Longitudinal Edge Beam Design (LFD) 1

[Hunt1999]

I am trying to remember how to perform a longitudinal Edge Beam Design for a continuous RC slab using AASHTO 17th Edition. 3.24.8 provides guidance on load determination, but I can not find any guidance on what width to assume for the beam that this load is applied to... Anyone remember how to do this?

 

[wiktor]

The width of the beam will be determined by the practical width to place reinforcement to resist youur calculated moment M=0.10PS

 

[Hunt1999]

Thanks for the response, but I am still a little unclear on this one. How do I limit this width to a practical value. If I am trying to determine how much additional steel is required beyond what I have already provided, it would seem that I need to define a specific width of beam to check. Practically speaking, if my interior strip design requires #8 @ 6", how many of those #8's (in the coping) do I consider effective in resisting the 0.10PS Load? Once I know how much of my longitudinal steel is effective, then I can determine how many additional bars to provide if necessary for the edge beam. I hope my confusion makes sense...

 

[edward1]

I have been modelling total width of hundreds of RC slabs simple and continuous for about 15 years. In the beginning I used edge beam design and generally used about 1' to 2' widths. Now I just use STAADPRO and model the whole slab and put the centerline of the outside wheel line 18" from the bridge parapet or guardrail; the next wheel line 6 feet over; the next wheel line 4 feet over; and the next wheel line 6 feet over etc. You will get a maximum moment on the edge of the slab for which you can design without the SWAG method!

 

[wiktor]

The code requirement is to have an edge beam, capable of carrying prescribed moment.
Calculate the area of steel required (M/0.80dxfs), and evaluate required width to place it. I do not know what's your slab depth, but for typical bridges your beam shall be 1.5 - 2' wide.
This will be your width. Then check the steel and concrete stresses using normal formulas. You can combine your edge beam reinforcemment with your regular slab reinforcement.
Your edge beam shall have separate stirrups.

 

[Hunt1999]

Thanks for your help. Last question, I am assuming that this steel needs to be continuous for the bridge length in the bottom of the slab only. Is this correct? Thanks again.

 

[wiktor]

For simple supported slab it's bottom only, but in order to have a "beam" put two of this #? rebars at the top.