Which members are intended to be included in the parameters FLEX and AXIAL for a braced bay when using Direct Analysis Method? Does this include just the braced columns, beams and bracing or the entire frame (including gravity only beams/columns)? I'm a little confused by the definition provided by STAAD. The definition STAAD provides is as follows:
FLEX, identification of members whose flexural stiffness is considered to contribute to the lateral stability of the structure, along with the initial value of τb that should be used. Members listed with FLEX will have their EI factored by 0.80 times τb while performing the global solution. The final member forces and code check will be with 100% of the flexural stiffness.
AXIAL, identification of members whose axial stiffness is considered to contribute to the lateral stability of the structure. These members will have their EA factored by 0.80 while performing the global solution. The final member forces and code check will be with 100% of the axial stiffness.
I am currently trying to model Example 2.1 in AISC Guide 28 in STAAD and analyze it using direct analysis method and compare the results to those provided in the example. I think it may be worth noting that the example usese effective length method, where I am using direct analysis method. Currently, my results for the moment in the beams and axial loads in the columns match that of the example (or are within 0.50%) except for the axial loads in the columns for combinations 5, 6 and 7. The results for these combinations have been up to 30% off. These are the only combinations that include the wind load; which is why I think the error might be coming from the columns in the braced bay. Any help is appreciated, thanks!
Lauren
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.
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
*1 1.0 2 0.75 3 0.75 10 1.0
LOAD 104 COMB4 = (D + 0.75L + 0.75LR) + N
1 1.0 2 0.75 3 0.75
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
…
to
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.
DOM ASD 6-28-16_modified.std