Last November, an innovative piece of structural engineering equipment was introduced to the LBJ Express project: a beam placement system – or beam truss – tailored specifically for the setting of beams for the future westbound general purpose lanes under Marsh Lane.
LBJ Express construction and design managers opted for this truss system because it was not possible to use conventional cranes under the Marsh bridge for setting precast beams.
The existing Marsh bridge (as well as other crossing bridges along LBJ) created constructability challenges to the under-passing bridges due to vertical clearance and crane placement constraints.
Traditional cranes typically used along the project weigh hundreds of tons and require clear level paths and large set up areas. In contrast, this custom built truss system moves along the caps of the bridge, ideal for low clearances and takes out the need for massive crane paths or pads.
Built at 270 feet long (8 ft. tall and 6 ft. wide), the bright blue truss system included an electrical generator with wireless remote control, two electrical trucks to move the truss along tracks set on the caps, and two electric winches to lift and place the beams.
THE PLANNING PROCESS
Working with VSL International, a construction engineering company known for conceptualizing designs to fit unique and complex engineering situations, the LBJ Express team spent months laying out plans to implement the truss method, after exhausting every feasible alternative to set beams at this compact site.
Contract documents were studied to develop overall geometric and structural requirements. Additionally, the limitation of space for the equipment and operations were discussed with engineers, construction staff, equipment suppliers and fabricators.
“We had many different subcontractors and elements going on simultaneously, which on one hand helped motivate different disciplines along, but on the other, also needed to fall exactly into sequence in order for this entire operation to work,” said Mark Hutson, Project Manager for Trinity Infrastructure, LLC, the general contractor of the LBJ Express Project.
Using the truss system for the first time along the project came with several challenges that Hutson admitted, but proudly overcame as operations progressed. For instance, cap construction, which was initially needed prior to the erection of the truss, actually occurred partially during the truss was launched due to time constraints, as crews had four remaining caps to install at the time.
Other challenges included finding space for beam trucks to maneuver in the tight, activity-packed site, which the team ended up resolving by constructing a very level and complex haul road to allow for the trucks to back into specific areas under the bridge to lift and place the next morning. Trucks with steerable back trailers were also used in order to enable sharper turns and meander around the tight corridors.
HOW IT WORKS
“The system had to be modular to allow for it be transported to the project site in pieces, assembled and de-mobilized from the site, and transported to storage or the next beam erection site,” said Dan Dock, Vice President of Engineering for VSL International.
The truss system was conveniently built on top of existing bent caps, which provided ample support for the system. Electric trucks ran on dual rails, which were bolted to the top of the concrete caps, and were what launched the concrete beams from one span of the bridge, to the next. The winches lifted each beam from the hauling rigs, precisely placing each one to an opening in the beam ledge.
The truss enabled each beam to be carried transversely, moving across the bent cap to the final location of each beam.
“The geometry of some of the caps as well as the reinforcing steel was adapted to allow for this system, which required the team to set rails on top of each cap so that the truss was able to move transversely,” said Carlos Gomez, Construction Coordinator for Trinity Infrastructure.
Dock added that this aspect of the truss system, and the way the beams were hoisted horizontally via the beam ledge was a custom design specific to the project.
IMPROVING OVERALL CONDITIONS
Not only was the truss option economically ideal for the geographical and structural make up of the construction site at Marsh, but it was also a more friendly way to set beams, providing the least impact to the public.
If regular beam-setting methods were used at this location, the sequence would have involved several main lane and frontage road closures, not to mention parking oversized cranes in very compromising positions.
“This innovative beam setting system was a challenge for all the parties involved, and a lot of early planning was needed to make this operation successful,” said Gomez. “Finally, the schedule was met as planned without requiring any lane closures at all on I-635.”
The truss provided flexibility in constraint-ridden construction schedules, allowing to perform the beam-settings during the day, free of lane closures. With built-in walkways, the truss also enabled construction personnel to monitor the beam settings up close, in a safe and efficient manner.
“There was a lot of moving parts and managing in the field,” Hutson said. “We had some difficulties, but overall I am extremely proud of how this operation came through.”
Beam-settings via truss for the westbound general purpose lanes under Marsh began late November, and continued every other night as weather permitted, through mid-January.
With the success of the beam setting at Marsh, LBJ Express plans to use the truss elsewhere along the project, for general purpose lane construction around Welch Road, Preston Road and Montfort Drive.
Later this year, the truss will again be needed at Marsh, this time to assist in the beam placements for the eastbound general purpose lanes bridge.
Special thanks to Mark Hutson and Carlos Gomez from Trinity Infrastructure, LLC, and Dan Dock of VSL International, for helping write this post.