This past Wednesday afternoon, I got the opportunity to go on a bridge tour of a few unique old-style bridges built in the town of Easton. It was organized by the ASCE (American Society of Civil Engineers) student chapter here at Lehigh, which I am the vice president of. The tour was given by Professor Yen, an Emeritus Professor in the civil engineering department at Lehigh, and Greg Kuklinsky, an alum of Lehigh very active in the Lehigh Valley and National ASCE chapters. It was also open to students from Lafayette College in Easton, just 20 minutes from Bethlehem.
When we arrived at the first bridge, we all put our hard hats on and walked over, underneath the bridge seen at the bottom of this post. There was major reconstructive work being done on the bridge to strengthen and widen it so that it can support heavy truck traffic on Route 22 crossing the Delaware River between Pennsylvania and New Jersey. Unfortunately for us the bottom section was covered in sheeting, probably for sandblasting and repainting.
The next bridge, called the Free Bridge, was just a quarter mile downstream. It was built in the late 1800’s and though it has gone through many redesigns, it still has just a 3 ton limit. In fact, there was a booth on the New Jersey side that would watch for and stop any truck that would try to cross over it. In the side view picture up top you can actually see the center of the bridge sagging, making the deck up top not level. The bridge was designed with two towers that contain the only transverse supporting members. The deck is simply steel grating, which is much more lightweight than concrete. However, it was pointed out that there are strips of concrete on the section of road above the supports to keep water from draining and eroding the piers underneath.
The diagonal members in both bridges are much smaller than all of the other ones because they are always in tension. When a member is in compression, it is subject to buckling which can either be reduced by shortening the length of the member or increasing it’s cross-sectional area. In the first bridge we saw, below, the top beams are extremely thick and relatively short because they are under so much compression. The other bridge however has just the towers that are in compression. The vertical members in both bridges are mostly in tension and so they can be made to be more lightweight. You can see that they are all hollow sections. In the Free Bridge, all of the steel connections are rivets.