ENGINEERING INFORMATION

BULLETIN

April 1999

PORTABLE STEEL BRIDGES

By: Allan R. Vanderpoel,PE

Almost ten years ago, E & M Engineers began a working relationship with A.D.M. Welding & Fabrication. A.D.M. started fabricating portable steel bridges at that time, and used our services as structural engineers to design the beam sizes, and wood deck details for a wide variety of bridges.

The bridges are unique in that they can be transported by truck to a site, and set upon temporary abutments to create a bridge in a matter of hours. The bridges are designed with a series of hinges to allow them to fold to a width so they can fit on a truck bed for transport. A typical bridge has six steel beams, with three of the beams and deck assemble being fixed, and the opposite three beams and deck assembly rotated with the help of the hinges to a position directly on top of the first three beams. The railing is attached to both outside sections, so upon delivery of the bridge to the site, and "opening" the bridge to its full width, it is ready for use.

E & M Engineers has worked with A.D.M. to come up with beam sizes for bridges from 20 feet to 70 feet long, with load capacities from 15 tons to standard HS25 highway loads. The individual sizing of beams has helped keep costs to a minimum for each bridge, and the shop fabrication has insured the highest quality of work. A typical bridge is 13 feet wide, adequate for one lane of traffic. In some cases, two bridges have been put in place side by side to allow two lanes of traffic.

To date, one-hundred and eleven of the portable bridges have been constructed by A.D.M. in less than ten years time. That figure surely makes A.D.M. one of the largest bridge building companies in this part of Pennsylvania and New York. We have specified their use for temporary detours around the construction of replacement bridges, and this cuts the time required for construction of a temporary structure as compared to doing all the work on site. E & M has worked with A.D.M. for specific requirements on bridges to meet the owners requirements, including meeting Pennsylvania State Forest and PADOT standards.

An example of the use of these bridges occurred after a heavy rainstorm in July of 1992. The storm washed out the abutment of the Mill Street Bridge in Youngsville, PA and caused the collapse of the structure. The only access remaining for many residents on the far side of this bridge was a narrow gravel road which curved for several miles over a hill. Youngville Borough contacted ADM almost immediately after it was learned that the Mill Street Bridge was washed out, and ADM delivered a bridge to the site the next day. Preparation was made for concrete blocks to be used as temporary abutments (which were dug into the banks), and after an emergency permit was obtained from Pennsylvania DEP, the portable bridge was unfolded, and placed across the creek within two days of the washout. The portable bridge remained on site until a new concrete box culvert could be designed and constructed, and performed without flaw throughout the project.

A major bridge replacement project in Allegany State Park in 1995 involved the replacement of the Highway 2 bridge just south of the Red House Lake area. In order to keep access to the sites on each side of this bridge, without a 30 mile detour, a temporary bridge adjacent to the main bridge was specified. Two ADM portable bridges were utilized for this temporary structure, and thus two way traffic was maintained throughout the construction project. An advantage of this type of temporary structure over a series of culverts placed in the creek is that a much larger waterway opening is maintained, with almost no road or culvert maintenance required. A previous temporary creek crossing in the park consisting of culverts placed side by side was washed out during one of the typical high flows experienced with these creeks.