Code Update: SMDI Updates Guide Specification for Highway Bridge Fabrication
High-Performance Steels (HPS) have been used in bridge projects for more than two decades, helping builders reduce weight, ease construction and improve design flexibility. HPS made a rapid entry into the bridge industry, with the first HPS 70W bridge completed in December 1997, only three years after starting the cooperative research effort to develop the material. Today, it’s estimated there are more than 500 HPS bridges in service in the United States.
Although HPS use isn’t a new trend, ongoing research by the steel industry has led to new, stronger grades of steel entering the market, such as HPS 100W, which was developed to have improved mechanical properties in heat-affected zones and is primarily for use in bridge girders, piers and support structures. By constructing bridges with the highest-strength material where the girder is being loaded, designers can reduce weight for more-efficient transportation results and improved erection times. As more bridge owners and contractors require the use of higher-strength steels, it’s important for fabricators to understand the differences in capabilities and processes when using various HPS grades, including 50W, 70W and 100W.
New Recommendations in 2018 Guide
Although codes are updated periodically to account for new materials, it’s important for specifications to evolve between updates so vital tools and information are available for safe and effective fabrication. Offering new guidance for the implementation of HPS 100W, the Steel Market Development Institute (SMDI) recently published an updated version of its “Guide Specification for Highway Bridge Fabrication with HPS 100W (HPS 690W) Steel for Non-Fracture Critical Applications.” This 20-page guide provides owners, designers and fabricators the latest recommended methodology to fabricate and weld structures using ASTM A709 or AASHTO M270, Grade HPS 100W (HPS 690W) steel. The 2018 edition supersedes the original guide published in 2012.
SMDI’s original guide for HPS 100W use in bridge construction was based on testing performed by LeTourneau University, the Naval Surface Warfare Center and manufacturers of welding consumables. The HPS 70W Fab Guide was used as a template to publish the recommendations for HPS 100W.
After six years of application in the marketplace, updates were needed based on industry feedback, experience and increased use of HPS 100W steel in bridges. The updated guide includes recommended procedures to achieve high-quality welds using HPS 100W steel; more-specific direction on the properties, grades and uses of weathering steels and applications of corrosion resistance in bridge design; and specific considerations for designers relating to the properties of HPS 100W.
When welding high-strength steels, fabricators must follow specific guidelines. Although HPS 70W allows for conventional heating practices, HPS 100W requires some higher preheats, regardless which type of diffusible hydrogen is used. The SMDI guide clearly explains a conservative approach to welding high-strength steels, clarifying maximum heats and preheating practices to ensure consistent and safe performance of the completed project. This guidance is based on continuing research conducted by SMDI, the Federal Highway Administration and the U.S. Navy.
The guide’s updated welding section also explains the techniques required for hybrid connections between HPS 100W and 70W or 50W steels. It includes clarification of maximum heat inputs along with other criteria for connecting different grades, making implementation of the highest-strength steels more time-efficient and cost-effective.
The SMDI guide also helps builders and designers account for differences in properties between HPS 70W and 100W steels. Although higher-strength steel allows for greater design flexibility, engineers also must account for changes in properties such as deflection criteria.
As state and federal codes are updated in coming years, SMDI expects recommendations and guidance in this version of the guide to be included in the AASHTO/AWS D1.5 Bridge Welding Code as well as state specifications for bridge fabrication. This information also is currently included in training sessions and seminars conducted at federal and local levels to help fabricators successfully upgrade or replace deteriorating bridges with HPS 100W steel.
“Guide Specification for Highway Bridge Fabrication with HPS 100W (HPS 690W) Steel for Non-Fracture Critical Applications, Second Edition,” is available for free download at