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dimension tolerances and W6x9

Question
State WI
Description Text

To All,



 



I got a request from a manufacturer to approved there shop
drawings for steel post in the thrie beam transition.



 



I when through their drawings, my drawings and your
drawings.  It appears that the manufacturer’s
one of the dimensions are about 3/32” of an inch off.



 



To me this does not appear to be an issue.  But, it does bring up a good questions:



 



  1. What
    dimension tolerances does MwRSF use in their drawings?  Should states have similar tolerances in
    their drawings?

  2. We
    allow the contractors to use W6x9 or W6X8.5 in the MGS thrie beam
    transition.  MwRSF has only W6X8.5 in the
    MGS thrie beam transitions.  Is W6X9
    acceptable in the MGS thrie beam transition?


Approach Guardrail Transitions (AGTs)
Road Closure Gates
Thrie Beam Guardrails


Date June 3, 2016
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Response
Response
(active)
It is not entirely clear what dimension on the post is 3/32" off the detail. However, the discrepancy could could from a couple of areas.

First, our plans show the actual dimensions of a W6x8.5 post to the nearest 1/16". These vary slightly from the actual decimal values for the post. Additionally, W-shapes are generally referenced or shown using simplified values rather than the actual values. Thus, a W6x8.5 is usually listed as 6" deep by 4" wide when the actual dimensions are 5.83" and 3.94", respectively. 

As you noted, the difference could also be due to W6x9 posts versus W6x8.5 posts. We have been using these posts for several years as it came to our attention that they are what is generally supplied for guardrail applications. They are very similar, and you may have received W6x9 posts in reality. 

We do not believe that the performance of the barrier is affected in any significant way using W6x9 versus W6x8.5. The overall dimensions are nearly identical and the section properties are only 10% different. Thus, we don't see any issues with using the W6x9 posts in lieu of the W6x8.5 posts.

We do not post dimensional tolerance for standard structural parts in our plans. The AISC steel manual does list acceptable tolerances for W-shapes. This would be the best place to start for tolerances. See attached.
Date June 3, 2016
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Attachment 3211_001.pdf
Response
Response
(active) 		We have further reviewed the dimensional tolerance issue noted previously. It appears that the discrepancy has to do with the location of the post bolt hole along the width of the post flange. 

Review of the plans, previous hardware guide details, and plans from full-scale crash tests suggest that the 3/32" you note is based on how the hole placement is measured. The original 1979 Guide to Standardized Highway Barrier Rail Hardware denotes the location of the hole as 1 1/8" from the center of the post. Later revision of in the 1995 AASHTO Guide to Standardized Highway Barrier Hardware denotes the hole location using both the 1 1/8" from the center of the post dimension and a dimension 3/4" from the edge of the flange. 

These two locations for the hole are not the same. Using the 1 1/8" from the center of the post dimension yields a distance from the center of the hole to the the edge of the flange of 0.845". This is the source of the 3/32" difference you noted. 

MwRSF has typically detailed our hole locations from the edge of the flange based on standard CAD practices. Also, this provides a slightly lower distance to the flange edge for structural loading. However, most manufacturers specify the hole locations from the center of the flange based on manufacturing procedures. We believe that there is little functional difference between these values and either is acceptable. 

It should be noted that the AISC allowable tolerances for W-sections allows for variations in flange width and hole placement that may affect the location of the hole. According the AISC, flange width can vary +1/4" to -3/16" and the web can be offset as much as 3/16" from the center of the flange. These variations along with manufacturing tolerances on the hole fabrication may place the hole closer to the edge of the flange than 3/4". To the best of our knowledge, the affect of the W-section tolerances on the post hole distance to the edge of the flange has not been a safety issue with respect to barrier performance. 
Date July 12, 2016
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Attachment manufactuer details.pdf Attachment TF13 Standardized Highway Barrier Rail Hardware (June 1979).pdf Attachment pwe01-04.pdf Attachment PWE0607.pdf