09.18.09 AISC Design for Stability Live Webinar Questions and Answers

09.18.09 AISC Design for Stability Live Webinar Questions and Answers

SEISMICQUESTION ANSWERDoes the Delta sub H deflection in a seismic design include the C sub D amplification factor?

The Delta sub H is a measure of the lateral stiffness of the frame. If, when doing a rigorous second-order analysis you would amplify the displacements so as to get increased second-order effects because of these inelastic displacements, then you would also want to amplify Delta sub H for the same reason.

Is the Delta sub H the elastic or inelastic deflection for seismic loads?

The Delta sub H is a measure of the lateral stiffness of the frame. It is an elastic deflection. If you are using the Direct Analysis method you can use the reduced stiffness. In that case the comparison for appropriate methods is to 1.7 rather than 1.5. But regardless of which EI you use, the amplification will be the same.

Does the response regarding braced frames apply to BRBF's since their drifts are more along the those for moment frames

The response was that for braced frames, K=1 by definition in the 2005 Specification. BRBF's respond very much like other braced frames so it is appropriate to use K=1.

SOFTWARE/MODELINGQUESTION ANSWERShould we include rigid end offsets in our models? Deformations within the panel zone in steel structures

is usually a contributor to frame drift. Thus, you would not want to use rigid end offsets, since that would remove any deformation within that panel zone from your calculations.

To your knowledge, have these code changes been incorporated in common structrual software (i.e. STAAD-Pro)?

I do not know the status of STAAD-Pro as far as updating to the 2005 specification but there are many packages that have been updated. You should check with your vendor.

With the new stability requirements, and when considering a moment frame with composite concrete floors with studs, is there sufficient reliability in using STAAD MASTER/SLAVE - RIGID for modeling the diaphragm with respect to HORIZONTAL displacements?

I am not familiar enough with STAAD to give you an answer. You should check with your vendor.

If your software package easily incorporates P capital delta effects but not P little delta effects, how would you recommend running a rigorous second order analysis using the Direct Design Method?

You can include P-little delta in your rigorous second-order analysis by modeling the compression members with nodes along their length. The number of nodes required depends on the type of end restraint and sway resistance. The forthcoming design guide indicates that for members with K greater than 1.0 it would normally be sufficient to use 5 nodes. It is also possible that a smaller number of nodes would be sufficient.

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Is there any advantage to using rigorous second order analysis as compared to amplified first order analysis, or is it just the sales pitch of some software vendors only and you do not get huge advantage in real life design problems?

A rigorous second-order analysis is often easier to implement since all of the nodes are automatically in equilibrium. With the amplified first-order analysis you have to give some thought to how you apply the amplification. For regular structures, the amplified first-order analysis will yield reasonable and acceptable results.

Do you feel ANSIS software adequately addresses the different analysis methods and applicable code issues?

I am not familiar with ANSIS so I can not comment on what it does as far as implementing the AISC Specification.

DIRECT ANALYSIS METHODQUESTION ANSWERIn the Direct Analysis Method, I can forget about k for leaning columns?

In the Direct Analysis method, all columns are designed with K=1. Thus leaning columns and all frame columns are treated the same, K=1. But, you must remember to include the leaning columns (gravity only columns) in your analysis so that their impact on the lateral load resisting system is reflected in the analysis results.

Direct Analysis Method does not differentiate between braced frames and moment frames, but you get a lot more sway (big delta) in moment frames. How does the direct analysis method addresses that?

You must do a second-order analysis, thus the increased drift in a moment frame over a braced frame is automatically included in your analysis

Does the Direct Analysis Method apply also to Composite Columns ?

Yes, per section I4 of the Specification.

MISCELLANEOUSQUESTION ANSWERCouldn't that out-of-plubmness condition be required only if the structure is recognized to have a significant out of plumbness condition ?

Out-of-plumbness always needs to be considered, but it may not have any effect. The notional load, Ni=0.002Yi, accounts for an initial out-of-plumbness at the maximum 1/500 permitted by the COSP. If you can be sure that a lesser out-of-plumbness will actually exist, Ni may be reduced accordingly. However, normal construction practice would suggest that specifying a tighter tollerence could be a expensive requirement. If B2<=1.5, the notional load (and thus the consideration of out-of-plumbness) only must be applied in gravity only load combinations. If B2>1.5, the notional load (and thus the consideration of out-of-plumbness) is applied in all load combinations.

What is the out-of-plumbness value limit up to which one wouldn't need to include the notional loads ?

Same answer as above.

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MISCELLANEOUSQUESTION ANSWERAlthough not covered in the seminar, page 13-2 of manual (bracing and truss connections sections) requires diagonal bracing members separated at sandwich ends, to have a minimum of two stitch fillers. No length parameters are provided, so is this applicable for all short braces such as double angle knee braces?

The stitches are only required if the member is treated as a double-angle member. If you design it as a pair of single angles, there is no need for complying with that requirement, and this might be a very viable solution for a short knee brace.

In a much earlier slide, showing the deflection of first order linear analysis and second order analysis, do we not limit the working stress ratio of elements such that the member stays in the linear elastic range, much below the point on the second order analysis graph where it diverges from a standard linear analysis, thus negating the need for second order analysis? The graphs have not been clear on this - thus raising the question.

Since we are looking for the ultimate strength of our structures, second-order analysis must be carried out at ultimate load levels, even though our structures are only actually used at load levels much below this. This is why we have the alpha factor of 1.6 for ASD. Thus, under service load everything might actually behave elastically but we still need to look at the amplified load levels. However, there is nothing in the 2005 Specification that limits the structure to the elastic range under actual service loading.

How would a tapered section treated in DA method? You would have to decide on just how you wanted to model it, depending on the software or analysis method you are using, just like for any other analysis. AISC will soon publish a design guide on tapered members that contains recommendations for stability assessment.

In slide 69, would the Pnt term generally include all gravity load stabilized by the frame in question?

Yes

If there are only small amouts of lateral loads, should the notional loads be used as a minimum?

The notional load is a minimum lateral load if B2 is less than or equal to 1.5 for the direct analysis method and the effective length method. If B2 is greater than 1.5, the notional load is added to all load combinations. For the First Order Analysis, the notional load is always added in all load combinations.

When determining Pe2, is the service wind load always used to determine the drift independent of whether you're using ASD or LRFD?

For calculation of Sigma Pe2, the lateral stiffness of the structure is determined. The ratio of Sigma H (the lateral force) to Delta H (the lateral displacement) is a constant. It doesn’t matter what force is used.

When is stability analysis required for any structure? You must always consider stability of the structure. If you look closely at what you have been doing all along, you should find that you have been considering stability since stability was implicit in the calculation of K, with all of its limitations and problems. What is new in the 2005 Specification is that you now have other ways to accomplish that consideration.

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MISCELLANEOUSQUESTION ANSWEREconomy-wise, if in a given structure you could use any of the methods we just went over, assuming equal unit prices for all shapes (weight being the only variable), which method would be most economical in your experience?

Generally I don't think you would find economy of the final structure to be a determining factor in selecting the method. Except that, if you use any of the simplified methods or the first-order analysis method, there might be some simplifying assumptions that make these methods conservative.

In AISC 9th Ed, design used K factors and Moment amplification; the 1st order analysis method here seems to use only moment amplification (i.e. K=1.0). Why is this?

In the 1989 ASD Specification (in the 9th edition Manual) the second order amplification was included in the interaction equation and K factors were to be determined. In the 2005 Specification, second-order amplification is accomplished with the B1-B2 method or through a rigorous second-order analysis and in Section C2.2a Design by Second-Order Analysis (the effective length method) you also determine K factors just as before. With the First Order Analysis Method, the notional loads account for the initial out of plumbness as well as the second order effects. Thus, there is no need to amplify except through the use of B1.

If the cards on slides 133 & 134 are new, how can they be obtained?

These are given out at our full day seminar. They can also be obtained on our web site, www.aisc.org, under epubs, under Specifications and Codes.

In the simplified example, is the 150K lateral load that PRODUCES the H/400 limit or is the 150K load and we want to LIMIT the delta to H/400?

Yes to both. We have 150 kip load and we plan on limiting the drift under that load to H/400. We don’t have a structure yet but when we get a designed structure, the drift under 150 kips better not be more that H/400 or we will have to go back and redesign.

You said not to interpolate on the simplified table method. Can we round up or down on values we enter the table with?

As long as you make conservative decisions you can, yes.

How does AISC define a story? That is one of the issues with the amplified first order analysis method since B2 is based on that definition. In a typical office building that is easy, each floor is a story and they tend to be nice and regular. For structures where stories are not easily defined, a rigorous second-order analysis method should be considered.

Is differential foundation settlement considered a first or second order effect?

I don’t think it makes any difference what you call it. If you are going to have differential settlement then you should be accounting for it. I would treat it as a first-order effect because there might be additional drift or deformation following the settlement, and that would be second-order in relation to the settlement.

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MISCELLANEOUSQUESTION ANSWEROn Slide 143, should the boxes circled in columns be slides 1 row to the left to include 16.3?

Yes. I think I answered this during the presentation but if I did not I will answer it here. The question is correct. I included columns for 20 and 30 when I should have included 10 and 20

Why is H/400 use for both LRFD and ASD? H/400 is a Service Load Consideration.

Because we are trying to measure the lateral stiffness of the structure. If we use a service load and the corresponding drift limit we will have a stiffness measure when we take the ratio. Regardless of the force we use, the ratio of force to drift is a constant.

Will it be available any design guide? We are in the process of developing a design guide. It will be available hopefully in early 2010.

Please repeat the reference article that talks about the foundation modeling analysis criteria

I went back and listened to the presentation and I did not have a reference on foundation modeling in the section I listened to. But, there is a reference that might be of interest: Nair, Shankar, "Modeling of Support Conditions at Bases of Tall Buildings," AISC Engineering Journal, 1st Quarter, 2000. Additionally, check out the Boxed Lunch presentation, "Got Stiffness? Designing Better Base Plates" by Barry Arnold found at www.aisc.org/elearning.

What atachments are required to allow the use of metal decking as continuous lateral bracing?

When the deck is perpendicular to the beam, puddle welds and shear stud connector welds are sufficient to provide lateral support. When the deck is parallel to the beam, there may not be enough strength and stiffness to justify the assumption of lateral support. However, in that situation there will usually be infill beams framing into the girder.

We noticed that in example 4, te ASD and LRFD solutions differ considerably (each are the result of H1-1a). Can the presenter comment on this disparity?

The issue identified in this question is the result of the ASCE-7 load combinations. Note that the magnitude of the ASD load is quite close to the magnitude of the LRFD load. Even though one would have expected the LRFD load to be significantly higher. In the 8 hour seminar we actually have an example where the column is designed for more load in ASD than in LRFD because of this same load combination. It has nothing to do with anything in the AISC Specification.

Where can I get one of the toy erector sets from the AISC Continuing Education Ad? http://bridgestreettoys.com/

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09.18.09 AISC Design for Stability Live Webinar Questions and Answers