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DCB2524
Posts: 4
Registered: ‎06-12-2012
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AISC Direct Analysis

450 Views, 6 Replies
06-12-2012 02:19 PM

I'm wondering how to implement AISC's "Direct Analysis Method" (per the 13th Edition steel manual) in Robot.  How do I get the program to reduce the flexural and axial stiffness of lateral members for the P-Delta analysis?  I'm new to Robot.  I've typically used RISA or STAAD, both of which provide a way to do this.  I don't see it in Robot.

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Distinguished Contributor
bjur
Posts: 147
Registered: ‎09-14-2011

Re: AISC Direct Analysis

06-12-2012 05:31 PM in reply to: DCB2524
Not sure if this will help but it makes for some good reading. There are a couple for charts that review this issue.
http://www.modernsteel.com/Uploads/Issues/December_2009/122009_Dec09_Steelwise_web.pdf
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Product Support
Artur.Kosakowski
Posts: 4,750
Registered: ‎12-17-2010

Re: AISC Direct Analysis

06-13-2012 11:14 PM in reply to: DCB2524

In Robot the solution would be to create a new material with reduced E value for these members.

 

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Artur Kosakowski
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Member
DCB2524
Posts: 4
Registered: ‎06-12-2012

Re: AISC Direct Analysis

06-21-2012 05:44 PM in reply to: DCB2524

I thought of that too, but there's one problem with it: E stays reduced during design.  I need to reduce E for the purposes of analysis and then revert back to the actual value of E for design, otherwise my results are too conservative.

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Product Support
Artur.Kosakowski
Posts: 4,750
Registered: ‎12-17-2010

Re: AISC Direct Analysis

06-21-2012 11:31 PM in reply to: DCB2524

The typical steel design process in Robot includes the following steps:

 

1. Creation of Design Groups

2. Group design

3. Replacement of sections

4. Verification of groups/bars

 

For this particular situation I would use the following approach

 

A. Definition of  the E reduced material; applying this material to selected bars; calculations of a model

1. Creation of Design Groups (mind that you can assign different material than assigned to bars ; in this case the one with not reduced E)

2. Group design

3. Replacement of sections (mind to have the E reduced material still assigned)

B. Re-calculation of the model

4a. Verification of groups

4b. Verification of bars - this cannot be done directly (material issue) therefore:

- create single bar groups using all/-1 (or E reduced material bar list/-1) syntax

- assign not reduced material to these groups

- run group verification

 

auto group.PNG

 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

 



Artur Kosakowski
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Member
DCB2524
Posts: 4
Registered: ‎06-12-2012

Re: AISC Direct Analysis

06-22-2012 03:16 PM in reply to: DCB2524

I think I understand what you're saying.

 

What material does Robot use for analysis? Does it use the material defined in the group or the material assigned to the bar?  How about for design/verification?  Would the following procedure work?

 

Step 1:  Create a new material with a reduced E value

 

Step 2:  Assign this new materal to the bars that need to have their stiffnesses reduced for analysis.

 

Step 3:  Push the "Calculations" button

 

Step 4:  Under "Design->Steel Members Design-Options->Members/Groups-Definition..." create groups (with the unreduced value of E) to be used for code checking/verification.

 

Step 5:  Verify the groups.

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Product Support
Artur.Kosakowski
Posts: 4,750
Registered: ‎12-17-2010

Re: AISC Direct Analysis

06-25-2012 12:11 AM in reply to: DCB2524

For static analysis materials assigned to each bars are used. For the group verification and design (Steel Design module) the material defined for the group is used instead.

 

The steps seem to be OK.



Artur Kosakowski
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