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W-Beam Guardrail Near Steep Slopes w/ Wood Plank Soil Containment System

Question
State WI
Description Text We are developing plans for the mill and
resurfacing of STH 145 from STH 100 to STH 167 in Waukesha and Washington
Counties.


We have a question for you regarding the beam guard on the NB approach to
B-67-217. The beam guard in question is between Sta 42+12 to 45+12, Rt.
The existing beam guard has 10' posts tightly spaced and has some timber
planking along the inside face more or less serving as a short retaining
wall. The beam guard, posts and planking are in reasonably good condition.
Our original concept was to replace the beam guard and 10' posts and saw off
the old posts at the top of the planking. During the review process it was
questioned whether this beam guard would be considered crash worthy? We
would like your opinion as to the best way to resolve this issue.


I am attaching the following for your review:
PD01 - Plan/profile showing the proposed beam guard (Sta 38+50 to 45+00)


DT03 - Steel Plate Beam Guard Special detail
0321 - photo
Special Beam Guard Detail - from the as-built plans
Keywords
  • Guardrail
Other Keywords none
Date April 9, 2010


Response
Response

First, there is concern with the placement of a ground-mounted, wood rub-rail system under the guardrail that may cause an impacting vehicle to vault upward as a wheel contacts the timber member, thus increasing the propensity for a vehicle to override the guardrail or become unstable during redirection. This result would especially be of concern when the wood member vertically extends greater than 4 inches above the ground-line for standard 27" tall guardrail systems.

 

The use of 6"x8" by 10' long wood posts in the noted situations may also result in premature post fracture and reduced energy dissipation when the drop in back slope in minimized (i.e., embedment maximized). However, the use of a 1/2-post spacing could garner back some of the reduced capacity if premature post fracture occurs. For cases where the soil drop is maximized, the soil may yield and allow post rotation prior to reaching a wood post fracture condition. To reduce concerns for wood post fracture in these special situations, it may be preferred to utilize long steel posts which would remain intact and dissipate energy during displacement of the barrier system.

 

As such, there are safety concerns with using a 27" tall, W-beam guardrail system when coupled with the exposed wood plank, complicated steep slopes, wood posts, and TL-3 impact conditions with higher C.G. passenger vehicles.
Date May 5, 2010


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