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Extra Blockouts

Question
State KS
Description Text

We have a project that will be removing the bridge rails and upgrading the bridge rails to a 32" corral rail (Kansas type of bridge rail). Attached to the old bridge rail is what you see in the picture.
It appears that the contractor some time ago was allowed to use triple and double blockouts to avoid the pavement and curb and gutter. We are trying to minimize the project cost and avoid reconstructing some of the pavement and curb and gutter in order to provide a more typical single blockout guardrail installation at this location.

I recall that double blockouts can be used but limited to a certain amount of post locations. Also, what about the use of triple blockouts, limited to one post? I appreciate your help.

Keywords
  • Bridge Rails
Other Keywords none
Date June 15, 2011
Attachment RIMG0110.JPG
Attachment RIMG0113.jpg


Response
Response

We have looked over the extra blockout issue that you sent. In the past, we have recommended no more than one triple 8" blockout installation very 50' for guardrail installations. This is based on concerns that the ability of the triple blockout to transmit load to the post would be compromised for large deflections. With regards to transitions, we have used a similar rationale and have limited the installation of triple blockouts to a single post in the transition at limited locations. For your installation shown, we believe that the number of consecutive triple blockouts is likely too many. In addition, the use of steel blockouts further complicates the issue, because they are more likely to buckle and fold under load during the impact and compromise the load transfer to the post.

 

Double 8" blockouts pose much less of an issue as we have tested them in certain systems with good results and the depth is only 4" more that the MGS blockout depth. That said the use of double steel blockouts still poses an issue due to block collapse. We would recommend that the steel blockouts be gusseted to prevent collapse under load. This would apply to the triple block installation as well.

 

I should also note that when steel blockouts are used, we are recommending the use of backup plates to reduce the potential for guardrail rupture.

 

For the installation shown, we would recommend moving up the posts closer to the curb if possible to eliminate the triple blockouts. In addition, if you are planning on replacing the bridge rail, we would recommend realigning the transition and bridge to reduce the number of extra blockouts needed.

Date June 21, 2011


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