Load Combinations Using Moving Loads

Question

I have run into this problem several times now and have not found a good solution for it yet. I am modling a large structure in which I need to consider several loaded lanes of traffic traveling accross it. To generate the moving truck loads I am using STAAD's vehicle definition and load generation features. The structure I am analysing is approximately 96 feet long and i need to consider all possible load patterns for 4 lanes. From what I have found there is no way to include load generations within a load combination. To get around this I have taken load generated for each lane and put them into their own envelopes (i.e. all loads that get generated for the moving load in lane one get enveloped). In order to make the necessary AASHTO Load combinations I then have to export all results out of the prgram into Microsoft Excel where i can take the enveloped results and combine them with the results from any of the other static loads that have been applied. This is a very tedious and time consuming process in which there is great potential for human error. Is there any other way that these load generations can be combined with other load cases within the STAAD.pro program so that I do not have to export the data out and do it manually? Also, I am not able to use the STAAD.pro BEVA package for this as it is not approved for use by the DOT we are working for. Any input or suggestions on this would be much appreciated. Thanks.

Matt Kline · Accepted Answer

Here is a model that will combine the moving loads into the load combinations. It is rather impractical on a large scale due to the number of load cases the moving loads generate and the associated analysis time, but it seems this is the only way to do it... 
 
 You need to create your "load combinations" as REPEAT LOADS. You then need to ADD LOAD these repeat loads to the generates moving load. You would also need to create multiple VEHICLE DEFINITIONS in order to get the correct load factor on the generated moving load.

STAAD SPACE 
 START JOB INFORMATION 
 ENGINEER DATE 14-Jan-16 
 END JOB INFORMATION 
 INPUT WIDTH 79 
 UNIT FEET KIP 
 JOINT COORDINATES 
 1 0 0 0; 2 0 0 20; 
 MEMBER INCIDENCES 
 1 1 2; 
 DEFINE MATERIAL START 
 ISOTROPIC STEEL 
 E 4.176e+006 
 POISSON 0.3 
 DENSITY 0.489024 
 ALPHA 6e-006 
 DAMP 0.03 
 TYPE STEEL 
 STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2 
 END DEFINE MATERIAL 
 MEMBER PROPERTY AMERICAN 
 1 TABLE ST W14X22 
 CONSTANTS 
 MATERIAL STEEL ALL 
 SUPPORTS 
 1 2 PINNED 
 DEFINE MOVING LOAD 
 TYPE 1 LOAD 1 
 DIST 0 
 TYPE 2 LOAD 0.75 
 DIST 0 
 LOAD 1 LOADTYPE Dead TITLE DEAD 
 SELFWEIGHT Y -1 
 LOAD 2 LOADTYPE Live REDUCIBLE TITLE LIVE 
 MEMBER LOAD 
 1 UNI GY -0.5 
 1 CON GY -10 10 
 LOAD 3 LOADTYPE Wind TITLE WIND +X 
 MEMBER LOAD 
 1 UNI GX 0.05 
 LOAD 4 LOADTYPE Wind TITLE WIND -X 
 MEMBER LOAD 
 1 UNI GX -0.05 
 LOAD 101 D+L 
 REPEAT LOAD 
 1 1.0 2 1.0 
 LOAD 102 D+.75(L+WX) 
 REPEAT LOAD 
 1 1.0 2 0.75 3 0.75 
 LOAD 103 D+.75(L-WX) 
 REPEAT LOAD 
 1 1.0 2 0.75 4 0.75 
 LOAD 104 0.6D+WX 
 REPEAT LOAD 
 1 0.6 3 1.0 
 LOAD 105 0.6D-WX 
 REPEAT LOAD 
 1 0.6 4 1.0 
 LOAD GENERATION 30 ADD LOAD 101 
 TYPE 1 0 0 0 ZINC 1 
 LOAD GENERATION 20 ADD LOAD 102 
 TYPE 2 0 0 0 ZINC 1 
 LOAD GENERATION 20 ADD LOAD 103 
 TYPE 1 0 0 0 ZINC 1 
 PERFORM ANALYSIS 
 PARAMETER 1 
 CODE AISC UNIFIED 2010 
 CHECK CODE ALL 
 FINISH

DSANJU · Answer

You can simply use STAAD editor to create load combination with the generated moving load. You may refer to the attached example model. In this model, the following primary loads have been created: 
 LOAD 1 LOADTYPE None TITLE DL 
 SELFWEIGHT Y -1 
 LOAD 2 LOADTYPE None TITLE IMPOSED LOAD 
 MEMBER LOAD 
 7 TO 11 18 TO 22 29 TO 33 40 TO 44 51 TO 60 UNI GY -4 
 LOAD 3 LOADTYPE None TITLE WL 
 JOINT LOAD 
 7 TO 30 FX 1 
 
 It also has the following moving load generation command: 
 
 LOAD GENERATION 10 ADD LOAD 1 
 TYPE 1 2.25 0 0 ZINC 3 
 
 Now, if you want to create a load combination with the generated load case 5, you can simply type the following command 
 
 LOAD COMB 15 COMBINATION LOAD CASE 15 
 1 1.0 2 1.0 3 1.0 4 1.0 5 1.0 
 
 The first integer number is the primary bload case number and the next one is the load factor. Here, the load factor is 1.0. I hope this helps.