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W-beam and Thrie Beam Rail Splice Slot Dimensions

Description Text We have recently received feedback from various states noting a discrepancy between the dimensions of the guardrail splice slot dimensions for W-beam and Thrie beam. 

Trinity fabricates their guardrail hardware to the AASHTO M180 specification. In that spec, the rail splice bolt slot is dimensioned as 29/32" x 1 1/8" (23.0 mm x 28.6 mm). 

Several MwRSF details have these slots noted as either 1" x 1 1/4" while older hardware guide details note a 24 mm x 30 mm slot. 

Can MwRSF provide some feedback regarding the discrepancies in the slot dimensions? 
  • Guardrail
Other Keywords none
Date February 10, 2017
Attachment 2015 AASHTO M-180 7-8.pdf
Attachment Material Certifications (B&W) for TRP-03-244-10 (pages 89-99 only).pdf
Attachment MwRSF Bullnose Thrie Beam Rails Section Details (Test# USPBN-3).pdf
Attachment MwRSF Splice Slot Details in FHWA letters.pdf
Attachment MwRSF TF13 (1994) RWM04a .pdf

Response We have reviewed the information you supplied regarding the variation in the guardrail splice bolt slot dimensions. We believe that the discrepancy in the slot size is a numerical error on the plans and drawings we have supplied. 

Originally, the standard hardware drawing for thrie beam and W-beam were based on the AASHTO M180 spec and the rail slot dimensions were shown as 29/32" x 1 1/8" (23.0 mm x 28.6 mm). Over time, the industry made a temporary push to switch to metric units. At that time, the hardware guide details of the slot dimension changed slightly due to conversion and rounding to 24 mm x 30 mm. Eventually, the metric units were abandoned. At that time it appears that the metric 24 mm x 30 mm slot was again converted and rounded to yield a 1" x 1 1/4" in many MwRSF plans.

MwRSF believes that the slot dimensions should correctly follow the original M180 standard. Additionally, review of MwRSF mill certs and material specifications indicated that the W-beam and thrie beam rail systems designed and evaluated at MwRSF were supplied, fabricated, and tested with guardrail the met the M180 spec and thus had the 29/32" x 1 1/8" (23.0 mm x 28.6 mm) splice slot. 

Thus, the other slot dimensions found in other details are errors made due to unit conversion and rounding. This error is only on the CAD details as the tested systems met the M180 spec. MwRSF is working to correct their CAD details and the Hardware guide details that indicate slot dimensions inconsistent with the M180 spec. Additionally, MwRSF would recommend that manufacturers, installers, and DOTs continue to use the original M180 specification as it is consistent with the actual tested barrier systems. 
Date February 10, 2017

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(402) 472-0965
Email: mwrsf@unl.edu
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