A prominent pedestrian bridge recently was closed in Boston. The bridge was closed due deteriorated structural members. The old structure served most of its life carrying vehicular traffic. So the wear and tear was caused by age and vehicular loading over time, not by pedestrian loading.
For distressed highway and rail bridges, instead of a full bridge closure, live loads may be limited to extend the structure’s usable life. The bridge can be rated and then posted for reduced loads. Many vehicular bridges are aging past their design lives, and they have supported greater volumes and heavier loads than originally anticipated. The option of posting a vehicular bridge buys some time before repairs can be made. But a similar approach is not available for pedestrian bridges.
But what if it was? Pedestrian bridges are subject to the same trends as vehicular bridges. Many have been around for decades and have exceeded their design life. Just as vehicular loads have increased, pedestrian loads have also increased substantially over time. These loads are perhaps greater than those originally expected in design.
If a pedestrian bridge could be posted with weight limitations, it might be possible to extend its usable life. The structure would need to be analyzed for specific pedestrian loads. If it was necessary to post the bridge, appropriate signs would be displayed at the abutments warning potential users of access limitations. Probably one graphic would be enough. But maybe a graphic and posting could be provided not only for pedestrians, but for pedestrians on bicycles. For bikers, load restrictions would tend to favor higher end carbon composite road bikes which are lighter than steel framed bicycles .
Similar to truck postings for different types of vehicles, it may be desirable to post for different body types. Procedures could be developed to account for mesomorphs, endomorphs and actomorphs, with appropriate calculations and methods for rating.
Probably it would also be necessary to account for pets – let’s assume that cats get a free ride along with small poodles. But larger dogs may not be permitted.
Enforcement would be tricky but not impossible. On highways, trucks must stop at weigh stations which determine if the vehicles are overweight.
Similarly, pedestrian weigh stations could be provided to measure overloads and help promote longer bridge life. For pedestrians that are found to be not in accordance with criteria, warning lights could flash and sirens could sound. Individuals not in compliance could then be referred to supplemental programs to assist in, say, snack reduction and exercise. This approach would mesh well with the overall trend in American health care, with its increased emphasis on preventative medicine. In this way, posting and load restrictions could lead to lower health care costs and could result in longer lives for the pedestrians as well as the bridges.