The Cosumnes River Bridge

Welcome to “Working Wonders With Wood”. My goal with this is twofold: 1) to utilize this space to spotlight recent projects that Western Wood Structures has been involved in, and 2) to make an attempt to keep people abreast of recent developments and changes in our industry.

Western Wood Structures is a sales and engineering company founded in 1969. Our product lines include timber bridges, heavy timber trusses, Arches, Glulam beams, and laminated decking with our primary focus on the commercial market. Many of the projects we sell are designed in house. We also fabricate some of the projects we sell in our shop as well as install some of them with our own field crews. Sometimes we act as a material supplier and sometimes as a sub-contractor depending on factors such as project location, current work load, and customer needs. It is not uncommon for a project to start out as conceptual and pass through every department here from sales to engineering to fabrication shop to our guys in the field after it becomes an order.

Western Wood Structures offers inspection services of existing timber buildings, primarily roof systems. We are often hired to make a site inspection, run an analysis, determine the cause of any problem, and make remedial recommendations. We can then design and implement structural repairs and code upgrades to timber beams and trusses which usually involve post tensioning, shear dowel reinforcement, or other strengthening methods. There will be more on this subject another time.

This first monthly entry will focus on one of our many outstanding timber bridges: The Cosumnes River Pedestrian Bridge outside of Rancho Murieta, California. This bridge measures 410’ in length with a center span of 200’ and side spans of 105’. It is 12’ wide and was constructed primarily for pedestrian traffic, golf carts, and bicycles but it is also designed to carry a 10,000 lb. maintenance vehicle. It was required by conditional use permit when property in the Rancho Murieta South housing development near Sacramento was developed for residential use.

Western Wood Structures worked under a General Contractor, Viking Construction of Rancho Cordova, CA. Quincy Engineers of Sacramento was responsible for the civil work as well as the design of the concrete abutments.

The main carrying members are Glulam Trusses. The main span features bowstring trusses while the outer spans are parallel chords. The top chords feature a reverse continuous curve to form a flowing appearance. The curved shape of the bridge was proposed by WWS to provide a deep section in the main span as well as a smooth transition to the outside spans. The reverse in curvature begins in the side span and the point of reversal takes place in the first top chord splice in the main span. A continual graceful curve from end to end was achieved and of course splices were introduced to allow for shipment and pressure treating. The curvature gives this massive bridge a sleek appearance. Glulam floor beams support the 2-1/2” thick Glulam deck panels to form the deck and the floor beams are located at panel points so the trusses are not point loaded.

The analysis determined the radii of the truss top chords and this information was transferred to our fabrication shop where they trusses were laid out, fabricated, and match-marked. All holes were drilled in the Glulam prior to pressure treating using the steel as templates which allows for a proper treating envelope around the entire member. This extra step will help to protect the member from insect or fungal attack. Western Wood Structures believes that a treated timber bridge will easily have a life span of 75 years if it is pressure treated, fabricated before treatment, and properly maintained.

Western Wood Structures crews assembled each truss on site including much of the deck prior to erection. Viking Construction was able to build a crane pad near the river to allow the crane to set up near the main span thus minimizing the reach of the main span which weighed almost 130,000 lbs. representing the heaviest lift Western Wood Structures has ever made. Each side span was just under half this weight. Jobsite assembly and erection took a 5 person crew six weeks to complete.

This magnificent treated timber bridge is not only functional and affordable but was also able to achieve all of the aesthetic requirements of the project. It provides the access needed for the residents while allowing greater utilization and appreciation of the community’s waterway.

We are very proud of the awards this bridge has won since its completion. It earned a Merit Award of Engineering Excellence from the Consulting Engineers and Land Surveyors of California [CELSOC] in 2008. And just last month it earned a Timber Bridge Design award from the California Region of Woodworks in the non-residential category.

I hope that you will check back monthly. We welcome your questions and comments.