AS 1170 Combinations 1

[cooperDBM]

I'm new to Australian codes. AS 1170.0-02 clause 4.2.2 (Strength Combinations) gives two similar combinations;
(b) Ed = [1.2G, 1.5Q]
(c) Ed = [1.2G, 1.5ΨlQ]

The second combination includes the long-term factor which is less than or equal to one. When would combination (c) ever govern over (b)?

Thanks for your help.

 

[IDS]

When the ultimate load capacity reduces over time.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[cooperDBM]

Thanks Doug,

I'm not following you. How is the ultimate (strength) capacity time dependent?

I understand that the imposed actions in (c) are long-term (i.e. sustained) loads. It seems that the imposed actions in (b) cover any load duration, or should they only be short-term (intermittent) loads? Are (b) and (c) meant to be used together?

Perhaps a quick example would illustrate the difference. Say we're designing a beam that supports a lab. Take G to be say 300 kNm at midspan, while the long-term live load (benches, equipment, etc) is 150 kNm, and the short-term live load (people, materials, etc.) is 200 kNm. Assuming distributed loads I take the long-term factor to be 0.6.

(b) Ed = 1.2(300) + 1.5(150 + 200)= 885 kNm - my interpretation?
(c) Ed = 1.2(300) + 1.5(0.6)(150) = 495 kNm

If (b) includes the long-term live load as well as other live loads then it should always govern.

Sorry if I'm missing the basic philosophy but my experience is North American.

 

[rowingengineer]

I believe Doug is referring to a material like timber, where in the design using AS1720 there is different factors on the timber design due to the loadings duration. ie a long term loading reduces the capacity of the timber (reduction factor or 0.57 (note this is for dead load and shouldn't be confused with live load but for illustrative purpose will do)) conversely a short term loading like wind allows for an increase in the capacity (factor of 1.00).

Hence the ultimate limit state capacity is time dependent depending on the loading type.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[apsix]

RE is correct, except the 0.57 factor is applicable for permanent + long term imposed, as well as for permanent only (with different load factors).

 

[cooperDBM]

I'm still wondering how combination (b) relates to combination (c)?

 

[apsix]

Combination (b) includes transient loads, (c) doesn't.
For timber the strength for loads that include transient loads is greater than for permanent/long term loads only.
Both combinations only need to be checked for materials with time dependant strength such as timber.

 

[cooperDBM]

Thanks apsix,

That answers it. I'm working with steel and concrete and didn't see the rationale.