I am designing a post-tensioned straddle bent. The superstructure has 6 girder lines. Is it possible to define a design truck using fixed axle spacing so it appears "stationary" to reflect the live load reactions from girders to a post-tensioned straddle bent? For example the bearing locations = axle fixed spacing from the left end are: G1-G2-G3-G4-G5-G6 = 63.5'-7.5'-7.5'-7.5'-7.5'-7.5'.
I did not understand question completely.
User can calculate the Critical LL reactions and apply those reactions manually on the bearings.
Then design the Straddle Bent , if the above situation is different then please do email me your issue and also related file to "Vinay.Mysore@Bentley.com".
For more information related to Leap Bridge Concrete program please check the wiki articles in the below mentioned Bentley Communities link.
https://communities.bentley.com/products/bridge_design___engineering/w/bridge_design_and_engineering__wiki/11174/leap-rcpier-substructure
The program assumes the straddle bent is a superstructure and applies LL as a design vehicle and runs it from end to end. In actuality the straddle bent supports 6 girder lines at discrete locations. The girder loads are applied as point loads for both DL & LL. If the LL is applied as part of a DL load combination the LL must be factored up and down to account for Impact, LL Load Factors and DL Load Factors.
If the girder spacing is 7'-6 and the first girder is 63 from one end, is there a way to define a design vehicle such that the axle spacing and axle loads match the layout of the girders? This way the program will perform the correct load factoring for the LL.
Also, does the program run the design vehicle completely off the superstructure? In other words the last axle is run to the end of the bridge or is the LL stopped when the first axle reaches the end of the bridge? I would like to space the first and last axles such that the truck was located at the bearing locations on the structure so it wasn't moved from end to end.
Thanks
Accurately determining live load effects on intermediate piers always represented an interesting problem. The live load case of loading producing the maximum girder reactions on the substructure varies from one girder to another and, therefore, the case of loading that maximizes live load effects at any section of the substructure also varies from one section to another. The equations used to determine the girder live load distribution produce the maximum possible live load distributed to a girder without consideration to the live load distributed concurrently to the surrounding girders. This is adequate for girder design but is not sufficient for substructure design. Determining the concurrent girder reactions requires a three-dimensional modeling of the structure. For typical structures, this will be cumbersome and the return, in terms of more accurate results, is not justifiable. In the past, different jurisdictions opted to incorporate some simplifications in the application of live loads to the substructure and these procedures, which are independent of the design specifications, are still applicable under the AASHTO-LRFD design specifications. The goal of these simplifications is to allow the substructure to be analyzed as a two-dimensional frame. One common procedure is as follows:
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