AASHTO Edge Beam

[RichBridge]

In the 17th Edition of AASHTO for Slab Edge Beam Design (3.24.8) a live load moment is given of 0.10PS where P=16k for HS20 and S is the Span Length.

Is this Live Load Moment to REPLACE the "normal" computed live load moment? Should impact be added to this Moment? And I assume it is still subject to the 1.3 & 1.67 "normal" load factors?

I have been conservative and have always just added this moment (sans impact, but with load factors) to the reinforcing steel required at the edges of slabs. So in effect I have been designing the edges of slabs for two live loads. I think the wording in 3.24.8.1 of "additionally reinforced" led me to add the live load moment instead of replace.

Thanks for any comments.

p.s. This was originally posted in the AASHTO Code Issues forum, but didn't get many replies. Sorry for the double-post.
Further investigation also found that for simple spans, 3.24.3.2 gives the LLM as 900S/ft lb-ft or 0.90S/ft k-ft (up to 50 feet). From Above if use P=16k for the Edge Beam, the Moment is 1.6S. If use a 2-foot wide Edge Beam, the LLM per foot is 0.80S which is less than the "normal" live load and will never control and no additional reinforcing will be needed. I realize that continuous spans, other loads than HS20, and using a narrower edge beam will change this, but it seems odd the edge beam moment will never be higher than the normal LL moment for a simple span using AASHTO estimations....which again would lead to adding the edge beam moment instead of replacing.

 

[wiktor]

Yes, this moment is a design live load moment for the edge beam, computed using empirical formula.
It is not replacing the "computed" live load moment, and should not be added or anyhow combined with the calculated live load moments.
It is clear that the code do not require amplification w/ the impact factor - the verbiage used is very specific. The edge beam may be a typical slab section additionally reinforced, deeper section, or combined with the curb or dripping edge.
If you have "computed moments" for the edge beam, based on the more rigorous analyses, these could be used for the design, with the impact factors, resulting in some questionable savings and a lot of questions from the reviewers and supervisors. (Why not just stick to AASHTO? – even posting such a question suggest that designer is not fully understanding the concept and idea of the code)
However, if the moments computed by the empirical method were larger (which is highly unlikely), it would be prudent to use these for the design.