Trends Summer 2017 | Page 16

Setting the Standards

A yres Associates has been actively involved with developing and advancing the guidance documents the Federal Highway Administration produces associated with scour prediction . Three documents in particular – Hydraulic Engineering Circulars ( HEC ) No . 18 , 20 , and 23 – establish the state of engineering practice related to scour , stream stability , and countermeasure design . With Ayres senior hydraulic engineer Pete Lagasse leading the development effort , these documents outline the regulatory requirements for how state Departments of Transportation handle bridge scour inspection and observations inspectors noted in the field . Ayres has been instrumental in developing and updating every revision of HEC-18 , HEC-20 , and HEC-23 since they were issued ; HEC-20 and HEC-23 are in their third edition , while HEC-18 is in its fifth edition . Ayres also developed reference manual HDS 7 , “ Hydraulic Design of Safe Bridges ,” which provides guidance on the hydraulic analysis and design of bridges and is in its first edition .

The team ’ s efforts have helped FDOT ’ s Unknown Foundation Bridge Scour Program become a nationwide model .
“ Florida is one of the states leading the effort in bridge scour evaluation ,” Sunna said . “ FDOT ’ s Central Office produced a procedural manual for evaluating bridges with unknown foundations , looking at some of the things that had been done . We then put together a team of national experts in all facets and methodically went through all of the bridges , going through all the steps of evaluation .”
Team members did “ a lot of engineering ” to evaluate bridges that were Scour hole at a bridge piling outside the normal parameters of the procedural manual , Sunna said . This included pioneering an innovative procedure known as the “ static / back-calculation ” method to examine bridges that could have otherwise failed due to lack of proper evaluation techniques .
“ Knowing the structural and geotechnical requirements at the time a bridge was designed , we try through finite element analysis to ‘ reverse engineer ’ a minimum embedment that would have satisfied the criteria at the time the bridge was built ,” Sunna explained .
What factors would they have considered when driving the piles ? What were the design equations at the time ? What loads was the bridge likely carrying then ? All of this helps them arrive at what a reasonable minimum foundation embedment should be for their analysis , which offers peace of mind to transportation managers and enhances safety for the traveling public .
The methods the team developed and published have been presented at national workshops on foundation reuse and are being touted nationally .
“ Other states are starting to follow suit ,” Sunna said . “ If you look at the scour evaluation of bridges nationally , you will find that most of the methods and procedures were developed by Ayres on the hydraulic and structural sides and our teaming partner GCI on the geotechnical side .”
Exposed bottom of footing due to scour