STAAD Direct Analysis Method FLEX and AXIAL Parameters

RE: STAAD Direct Analysis Method FLEX and AXIAL Parameters

Sye — Wed, 29 Jun 2016 20:21:22 GMT

While taking a look at the model, I noted a few points

Point 1

The error was stemming from the fact that MY MZ releases were applied to these members in addition to the TRUSS specification. So you need to change the MEMBER RELEASE as shown next

MEMBER RELEASE

*4 5 9 10 14 15 19 20 24 25 28 30 START MY MZ

*4 5 9 10 14 15 19 20 24 25 28 30 END MY MZ

4 5 9 10 14 15 19 20 24 25 START MY MZ

4 5 9 10 14 15 19 20 24 25 END MY MZ

Point 2

In your DIRECT ANALYSID definition, the FYLD values specified as 7200/ 5184 are not consistent with the last unit command before that, which is Inch Kip

DEFINE DIRECT ANALYSIS

FLEX 1 ALL

AXIAL ALL

FYLD 7200 LIST 1 2 4 TO 7 9 TO 12 14 TO 17 19 TO 22 24 TO 27

FYLD 5184 LIST 28 30

So these should be changed to

FYLD 50 LIST 1 2 4 TO 7 9 TO 12 14 TO 17 19 TO 22 24 TO 27

FYLD 36 LIST 28 30

Point 3

You have specified the notional load 10 and have used that in various REPEAT LOAD cases. However the load case 10 only consists of notional load generated based the D case ( load case 1 ) only. When you are combining D+L or D+L+LR with appropriate factors, the notional load would have to be based on the vertical loads for each case. So instead pof having the load case 10, you specify the notional load as part of each load case itself. This is illustrated below for two cases 102 and 104. Similar changes has to be made for the other cases too

*LOAD 102 COMB2 = (D + L) + N

*REPEAT LOAD

*1 1.0 3 1.0 10 1.0

LOAD 102 COMB2 = (D + L) + N

REPEAT LOAD

1 1.0 3 1.0

NOTIONAL LOAD

1 X 0.002 2 X 0.002

Similarly for load case 104 it would be

*LOAD 104 COMB4 = (D + 0.75L + 0.75LR) + N

*REPEAT LOAD

*1 1.0 2 0.75 3 0.75 10 1.0

LOAD 104 COMB4 = (D + 0.75L + 0.75LR) + N

REPEAT LOAD

1 1.0 2 0.75 3 0.75

NOTIONAL LOAD

1 X 0.002 2 X 0.0015 3 X 0.0015

Point 4

If the design is to be carried out based on the AISC 360-10 and following the ASD approach, the code assignment should be changed from

PARAMETER 1

CODE AISC

…

PARAMETER 1

CODEAISC UNFIED 2010

METHOD ASD

…

Point 5

Since the code check cannot be carried out for the bracing rods, instead of using CHECK CODE ALL you should use

CHECK CODE MEMB 1 2 4 TO 7 9 TO 12 14 TO 17 19 TO 22 24 TO 27

Point 6

The parameters LY and LZ for braces are not required as these cannot be designed anyway.

LY 0.0833333 MEMB 28 30

LZ 0.0833333 MEMB 28 30

A modified file is attached for your reference.

communities.bentley.com/.../DOM-ASD-6_2D00_28_2D00_16_5F00_modified.std

RE: STAAD Direct Analysis Method FLEX and AXIAL Parameters

Lauren Ruchti — Wed, 29 Jun 2016 14:13:02 GMT

I tried to give the truss specification to the rods; however, STAAD gave me an error message almost immediately when I ran the analysis. I think this is because I am using the direct analysis method. I have uploaded the file using Bentley Secure File Upload.

RE: STAAD Direct Analysis Method FLEX and AXIAL Parameters

Sye — Wed, 29 Jun 2016 01:04:37 GMT

It is OK to define the braces as prismatic circular sections. However you may assign the TRUSS specification to these rods as otherwise these would take up moments/shears. If you still have doubts after making these changes, please upload the .std file and I will take a look.

RE: STAAD Direct Analysis Method FLEX and AXIAL Parameters

Lauren Ruchti — Tue, 28 Jun 2016 14:03:56 GMT

Thank you, that seems reasonable to me. I tried running another analysis and the results did come a little closer but are still about 30% off. I agree that the two methods would have some variations between them but I didn't think it would be as significant as they currently are.

Another area that I'm a little unsure of in this model is how the bracings are modeled. In the example they are specified as 1" and 1.5" tension only rods. To model this I deleted the bracings that would be in compression from the model and modeled the ones that would be in tenion as prismatic circular members. I noticed that in the deflected shape of the structure, the bracing has significant deformation. Is this expected or is there a better way to model these shapes?

Thank you again.

RE: STAAD Direct Analysis Method FLEX and AXIAL Parameters

Sye — Mon, 27 Jun 2016 20:23:38 GMT

I think to prevent artificial modification of structural stiffness leading to unintended distribution of forces, you should apply the FLEX and the AXIAL to all members in your structure and NOT just the ones contributing towards the lateral stability. The section AISC C2.3(1) of the AISC 360-10 specification, has a user note related to this.

The DIRECT ANALYSIS and EFFECTIVE LENGTH methods are significantly different and so I would not expect the results to match.