Bending tests of bridge deck planks

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Abstract

This paper presents results of timber plank tests, for planks used in highway bridge decks. Tests were carried out to determine the flat-wise use factor, which is represented by the ratio of modulus of rupture (MOR) for flat-wise loading to MOR for edge-wise loading. Four sizes were tested. The tests confirmed that MOR for flat-wise loading is considerably larger than for edge-wise loading for the larger planks tested and that flat-wise use factor increases for larger plank width. The flat-wise use factor in the current bridge design code is overly conservative for typical plank sizes. New flat-wise use factors are recommended for the design of wood plank decks for highway bridges. The recommended values vary from 1.10 to 1.6.

Introduction

In a recent study by Nowak et al. [1] dealing with the design criteria for plank decks for highway bridges, it was found that there is no in-grade test data available for flat-wise bending. The flat-wise use factor is defined as the ratio of the modulus of rupture (MOR) for flat-wise load application to MOR for edge-wise load application. The need for rational design criteria for plank decks was identified by the AASHTO Subcommittee on Bridges, Committee on Timber Bridges, as a priority item requiring an urgent solution. The current flat-wise use factor in AASHTO [2] seemed to be overly conservative for typical plank sizes. Therefore, tests were carried out at the University of Michigan to verify the flat-wise use factor for bridge design codes [2], [3]. The objective of this paper is to present the testing procedure and test results.

A typical plank deck consists of planks laid flat-wise and placed on supporting stringers, as shown in Fig. 1. There are two categories of plank decks depending on the direction of planks vs. direction of traffic: transverse deck and longitudinal deck. A typical transverse plank deck is shown in Fig. 2. The span length of stringers is usually 5–6 m, in older structures it can be up to 11 m. Stringers can be spaced at 300–600 mm (center-to-center), but mostly 300–450 mm. Stringers are made of sawn lumber, typical Southern Pine size is 150×450 mm, or Douglas-Fir (larger size). The planks are typically 100×250 mm or 100×300 mm, and their length can be 3.5–11 m. Planks are nailed to stringers. A longitudinal plank deck is shown in Fig. 3 and is similar to transverse plank decks in terms of plank and stringer size and stringer spacing. The major design parameter determined by the designer is the spacing between stringers.

The design provisions for plank decks in AASHTO [2] and AASHTO LRFD Code [3] include specified live load (wheel load), load distribution, and material strength. The latter is specified as allowable stress in [2] and base resistance in [3]. For typical bridge planks, [2] specifies a flat-wise use factor of 1.10. The LRFD Code [3] does not specify any flat-wise use factor.

Section snippets

Description of tests

The tests were carried out on the most common species used for plank decks in Michigan, Red Pine. Four sizes were ordered, two of them are typical for bridge planks: 100×250 mm and 100×300 mm, and the other two sizes have good potential for future applications: 100×150 mm and 100×200 mm. Material was supplied by Woodstock, Inc., from West Branch, MI, USA. The planks were pressure in-kiln treated to retention of 10 kg/m3 with CCA Type C (Chromated Copper Arsenate). A special request was made to

Test results

For each specimen, the ultimate force, P, was determined as the force at failure. The statistical parameters of P, in particular the mean and coefficient of variation, are given in Table 3. The corresponding bending moment, M, and modulus of rupture, MOR, were calculated using the following formulas:MOR=MS

whereM=P2L3

for edge-wise tests,M=PL4

for flat-wise tests,S=bd26\\L=test span equal to 2134 mm for edge-wise tests, and 914 mm for flat-wise tests;b, d=dimensions (width and thickness) of the

Proposed flat-wise use factors

The flat-wise use factors were calculated as the ratio of MOR for flat-wise load to MOR for edge-wise load. The calculations were carried out for MOR corresponding to the lower 15th, 10th and 5th percentile. Actual dimensions and dressed sizes are considered, with results listed in Table 4. The recommended values of flat-wise use factors are also presented in Table 4. The recommended flat-wise use factor is noted to increase with plank width.

Conclusions

The wood plank tests performed at the University of Michigan confirmed that the modulus of rupture (MOR) for flat-wise loading is considerably larger than for edge-wise loading for the larger plank sizes tested. Flat-wise use factor increases for larger plank width. The flat-wise use factor in the current bridge design codes is overly conservative, for typical bridge plank sizes.

New flat-wise use factors are recommended for the design of wood plank decks for highway bridges. The recommended

Acknowledgements

The presented study was sponsored by Forest Products Laboratory, USDA Forest Service, and the University of Michigan. The tests were performed in the Structural Laboratory of the Department of Civil and Environmental Engineering, at the University of Michigan, and carried out in cooperation with Forest Products Laboratory, USDA Forest Service. Thanks are due to students and staff of the University of Michigan for their help, in particular Research Assistants: Sangjin Kim, Chan Hee Park, Kirk

References (4)

  • Nowak AS, Kim S-J, Saraf V, Ritter MA. Load distribution for plank decks. Report submitted to the USDA Forest Service....
  • Standard Specifications for Highway Bridges. Washington, DC: American Association of State Highway Transportation...
There are more references available in the full text version of this article.

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