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Question
State	WI
Description Text	FHWA's NHI Roadside Design Class allows for extra block outs to be installed within a given run of beam guard. WisDOT has adopted this within its current standard detail sheet for regular beam guard. With MGS using larger blockouts, would the same guidance that is in the NHI class still apply?

System Performance Evaluation

System Types
W-beam Guardrails

Applications
Midwest Guardrail Systems (MGS)

System Features
Blockouts

Date November 5, 2010
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Response
Response (active)	Historically, MwRSF may have allowed the use of up to triple 8-in. deep blocks at a few locations within a run of metric-height W-beam guardrail, thus resulting in an offset of 24 in. However, it is uncertain as to whether the use of three 8-in. deep blocks may be too excessive when used continuously with metric-height W-beam guardrail. In the metric-height W-beam long span guardrail, MwRSF incorporated the use of double, 8-in. deep wood blockouts with the three CRT posts adjacent to the long span. The MGS utilizes 12-in.deep blocks for standard applications as well as for special applications. For example, the MGS long span design utilizes one 12-in. wood block with three CRT posts instead of two stacked 12-in. deep blocks. For the MGS, it would seem reasonable that the use of two 12-in. deep stacked blocks could be accommodated at a few locations as well, thus also resulting in a rail offset of 24 in. However, it is uncertain as to whether the use of two 12-in. deep blocks may be too excessive when used continuously with the MGS. Thus, based on previous testing of systems with deep or extended blockouts and an analysis of the contact lengths of typical MGS testing, MwRSF would recommend the following: 1. Double standard blockouts or combinations of blockouts up to 16-in. deep may be used continuously in a guardrail system. 2. Triple standard blockouts or combinations of blockouts up to 24-in. deep should be limited to one in any 75 ft of guardrail.

Response

Response
(active)

Historically, MwRSF may have allowed the use of up to triple 8-in. deep blocks at a few locations within a run of metric-height W-beam guardrail, thus resulting in an offset of 24 in. However, it is uncertain as to whether the use of three 8-in. deep blocks may be too excessive when used continuously with metric-height W-beam guardrail. In the metric-height W-beam long span guardrail, MwRSF incorporated the use of double, 8-in. deep wood blockouts with the three CRT posts adjacent to the long span.

The MGS utilizes 12-in.deep blocks for standard applications as well as for special applications. For example, the MGS long span design utilizes one 12-in. wood block with three CRT posts instead of two stacked 12-in. deep blocks. For the MGS, it would seem reasonable that the use of two 12-in. deep stacked blocks could be accommodated at a few locations as well, thus also resulting in a rail offset of 24 in. However, it is uncertain as to whether the use of two 12-in. deep blocks may be too excessive when used continuously with the MGS.

Thus, based on previous testing of systems with deep or extended blockouts and an analysis of the contact lengths of typical MGS testing, MwRSF would recommend the following:

1. Double standard blockouts or combinations of blockouts up to 16-in. deep may be used continuously in a guardrail system.
2. Triple standard blockouts or combinations of blockouts up to 24-in. deep should be limited to one in any 75 ft of guardrail.

Date November 5, 2010
Previous Views (76) Favorites (0)

Response
Response (active)

Date November 5, 2010
Previous Views (76) Favorites (0)

Attachment RSD Session 11 - Special Consderations.pdf

Additional Blockout Depth