Allowable Compressive Design Stresses for Pressure-Treated Round Timber Foundation Piling-PDF

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The paper argues that existing allowable compressive design stresses for pressure-treated round timber foundation piles—about 1,200 psi for southern pine and 1,250 psi for Douglas-fir—are supported by research and decades of successful use. It also recommends improving the testing and design methods used to establish these values.<br /><br />Earlier pile-tip tests sometimes produced artificially low strengths because specimens were too slender and failed prematurely. University of Colorado tests found average crushing strengths of about 2,840 psi for southern pine and 2,860 psi for Douglas-fir. Tests also showed that soil confinement can increase pile strength, by roughly 20% on average in the reported trials.<br /><br />The author argues that pile design should not rely on lumber-based criteria, because piles are round products generally selected from dense, straight utility-pole stock. Instead, design methods should follow the utility-pole approach in ANSI 05.1. For unconfined compression tests, pile specimens should use a standardized length-to-diameter ratio near 2:1, with further research needed to refine the ratio and account for soil confinement.<br /><br />The proposed design basis is a 2,800 psi crushing strength for treated pile tips, reduced by a 0.90 variability factor and a 1/2 safety factor. This yields a single recommended allowable stress of 1,250 psi for southern pine and coastal Douglas-fir, apart from consideration of driving stresses. The paper notes potential additional strength from soil confinement, increasing strength toward the pile butt, group load sharing, and oversizing.<br /><br />The paper also calls for better quality control and certification of pile strength, more research into allowable driving stresses, and investigation of nondestructive testing. It emphasizes that treated timber piles have a long record of reliable service when properly installed and kept in suitable conditions.

Keywords

pressure-treated timber piles

compressive design stress

southern pine

Douglas-fir

pile-tip tests

crushing strength

soil confinement

utility-pole standards

ANSI 05.1

allowable driving stresses