Subject: INFORMATION: W-Beam Guardrail Installations in Rock and in 

Mowing Strips 

 /Original signed by/ 

From: John R. Baxter, P.E. 

 Director, Office of Safety Design 

 Office of Safety 

 

To: Safety Field 

Federal Lands Highway Division Engineers 

Strong-post (both wood and steel posts) w-beam guardrail is designed to absorb some crash 

energy through post rotation in the soil prior to post failure. Restraining these posts by 

setting them in narrow holes drilled into solid rock, by setting them in concrete, or by 

placing a “mowing strip” around the posts can lead to early post failure, placing more load 

on the rail element itself and possibly leading to rail rupture and subsequent penetration by 

an impacting vehicle. Such behavior has been modeled using finite element analysis and 

verified in a limited number of full-scale tests. Two papers were presented this year at the 

TRB meeting in Washington, D.C. that addressed concerns related to w-beam guardrail 

performance when its support posts were restrained from deflecting upon impact. 

The first of these was the MwRSF report entitled “Development of Guidelines for 

Placement of Guardrail Posts in Rock” by Rohde and Herr. The authors’ final 

recommendations for guardrail placement in rock are summarized in Enclosure 1. Simply 

stated, posts in solid rock should be set near the roadside edge of 530-mm to 580-mm 

diameter shafts drilled 610-mm (24-inches) deep, and backfilled with a compressible 

material (ASTM C33 coarse aggregate, size no. 57) so the post can rotate back 

approximately 380 mm (15 inches) at the ground line upon impact. For locations where the 

solid rock is below the surface, the size and depth of the drilled shaft will vary as noted as 

the enclosure, depending on the depth of soil above the solid rock. 

The second paper was the TTI report entitled “Evaluation of Guardrail Systems 

Performance When Encased in Pavement Mowing Strips” by Seckinger, Abu-Odeh, Bligh, 

and Roschke. The authors’ summary recommendations for the design of mowing strips 

around w-beam barrier are shown in Enclosure 2. As noted, a minimum “leave-out” area in 

the mow strip that will allow at least 180-mm (7 inches) of post deflection at the ground 

line is recommended. This area is then backfilled with a low-strength concrete mix. 

 

Memorandum

Date: March 10, 2004 

In Reply Refer To: HSA-10/B64-B
2

Concrete with a 28-day compressive strength of .85 Mpa (120 psi) was used in the crash 

tests, but any suitable backfill material having equal or lesser compressive strength may be 

used. 

The AASHTO Task Force for Roadside Safety will review these recommendations and is 

likely to incorporate them in whole or in part into the next edition of the Roadside Design 

Guide. A secondary benefit of the recommended installation practices is increased ease of 

removal and replacement of any posts damaged in a crash. State DOT’s should review 

their standard plans and specifications for strong post w-beam in light of these research 

findings and consider revisions where deemed appropriate. Electronic copies of both 

reports are included on the 2004 TRB CD that was given to all registered attendees. 

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