Flexural Performance of Nail-Laminated Timber Crane Mats

Abstract

A crane mat is a temporary platform used to stabilize heavy equipment and vehicles, while allowing for passage over various ground conditions. Crane mats are typically used as temporary roadways for pipeline installation, oil mining, electric power transmission, and general construction. Traditional designs range from solid timber mats held together with steel bolts to plastic and rubber composite mats. Nail-laminated timber (NLT) is an economically available alternative type of crane mat design. Nail-laminated timber structural elements are created by placing several pieces of dimensional lumber on the thinner edge and fastening the individual laminations together with nails or screws along the wide face of the lumber. Although NLT is an age-old timber construction method, its benefits are being rediscovered as wood technology continues to progress. This research was conducted in collaboration with Savanna Pallets, Inc. and the University of Minnesota Duluth Natural Resources Research Institute (NRRI) as part of a larger project. Specifically, this research had three specific goals. The first was to evaluate if NLT can be effectively manufactured using underutilized, small diameter, local wood species in collaboration with Savanna Pallets, Inc. Each specimen was fabricated with various fastener types, fastener spacing patterns, joint locations, and moisture contents. Through a series of design phases, it was determined that NLT can successfully be manufactured by fastening low grade lumber with either nails, screws or scrails. The second goal was to experimentally determine the NLT crane mat flexural performance and compare the results to current design recommendations for structural applications. Each specimen was tested to determine the maximum applied load, corresponding deflection, and modulus of elasticity (MOE) when subjected to static bending. The Nail-laminated Timber Design Guide (2017) and the International Building Code (2018) were used for theoretical predictions because there are currently no standards or specifications for crane mat design using NLT. The theoretical predictions for flexural strength, deflection at the maximum applied load, and MOE were typically conservative when compared to the experimentally measured values. The third goal was to determine the specific lumber and fastener configuration that produced a finished product which was economical for Savanna Pallets, Inc. The experimental test results suggested that a random distribution of butt joints in individual laminations of lumber yielded a greater flexural strength than butt joints placed in an organized, symmetric fashion. Furthermore, NLT specimens fastener together with multi-purpose framing screws exhibited the highest flexural strength and largest deflection at the maximum applied load out of the three types of fasteners. The experimentally measured MOE was similar between the three fastener types, but much lower for specimens constructed with a symmetric layup compared to specimens constructed with butt joints in a randomized pattern. Additional research with a larger sample size for each unique design is recommended to better understand the flexural behavior of NLT crane mats constructed with low grade lumber.