AISC Direct Analysis

[Anonymous]

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.

[bjur]

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

[Artur.Kosakowski]

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

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[Anonymous]

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.

[Artur.Kosakowski]

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

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.

[Anonymous]

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.

[Artur.Kosakowski · Accepted solution]

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.