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

ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

(OP)
Hello

I have a 7-story concrete structure with a combined lateral resisting system in the x direction of shear walls and ordinary moment resisting frames.

based on section 9.5.5.7.2 of ASCE 7-02 my thetamax = 0.1
which also happens to be the limit at which you should consider p-delta effects.

My issue stems from the theoretical limit on my inter story deflections now set at hsx/1188-2865, in order to meet a thetamax =0.1, which is considerable higher than the limit from table 9.5.2.8 of hsx/100.

Can anyone provide any guidance on how to procede because as it stands now I am at h/400, considering p-delta, but my stability coeeficient, theta, exceeds theta max by a considerable amount. Is the structure really going to be unstable under these circumstances?

thank you for any help

RE: ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

Can you clarify what you mean by:

     hsx/1188-2865

??

Tell us your ratio of Px/Vx and your Cd value.

If you take a Table 9.5.2.8 allowable story drift Δ = 0.01hsx and a Cd = 4.5 (I'm guessing here) and then plug it into the Θ equation:

 ((Px/Vx)(0.01hsx)) / ((hsx)(4.5))   <= 0.10

The hsx value cancels out and you get:

Px/Vx  <=  45

Θmax = 0.5 / (0.8 x 4.5) = 0.139   using β = 0.8

or if Cd = 2.5 then Θmax = 0.25.

According to ASCE 7, yes - if you exceed Θmax you are unstable.

Verify your calculation values.



 

RE: ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

(OP)
By hsx/1188-2865 I mean to meet theta max my inter story drifts must be of magnitude hroof/1188 to hbottom story/2865 or 0.14" at the roof to 0.08" at the bottom level.

My Px/Vx ratios vary from 119 at the roof to 287 at the lowest level and Cd = 5
 

RE: ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

With a very high Px to Vx ratio, perhaps you should calculate an exact [&beta;} for the Θmax.  Don't know if this would do you any good or not.

Those ratios seem to imply you have a very low seismic demand.   

RE: ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

(OP)
I think I may be calculating Vx wrong reading the code it sounds like Vx should be the sum of the applied seismic story shear at level x up to level n, n being the highest level.

I am comparing my results with a ram frame analysis and it appears ram frame is only considering the the applied story shear for its theta calcs.

Can anyone confirm that Vx should be the sum as oulined in the first paragraph of this post?

Either way I will need to do further investigation at the very least at my upper levels to make sure beta is such that theta max is higher than theta.

RE: ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

Yes, Vx is the shear at that story, which would be the sum of shears for all stories above the story in question.

RE: ASCE7-02 Table 9.5.2.8 vs Section 9.5.5.7.2

(OP)
thank you for all your help JAE
 

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