Problem Statement No. 2012-AK/38
|TTI Research Supervisor:
William F. Williams, P.E.
Texas A&M Transportation Institute
Texas A&M University System
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Jeff C. Jeffers, P.E.
Statewide Traffic & Safety Engineering
Alaska Department of Transportation and
3132 Channel Drive
P.O. Box 112500
Juneau, AK 99811-2500
Many locations in Alaska are remote and without highway access. To minimize shipping weight, timber decked bridges are often used. The Alaska DOT&PF policy for temporary timber bridge decks requires portable concrete barrier (PCB) as bridge rail meeting TL-3. Transporting PCBs to villages and communities off the road system can be expensive and another bridge rail solution is sought for temporary construction on low-volume roads that meets crashworthiness standards. The current specification requirements for temporary bridges in Alaska are provided as follows.
Timber deck bridges are sometimes installed as temporary structures until a permanent structure can be built. Because of seasonal working conditions, construction may require two construction seasons. A light-weight cost-effective alternative to transport of PCBs is sought.
W-beam and thrie beam bridge rail have been crash-tested, but neither system has been approved for use on a timber deck bridge structure. However, the recent Roadside Safety Pooled Fund TPF-5(114) project Crash Testing and Evaluation of W-Beam Guardrail on Box Culvert have provided good results for using a W-beam bridge railing system anchored to a rigid concrete culvert system with shallow soil cover.
Several bridge rail options were considered for this project. Many of these options considered strong posts with flexible and stiff railing systems. The option of mounting W6x9 steel posts on base plates through-bolted to solid timber plank deck could be shown to be equivalent to the W-beam on box culvert with some failure or yielding mechanism for the post. For this project, a top of rail height of 31 inches was selected since this height has been successfully crash tested for MASH Test Level 3. Typical details of a W-beam bridge rail design with posts bolted to a 12-inch thick timber deck is shown in Figure 1. We understand that these details were previously proposed by a contractor for a timber bridge in Alaska. Considering this proposed bridge rail design, a crashworthy transition would be required if the details shown in Figure 1 are acceptable with respect to MASH TL-3 specifications. This transition would be required to transition this stiff post design to a flexible w-beam guardrail system off the bridge.
Figure 1 – W-Beam Bridge Rail for Temporary Timber Deck Bridge Installations
The purpose of the study was to perform engineering strength calculations and develop details of a W-beam guardrail system that utilizes steel posts with steel base plates bolted through a nominal 6-inch thick timber deck. We understand that the deck will have timbers laminated in the transverse direction and the design will be used in Test Level 3 applications. However, due to the 6-inch deck thickness and the condition of the deck system, the design may be limited to TL-2 applications. However, 8-inch deck thickness or greater will be considered if the 6-inch thick deck is not acceptable for TL-3. The deck systems are laminated bolt-through systems. We understand the availability of the 6-inch thick transverse laminated deck panels are more available in remote areas in Alaska. Details developed for this project are provided as follows:
Based on the engineering design and details provided herein, additional full-scale testing of this design is needed to determine if this design is acceptable for MASH TL-3. Full-scale component testing of the post connection to a 6-inch thick transverse laminated timber deck is needed to validate the post design. The condition of the timber deck could greatly impact the performance of this design with respect to the MASH specifications. It is recommended that full-scale component testing be performed in the next phase to determine the strength of the post connection to the timber deck. Similar details provided herein were successfully crash tested for MASH TL-3 for the TXDOT T631 Bridge Rail design (TXDOT Project 490024).