AASHTO LRFD Table A4-1 Live Load Significantly Low 1

[Tyler_M]

Hello there,

I am designing a concrete deck for a bridge. In the past I have used the AASHTO LRFD Table A4-1, which gives you a live load moment to design for based on your girder spacing and the distance from your CL of girder to the design section.

I have found that if you calculate the moment applied to your deck based on the AASHTO design truck (16k wheel loads, 6ft apart) using the AISC 3-23 tables, the load is 3-4 times greater than what table A4 suggests.

Can anyone explain this?

The only difference I can see is that AISC is looking at a simply supported beam while A4 is continuous, but can that alone be the difference?

Let me know if there is any more information I can provide.

 

[BridgeNerdGuy]

If you just solve a continuous beam loaded with the Design Truck and compare moments to the table you will notice the table values are much lower. Why? You need to determine how much of the deck resists this load, Your value needs to be divided by the strip width in 4.6.2.1.

Table A4-1 reports values on a unit or per foot basis. It uses the equivalent strip widths as described in 4.6.2.1 to distribute the Total Live load effect in the longitudinal direction. I am unsure which AISC case you are comparing too but the AASHTO table is for a minimum of 3 girder lines (2 spans) and the Table values include dynamic load analysis and multiple presence.

The page right before the table lists out all the assumptions, analysis methods, and locations of critical sections used to generate the table

 

[BridgeSmith]

If that table bothers you, you'll really be horrified when you see section 9.7.2 - Empirical Design for decks. It requires less than half the reinforcing of the traditional deck design (Section 9.7.3) Table A4-1 is based on, if certain requirements are met (full composite, 4 ksi concrete strength, etc.). See the commentary (C9.7.2.1) for some explanation of why it works.