Lever Rule - pedestrian load distribution 1

[timmyo131]

I am new to structural bridge design. I have been tasked to design the replacement for an existing adjacent box beam bridge. I am using PennDot's PSLRFD design software. Within the "PLD - Pedestrian Live Load Command" tab, the program asks for the portion of the ped live load that is acting on the girder you are investigating. The program's user's manual and AASHTO/DM-4 state that the user must calculate this using the lever rule. I cannot find a good example of how the lever rule is applied. I know it has to do with assuming each of your girders is a support and then applying plastic hinges to the interior supports, but that's about as far as my knowledge goes on the subject. If anyone has a good explanation or a good reference for the use of the lever rule, I would greatly appreciate it.

My other concern is that my sidewalk isn't the typical sidewalk that sits on the actual bridge deck. My sidewalk is aluminum grating that hangs off the side of my bridge deck. The edge of the grating sits on the very outside edge of my barrier (I've attached a diagram). So I basically have a line load acting on the extreme edge of my deck. In reality, it seems like the fascia beam would take on this load in its entirety, but I obviously don't want to violate the code.

 

[bridgebuster]

The lever rule assumes there is a hinge in the deck at the first interior stringer. The loading on the fascia stringer assumes that the deck is simply supported between the fascia and the fist interior member. Look at PDF page 134 of the attachmment.

If I'm looking at your sketch correctly, the bridge consists of adjacent box beams. If that's the case, the lever rule doesn't apply. Half of the sidewalk load goes to the steel beam. On the box beam, you can apply the sidewalk to the edge beam, distribute it equally if you have a composite slab, or use AASHTO (Std Specs) Article 3.23.4.

 

[timmyo131]

Thank you for your response. You were correct, my bridge is an adjacent box beam structure. I am also designing it as a composite deck. So just to be clear, are you saying that because it's composite, that I should distribute my pedestrian live load equally to all 4 girders? This seems unrealistic to me because that line load is on the extreme edge of my deck. I can't see that load being distributed equally to the opposite fascia girder that is 16 feet away. I could understand maybe distributing it between two girders (the fascia girder and adjacent interior girder), but all four seems like you're assuming that deck to be more rigid than it probably is in real life.

Also, I can't find AASHTO article 3.23.4. The last article I see in chapter 3 "Loads and Load Factors" is 3.15 - Blast Loading.

 

[bridgebuster]

I was referring to the Standard Specs, not LRFD. Sorry for the confusion.

You can distribute the live load to one or two beams; no one will fault you for it. Put it into one beam and if it doesn't quite work try two. As my 9th grade algebra teacher Mr. Eller would say, "young man, make a decison."

On a composite structure, the specs permit the distribution of superimposed loads to all members. Is it realistic? Some ay yes; some say no. I remember years ago on some designs for Mass DPW their policy was to distribute 60% of the sidewalk and parapet loads to the fascia and 40% to the interior stringers. On a non-composite bridge, the fascia would pick up these loads based on tributary area.

Without looking into the pre-70's AASHTO/AASHO I don't know if the lever rule was actually required. However, when I started working back then all the old-timers used it.