topic Re: Direct analysis method in Robot Structural Analysis Forum

Direct analysis method

HoshangMustafa — Tue, 11 May 2021 22:31:21 GMT

Hi all

Please find the attached file.

This is to model Example 3.1 of AISC design guide 28. I modeled bracing as tension member as suggested in post 4 in this thread:

https://forums.autodesk.com/t5/robot-structural-analysis-forum/steel-bracing-advanced-bar-properties-instability-type-1-error/m-p/9738739

So, how to manage Direct analysis method with this model?

Description of Framing
All lateral load resistance is provided by the tension only rod bracing. The tension rods are assumed to be pin connected using a standard clevis and pin (see AISC Manual Tables 15-3 and 15-7). Beams within the braced frame are bolted into the column flanges using double angles (see AISC Manual Figure 13-2(a)). A single gusset plate connecting the tension rod is shop welded to the beam flange and field bolted to the column flange (see AISC Manual Figure 13-2(a)). All other columns outside the braced frames are leaning columns with simple beam-to-column connections.

Re: Direct analysis method

Krzysztof_Wasik — Wed, 12 May 2021 08:56:04 GMT

Hi @HoshangMustafa

As far as I could check you do not have ULS combinations in your model. This is the main reason why DAM analysis cannot be run

Re: Direct analysis method

HoshangMustafa — Wed, 12 May 2021 09:06:23 GMT

As far as I could check you do not have ULS combinations in your model.

How to have ULS combinations since I have tension members (bracings)?

Re: Direct analysis method

Krzysztof_Wasik — Wed, 12 May 2021 09:10:12 GMT

Hi @HoshangMustafa

Define combinations manually or use Automatic combination in Manual combination mode (as described here)

Re: Direct analysis method

HoshangMustafa — Wed, 12 May 2021 10:21:40 GMT

Thanks.
Another question:
All other columns outside the braced frames are leaning columns with simple beam-to-column connections.
You know a leaning column is a column carrying vertical load only. How to make these columns be leaning columns? Should I assign compression bars to them? Or there are other work arounds? Compression bar will induce no convergence of nonlinear for wind load.

Re: Direct analysis method

Krzysztof_Wasik — Wed, 12 May 2021 11:16:00 GMT

Hi @HoshangMustafa

I understand that you do not want to transfer moments from beams to columns.

In that case you can use releases with RY releases in beam ends or if you have two continuous elements (continuous beam and column) intersecting compatible nodes can be used (with RY released).

Re: Direct analysis method

HoshangMustafa — Wed, 12 May 2021 11:37:18 GMT

Thanks.

The analysis calculation suggests using DSC algorithm. I know this option would reduce stiffness element. Is it reducing Iy only? Is it ok to use this option?

Re: Direct analysis method

Krzysztof_Wasik — Wed, 12 May 2021 11:44:45 GMT

Hi @HoshangMustafa

DSC is suggested for dynamic and nonlinear analysis when structure consists of releases. It changes simulation method used for releases definition but does not change members stiffness.

Refer to DSC description for more information.

Re: Direct analysis method

HoshangMustafa — Wed, 12 May 2021 14:30:30 GMT

Thanks @Krzysztof_Wasik

I've made releases on beams, ticked on DLC and the calculation shows that Additional stiffness reduction is required. Is it Tau_b coefficient?

Also the results for moment for the beam (on the roof) shows 85.02kipft for DAM - COMB1 + NL X+, while in the AISC design guide 28 it's 2580kipin.

Re: Direct analysis method

HoshangMustafa — Wed, 12 May 2021 14:51:32 GMT

I set beams 25 and 27 to be Ry released in Beginning and End. The results seem to be reasonable. Only one issue with required additional stiffness reduction. Any help would be appreciate.

Also it would be helpful if notional loads displayed as Forces generated automatically.

Re: Direct analysis method

HoshangMustafa — Wed, 12 May 2021 19:34:01 GMT

Hi @Krzysztof_Wasik

I wrote my considerations in posts 9 and 10.

Re: Direct analysis method

Krzysztof_Wasik — Wed, 12 May 2021 21:20:46 GMT

Hi @HoshangMustafa

I have registered question about warning about necessary reduction is reported in Developers Database (ticket RSA-39177). Warning disappears when option "Add additional loads" is selected (instead reduction 0.8 for steel elements)

Bending moments in beams are bigger than presented in reference table, (for COMB1) when structure is calculated without DAM, After adding notional loads in DAM moments cannot be smaller.

I understand that after model modification (described in message 10) you have obtained reasonable results for moments..

Re: Direct analysis method

HoshangMustafa — Wed, 12 May 2021 21:56:11 GMT

Thanks @Krzysztof_Wasik

Warning disappears when option "Add additional loads" is selected (instead reduction 0.8 for steel elements)

I tried this and the notional load coefficient became 0.003 instead of 0.002. The equation for the notional load in AISC is:

Ni=0.002Yi

I don't know if 0.003 is acceptable instead of 0.002. I think this 0.002 is based on tolerances in manufacturing.

Also, in the posts mentioned above, I suggested displaying notional loads as Forces generated automatically.

Re: Direct analysis method

Krzysztof_Wasik — Wed, 12 May 2021 22:09:10 GMT

Hi @HoshangMustafa

Using additional loads is workaround I can propose to avoid warning about stiffness reduction. I am waiting for explanation regarding this warning.

Re: Direct analysis method

Krzysztof_Wasik — Wed, 12 May 2021 22:18:26 GMT

HI @HoshangMustafa

I have posted your comment about notional loads presentation in Robot Structural Analysis Ideas board.

Re: Direct analysis method

HoshangMustafa — Fri, 14 May 2021 08:41:45 GMT

Hi @Krzysztof_Wasik

In that case you can use releases with RY releases in beam ends or if you have two continuous elements (continuous beam and column) intersecting compatible nodes can be used (with RY released).

How can I use compatible nodes (with RY released) for two continuous elements intersecting?

Re: Direct analysis method

Krzysztof_Wasik — Fri, 14 May 2021 12:38:43 GMT

Hi @HoshangMustafa

I have in mind for instance this kind of screw connection. RY moment is not transferred from beam to column, but both members are continuous

Re: Direct analysis method

HoshangMustafa — Fri, 14 May 2021 19:09:34 GMT

Hi @Krzysztof_Wasik

My last post was:

In that case you can use releases with RY releases in beam ends or if you have two continuous elements (continuous beam and column) intersecting compatible nodes can be used (with RY released).

How can I use compatible nodes (with RY released) for two continuous elements intersecting?

So, how this can be achieved step by step in Robot?

Re: Direct analysis method

Krzysztof_Wasik — Fri, 14 May 2021 19:13:34 GMT

Hi @HoshangMustafa

In this case there is no need to use releases. I have suggested releases or compatible nodes depending of what you need in model.

Re: Direct analysis method

HoshangMustafa — Fri, 14 May 2021 20:06:33 GMT

Thanks, @Krzysztof_Wasik

In this case there is no need to use releases. I have suggested releases or compatible nodes depending of what you need in model.

Suppose I need to use compatible nodes in the case of two intersecting continuous bars. This is also stated in the help file:

How can I use compatible nodes in the case of two intersecting continuous bars, step by step?