 

[Boiler8019]

Cd has nothing to do with creep. You account for creep in long term deflection calculations.

Cd has to do with the fact wood can support larger stresses the shorter the period of time they are applied. This is unique to wood.

I recommend reading Design of Wood Structures by Breyer, Fridley, Cobeen. It is an excellent reference.

 

[COEngineeer]

It is the time of continously loaded.

 

[Boiler8019]

Forgot to add, you apply loads via load combos. You apply the Cd value to the combo based on the SHORTEST duration within that combo.

Therefore if you have combo D+L+S for a floor, and you say the live load is an occupancy live load, then Cd=1.0. I wouldnt recommend reasoning a 10day interval on an occupancy live load that is applied on and off within a period of 10 years. I just dont see how you'd ever have that kind of certainty.

Also note that localities sometimes reduce the Wind/Earthquake Cd value to 1.33.

 

[UcfSE]

See 2001 NDS 2.3.2 for the explanaiton of the duration factor. Notice the term "cumulative duration", not continuous.

I agree with Boiler.

 

[Buzzbromp]

Thanks for the responses.

I'm ok with the load combintations but i'm still unsure with what factors to use with the specified loads. I have read 2.3.2 of NDS, and about the description of "When the cumulative duration of the full maximum load does not exceed the specified time period, all tabulated design values shall be multiplied by the appropriate load factor Cd from Table 2.3.2 to take into accouint the change in strength of wood with changes in load duration."

However, how can you be sure of this time realistically if it is over the lifetime of the member? The table lists snow loading as a two month period, with a factor of 1.15. I'm sure local codes have their own specifications, but i would think snow on a roof would last more than 2 months in the lifetime the structure, so why would they list that as a 2 month cumulative duration and not constant duration? Or what if you design something for a construction load of 7 days, and then the construction takes a lot longer than expected?

Another example is in the design of timber fender systems. These that take ferry loads consistantly, once a day, therefore, you would not use the impact load factor of 2.0. From the responses, I would use the 10 minute factor of 1.6 for this scenario. So, when would you actually use an impact factor, knowing that it will only happen once in its life?

I suppose my question is, how do you feel comfortable with using the load duration factor properly if it is over the lifetime? By being overly conservative all of the time? I think i'd use 1.0 most of the time.

 

[Lion06]

buzzbromp-
You are correct, there will be snow on the structure for more than (2) months. However, the catch is how long the design load will be on the structure. It is certainly reasonable to assume that the maximum design snow load will only be on the structure for (2) months.
Similar for wind - There is almost always some wind blowing on the structure, but you only count the time in which the design wind is blowing on it (you wouldn't consider it as a Cd of 1.0).

 

[COEngineeer]

Thats what I mostly do.. just use 1.0 and even .9 if I support concrete on wood framing (.9 for DL).