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Two weeks ago we ran an article on UPR's ideal compressive strength. This prompted Brett Bruton of Utility Service Agency in North Carolina to ask for clarification: "The logic of matching UPR to wood’s strength makes a lot of sense. You don’t mention what the compressive strength of UPR is. I think that I heard Wade say that the compressive strength of UPR is 1500 psi. Is that correct? If my memory serves me well, the bending strength of pine (at new) is 8000 psi, and the compressive and tensile strength of pine is about 12,000 psi (at new). So if I heard Wade right about UPR being at 1500 psi, it is very much lower than the strength of wood. This would contradict what the Friday Fax just said. Please clarify this for me." Sheri Dahlke, Technical Director for American Polywater, supplied the answer: "I've attached our Technical Data Sheet for the UPR Pole Repair. The compression strength is listed on the front page. Matching the compression strength of wood is a bit tricky since there are actually two values: with the grain and perpendicular to the grain. Most--not all--utility poles are made from Southern Yellow Pine. The compressive strength for pine is typically around 5,000 - 6,000 psi with the grain and 600-900 psi perpendicular to the grain. So the strength of the grain depends on the angle of the stress. 'Hankinson's Formula' shows that the overall strength is closer to the perpendicular measurement-- from 20 to the 90 degrees (perpendicular). See the graph in this link: http://worldwideflood.org/ark/design_calculations/wood_strength.htm. Overall, it is better to match the wood strength perpendicular to the grain. This eliminates any hard, bend point in the case of wind shear. In the specific case of increased weight from ice build-up, there may be some strength loss. However, there is another benefit to matching the perpendicular wood strength: gaffability. Many utilities climb rather than using a bucket lift. The UPR repair is gaffable--the gaff will penetrate UPR repairs very similarly to wood. The gaff will cut into the UPR repair and hold with minimal slippage or plowing. Our laboratory is working on compression studies this year. It's actually a pretty complex subject and we are still learning more. Regards, Sheri Dahlke, Technical Director" This complexity on wood grains and compressive strength is an opportunity for Polywater to differentiate itself with technical knowledge and to market premium products supported by documented performance advantages. Thanks for the excellent question, Brett. |
