As a key element in repairing a century-old bridge, a temporary detour bridge will be carrying traffic for the next year on the Mamalahoa Highway (Highway 19) on the Island of Hawaii, over the Umauma Stream. Acrow Bridge, a leading international bridge engineering and supply company, installed the 390-foot, three-span steel bridge being used as a bypass during repairs to the historically significant steel girder and trestle bridge. 

The existing steel girder bridge was built over 100 years ago to carry the railroad for the sugar cane industry.   When the rail line closed down, the Hawaii Department of Transportation (HDOT) repurposed the structure, by adding some steel girders to make a two-lane wide roadway bridge out of the old railroad frame. This roadway bridge has been used for more than 60 years until recent tests indicated a need for major repairs.  Traffic was limited to one lane for the safety of the public while they determined a proper repair plan. 

A Detour Adds Safety

The project was going to include repairing the girders and the deck on top using the common "phasing method," in which repairs proceed on half a bridge and traffic alternates on the half not being worked on.  However, lead contractor, Hawaiian Dredging Construction Company, convinced HDOT that an adjacent detour bridge would be a better and safer way to do these repairs.  The option can reduce inconvenience to users, as well as enhancing safety for construction workers by keeping traffic away from the work zone.  The use of detour bridges has grown significantly in recent years as contractors find they are able to stay ahead of schedule and lower construction costs when using a temporary bridge as a detour.

Acrow's Tom Pinder, Western U.S. and Heavy Haul Sales Manager, said the use of the temporary bridge detour was especially beneficial in this case.    

"Once the contractor was able to open up the old bridge deck and inspect the girders underneath it was decided that the entire bridge would need to be replaced as the old steel was just not sufficient to be used any longer," Pinder said. HDOT decided to replace, rather than repair those girders, something that "would have been near impossible to do if the traffic was on half the bridge as it might have been if phasing was used."

A Surprise Foundation

Although the modular design of temporary detour bridges makes installation quick and simple, preparation for the project is not always straightforward.

The right of way at the bottom of the valley where the Umauma Stream flows was over 100 feet deep and quite narrow.  Environmental requirements and property line limits further restricted the area available for footers.

Testing at the base brought a surprise.  "The base of the area is solid rock or so everyone thought," said Pinder. It turned out that the rock was a thin cover over "poor soils" meaning the dirt underneath the thin rock layer was not stable enough itself to support the bridge and towers.  Micro-piles were required to be put in to stabilize the ground.

"Additionally, the footprint allowed at the bottom, where the tower footers were placed, was very restrictive and the set distance between the towers for each side was eccentric, and not centered for the bridge support as they normally would be," Pinder said.  "The tower heights of 85 and 90 feet along with the seismic design requirements necessitated our working with KSF Engineering in Honolulu on special footers and stabilization guy wiring."   The reinforced tower design was detailed out using finite element analysis, a sophisticated form of computer modeling.

Installing the Bridge

Once the route was done and the towers were ready, the bridge was rolled out across the 390-foot gap and then lowered on to the abutments and towers. Total time for all of this was four weeks including some delays due to torrential rains that slowed the work.

The typical crew size of four, with one foreman, and a small crane with operator can install 18 to 22 tons of bridge frame in one shift per day.  In this case, the Acrow crew used two large Manitowoc cranes (a 888 model and a 12000 model) the contractor had on site for the repair project.

"The rest of the tools needed to assemble an Acrow are simple hand tools, wrenches and pry-bars," said Pinder.  "The Acrow Bridge uses super heavy duty forged steel pins that connect the structure together. There are no "˜slip critical' bolts the bolts only hold on bracing, they do not hold up the structure, the pins and truss panels do that." 

Once the bridge is set in place, Pinder explained, the weight of the bridge sets these pinned connections very tightly.  To be certain that they don't move during construction a ring clip is used on each side to keep them in place. Once the structure is up, it is ready for traffic.  Pinder said, "The bridge decking used has an epoxy-aggregate non-skid coating so it was ready to drive on immediately after the final inspection."

Bill Killeen, President and CEO of Acrow Bridge, added, "As the Mamalahoa Highway installation demonstrates, Acrow modular detour bridges are durable, easy to assemble, disassemble, transport, store and customize. As "˜temporary' bridges that can be rented or purchased, they are a logical choice for state DOTs, and contractors use them to stay on or ahead of schedule and control costs, while providing a safe and dependable route for local residents and area businesses."

Today, the replacement work is ongoing for the new Umauma Stream Bridge, which will equip the Highway 19 roadway for traffic increases for the next 75 years or more. For esthetic and historic reasons, the trestle work that held up the old bridge is going to be left in place and refurbished underneath the new bridge. It won't hold up the new bridge but will be there to speak to the history of the route and of the Sugar Cane Railroad that was once there. "From a bridge guy's perspective, that old trestle is a thing of engineering beauty," said Pinder. 

When reconstruction of the permanent bridge is completed, the Acrow temporary bridge will be removed as it was installed, by a small work crew, the components will be removed by container truck to storage to be ready for use in future projects.