INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

AS 1170 Combinations

AS 1170 Combinations

(OP)
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.

RE: AS 1170 Combinations

(OP)
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.





RE: AS 1170 Combinations

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."

RE: AS 1170 Combinations

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).

RE: AS 1170 Combinations

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

RE: AS 1170 Combinations

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.

RE: AS 1170 Combinations

(OP)
Thanks apsix,

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close