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Working width for MGS stiffness transition

Question
State	IN
Description Text	We have a question from a designer about the working width of the MGS Approach Guardrail Transition (AGT). The question is, given the location of a cantilevered sign (see picture attachment), within the AGT, would 4 ft of working width be acceptable. The crash report TRP-03-210-10 states that the impact point was chosen to produce maximum pocketing. It appears, after looking though the tables in appendix C that the maximum pocketing coincides with the maximum deflection. Is that correct? The report also states that the maximum working width is ~52 inches when impacted at the critical impact point by the truck. My thought is, given the cantilevered sign is located on the outgoing end of the bridge and within the nested thrie-beam section of the AGT, the risk of an opposing traffic vehicle impacting the AGT at the critical impact point at 62 mph is low. It would seem the vehicle would have time to slow down or redirect within the lane and shoulder (~19) before impacting the guardrail. As for the adjacent traffic, the vehicle would impact either the concrete bridge railing or nested thrie-beam sections first and it would seem the working width would be less than 52 inches for that type of hit. Basically I am asking is this good engineering judgment to approve this 4 ft working width for this location or am I missing something. As always, thank you for your time.

Question

State

Description Text

We have a question from a designer about the working width of the MGS Approach Guardrail Transition (AGT).
The question is, given the location of a cantilevered sign (see picture attachment), within the AGT, would 4 ft of working
width be acceptable.

The crash report TRP-03-210-10 states that the impact point was chosen to produce maximum pocketing. It appears, after
looking though the tables in appendix C that the maximum pocketing coincides with the maximum deflection. Is that correct?
The report also states that the maximum working width is ~52 inches when impacted at the critical impact point by the truck.

My thought is, given the cantilevered sign is located on the outgoing end of the bridge and within the nested thrie-beam
section of the AGT, the risk of an opposing traffic vehicle impacting the AGT at the critical impact point at 62 mph is low. It
would seem the vehicle would have time to slow down or redirect within the lane and shoulder (~19) before impacting the
guardrail. As for the adjacent traffic, the vehicle would impact either the concrete bridge railing or nested thrie-beam
sections first and it would seem the working width would be less than 52 inches for that type of hit.

Basically I am asking is this good engineering judgment to approve this 4 ft working width for this location or am I missing
something. As always, thank you for your time.

System Performance Evaluation

System Types
Approach Guardrail Transitions (AGTs)

Applications

System Features

Date August 28, 2019
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Attachment AGT layout with overhead sign.jpg

Response
Response (active)	The system deflection would be expected to decrease for impacts closer to the bridge rail, and they would increase for impacts further upstream. This trend was quantified within Appendix C of the noted report on pages 209 and 210. We selected the CIP to be node 93, or 75” upstream of the w-to-thie transition segment, which had a predicted deflection of 36.4” (testing had a maximum deflection of 33”, so we were reasonably close with the estimated system performance). If the impact point was moved further downstream into the thrie beam area, higher node numbers, the predicted deflections go down rather quickly. It’s difficult from your photo, where the sign support falls within the AGT, but you could use Table C-5 on pages 209-210 to get a predicted deflection. Note, the location of the maximum deflections are about 9-10 ft further downstream of the impact point, you would need to select an impact point upstream of the sign support location (about 12 nodes). The figure on Page 195 can be used to identify nodes on the system that would correspond to the location of your sign support. Finally, the working width was the result of the system deflection plus the width of the system, so if you add ~19” to the estimated deflection, you would get an estimated working width. I agree that for impacts from vehicles exiting the bridge, the impacts would be in the stiffened end, thus reducing the deflections as compared to impacts at the same location for vehicles traveling toward the bridge. However, the photo shows this bridge on an undivided, 1 lane each way, roadway. I would think that the AGT is within the clear zone for reverse direction traffic (going left toward bridge), and these impacts should also be considered. Engineering judgement can always be utilized to justify site specific issues. It all depends on what the engineer is comfortable with, and you can make a pretty good argument here. Your 4-ft offset is very close to the 52” WW described in the testing. A 4” difference may not justify moving the system or sign, especially when the working width was determined on system deflections, not the vehicle itself. Also, depending on the location of the sign relative to the AGT, you may meet the predicted working width anyway.

Response

Response
(active)

The system deflection would be expected to decrease for impacts closer to the bridge rail, and they would increase for impacts further upstream. This trend was quantified within Appendix C of the noted report on pages 209 and 210. We selected the CIP to be node 93, or 75” upstream of the w-to-thie transition segment, which had a predicted deflection of 36.4” (testing had a maximum deflection of 33”, so we were reasonably close with the estimated system performance). If the impact point was moved further downstream into the thrie beam area, higher node numbers, the predicted deflections go down rather quickly.

It’s difficult from your photo, where the sign support falls within the AGT, but you could use Table C-5 on pages 209-210 to get a predicted deflection. Note, the location of the maximum deflections are about 9-10 ft further downstream of the impact point, you would need to select an impact point upstream of the sign support location (about 12 nodes). The figure on Page 195 can be used to identify nodes on the system that would correspond to the location of your sign support. Finally, the working width was the result of the system deflection plus the width of the system, so if you add ~19” to the estimated deflection, you would get an estimated working width.

I agree that for impacts from vehicles exiting the bridge, the impacts would be in the stiffened end, thus reducing the deflections as compared to impacts at the same location for vehicles traveling toward the bridge. However, the photo shows this bridge on an undivided, 1 lane each way, roadway. I would think that the AGT is within the clear zone for reverse direction traffic (going left toward bridge), and these impacts should also be considered.

Engineering judgement can always be utilized to justify site specific issues. It all depends on what the engineer is comfortable with, and you can make a pretty good argument here. Your 4-ft offset is very close to the 52” WW described in the testing. A 4” difference may not justify moving the system or sign, especially when the working width was determined on system deflections, not the vehicle itself. Also, depending on the location of the sign relative to the AGT, you may meet the predicted working width anyway.

Date August 28, 2019
Previous Views (163) Favorites (0)