Difference between Longitudinal and Transverse LL DF 2

[yjung]

Could anyone explain me what the difference between Distribution of Wheel loads in Longitudinal beams and transverse beams is?

I am using everyday but always confused.

For example,
In AASHTO 3.23.4.2 says "In calculating bending moments in multibeam precast concrete bridges, ,,,,, no longitudinal distribution of wheel load shall be assumed.".
In AASHTO 3.23.4.3 says The live load bending moment for each section shall be determined by EQ. 3-11....

I am confused.

What does "load fraction" mean? What does "DF" mean?

Confused...

Please help.

 

[CanyonRat]

Section 3.23.4 is only for bridges that utilize the top flange as the driving surface such as deck bulb-tees or PS Slabs.

Section 3.23.4.2 is saying that wheel loads on other girders don't effect the girders next to it.

Section 3.23.4.3 gives you the distribution factor (aka load fraction or wheel fraction) to be used for the girder design of these types of girders. The basic AASHTO truck has a 6' wheel spacing. If the top flange is less than this then really only one wheel line can fit on the girder. However, a DF is used to model other real world effects and to design the girder. These equations also take into account diaphragms. The DF using these equations should end up being less than the DF's in Section 3.23.2 which are the normal ones.

Shear keys and PT rods are used to try and 'tie' the girders together somewhat and the DF's help account for this.

It's all confusing but I think the LRFD code helps clarify a few things about this.

MikeD

 

[yjung]

Hello Mike,

Where did you learn all these DF and LL and so on?
Any books? or school? experience?

 

[CanyonRat]

12 years experience with bridges; Worked for a state dot bridge program for 3 yrs with more experienced bridge engineers who knew the AASHTO code inside and out; State DOT's are on the AASHTO Committee and therefore have resources and in depth knowledge of the code. There were also several bridge designers with plenty of examples to look at.

A good reference for PS Girders is the PCI Bridge Handbooks (2 Vols) available from their website.

A good reference for steel girders is the NSBA website for the Highway Structures Design Handbook Second Edition.

There are some good bridge design textbooks out there. Try ASCE or Amazon.com

You need to have a good understanding of the live load. The first bridge design I completed was done by hand and not using a computer...

MikeD

 

[yjung]

Thanks Mike,
I have almost 3 years experience( i have asked questions here since that time) and passed CA PE last October. However, I am confused many times with Live loads.
I think if i could understand Live load completely, i would be a good bridge engineer.

Thanks again Mike.

I wish you can answer me more late.

 

[koo]

You are not alone.

"Bridge Engineering" by Demetrios Tonias is a good introductory book and explains DF well.

AASHTO Guide Specification and NCHRP report No. 187 about wheel load distribution are also very helpful.