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|Description Text||We have a question regarding the use of extra blockouts in a steel post transition to a bridge parapet. The subject location has an existing manhole with a conical top section that interferes with driving the second, third, and fourth posts from the end of the bridge parapet. These posts spaced at 1 ft 6 ¾ inches. A copy of our Highway Standard 630031 is attached, and depicts the particular bridge transition design. Also, several photos are attached that show the bridge rail transition, the curb alignment around the edge of the existing round inlet grating, the conflicting manhole, and the area immediately behind the curb. Please note that in photo “image009.jpg” the posts that are not driven, near the left side of the photo will be driven according to plan after some conflicting sleeper slab is removed at the joint of the bridge approach slab and the pavement.
It does appear feasible to add blockouts to produce a total offset from the back of the doubled thrie beam to the face of post of 24 inches (3 @ 8 inch blockouts or 2 @ 12 inch blockouts).
We have searched the consulting website for similar questions, but do not find one that is right on target for this issue. We do find the following:
June 15, 2011, ID = 205. Question about use of triple 8 inch steel blockouts in transition. This question is similar to our case, but is applicable to a w-beam section rather than the thrie beam application on our Highway Standard.
November 5, 2010, ID = 267. Question regarding use of extra blockouts in a run of guardrail (MGS.) This question is for guardrail, rather than bridge transition.
September 7, 2006, ID = 456. Question regarding use of extra blockouts in a w-beam transition to a bridge rail. The response acknowledges use of triple 8 inch blockouts with a thrie beam transition. However, this information is form 2006 and predates several changes to the bridge transition.
Our proposal is to use triple 8 inch blockouts or double 12 inch blockouts at posts 2, 3 ,and 4. Also, we have discussed ideas to compensate for possible increased deflection here:
- Adding a section or nested section of thrie beam rail across the backs of these three posts.
- Adding some form of diaphragm between the extended blockouts of posts 2, 3, and 4 (cross bracing, solid wood blocks, etc.)
Thanks for your help in advance.
|Other Keywords||Extra Blockouts|
|Date||November 25, 2014|
In the past, we have considered the use of deeper blockouts in limited cases dependent on system in question. We have used 16” deep blockouts in certain systems, but we have not used 24” deep blockouts in system due to concerns that the additional blockout depth may begin to affect the way the guardrail post is loaded and may increase the potential for later-torsion buckling of the post rather than the desired post loading modes of strong axis bending and rotation of the post through the soil. As such, we have limited these extended blockouts to a single post in a run of guardrail in order to deal with obstacles or other issues.
As you noted in the message you sent, we have allowed deeper blockouts in approach guardrail transitions in the past. The concern for altering the post loading is less prevalent for the transition posts as they tend to be closer spaced and deflect less, which lowers the concern for buckling of the post.
Thus, we believe that it would be possible to use large blockouts for post nos. 2-4 shown in your detail without adversely affecting performance due to the special circumstance you are faced with. However, for general installations we would recommend using the tested configuration as the use of the deeper blockouts has not been formally investigated or tested.
We have conducted research for WisDOT in the past on a related issue of spanning obstacles in a transition and came up with some potential solutions. Take a look at the report below. There is an option in it for deeper blockout posts with a beam spanning the gap that may work for you as well.
A couple of other items to note. First, I am not familiar with the curb section that you are using with the transition. I believe this transition was testing with a 4” wedge curb. As such, other 4” curbs may work with the transition as well, but higher curb sections may require further investigation for use in the transition. The exact dimensions are not listed on the detail.
The detail you have shown also appears to be longer than the transition sections we have tested to MASH with the MGS system. You may have a rational for using a longer transition section, but I wanted you to be aware that the transition may be able to be shortened.
|Date||December 2, 2014|
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