View Q&A

Question … given that many states have steel posts (W6x8.5#/9# as well as W6x15#) which serve multiple functions for guardrail applications, we were seeking clarification/guidance from MwRSF group as to the minimum amount of distance for post bolt holes – vertically?    This is in order to not jeopardize a tearing of the steel materials between them during a MASH impact event?

Attached is a TTI report where a post was utilized which features 6 post holes on each side of the post – please see page 20, Fig 5.4 (pdf page 32 of 75).   

Also attached is TF13 PWE01 – 04 drawings, in which note #2 indicates additional holes may be added to posts for various reasons, as needed. 

• Note that PWE04 shows a vertical post bolt hole distance on W6 x 8.5#/9# post as 3-7/8” Center-to-Center (“C-t-C”).

Other givens:

• From a manufacturing perspective, typically a minimum of 1/2” from the edge of one hole to the edge of another hole is needed, in order to ensure the holes do not propagate a tear between them, during the high speed operation we utilize to punch the post bolt holes in guardrail posts made from typical W6 structural beam.

• I note that on the TF13 drawings – as well as previous MwRSF drawings within various test reports:
  
  
  
  • a reference of ¾” from the CENTER of the post hole to the outside edge of the post is noted as typical.  
    
    
    
    
    • Thereby, if utilizing a 13/16” diameter post bolt hole, the “net” amount of steel from the EDGE of the post bolt hole to the edge of the post flange could be less than 11/32” (0.344”).
    
    
    • Perhaps as little as ¼” (0.25”), due to allowable ASTM A6 tolerances of the flange width associated with the production of the hot formed W6 structural beam (raw material) being utilized to produce guardrail posts.
    
    
    
    
  
  
  
  

• The flanges (where post bolt holes are located in guardrail applications) of the W6x8.5# posts are nominally 0.195” thick, W6x9# are nominally 0.215” thick and W6x15# are nominally 0.265” thick – please see attached ASTM A6 excerpt.

• The typical W/T guardrail is manufactured from 12ga (0.105” nom) and in some cases as thick as 10ga (0.135” nom).   At times, laminated 12/10ga installations take place and/or splices are still located on the posts in some applications.

• The post bolt slots in the W/T guardrail are typically ¾”H x 2-1/2”W.


• A 5/8”d button head w/shoulder ASTM A307 bolt is utilized for the attachment – of various lengths.  
  
  
  
  • The outside diameter of the “button head” (dome top) is 1-7/16” for standard GR Bolts and 1-5/16” (horizontal dimension) for clipped GR Bolts.  
  
  
  
  


• Whereas the post bolt holes in the indicated W6 posts are typically either ¾” or 13/16” in diameter.

Back to the question at hand … could you please provide guidance as to the minimum amount steel (vertically) that should be left between post bolt holes in the various W6 posts utilized in guardrail applications, to ensure the post is not compromised in a way that would change the W/T guardrail release during an “as designed and tested” MASH/350 impacts?   As you know the W/T guardrail is typically released from the posts by the post bolt “button head” releasing from the slot in the guardrail.

As information, we have been asked to manufacture a W6 guardrail post with only 1-1/16” of steel between the vertical outside edges of two post bolt holes in a W6 x 8.5#/9#/15# post and would like to ensure there is not a concern from a performance stand point with doing so?    We also note that some states regularly specify 13/16” post bolt holes in the indicated W6 posts to be 3” (C-t-C) from each other, for future asphalt overlays – this production request is slightly less than that at a C-t-C spacing of only 2-7/8” … resulting in ~1-1/16” of steel between the series of vertical post bolt holes in the flanges of a W6 post.  

Since the horizontal amount of steel related to post bolt holes in W6 posts to the flange edge is typically ~5/16”, we do not anticipate an issue with this request for a vertical gap of only 1-1/16” of steel between post bolt holes in W6 posts.    Thanks for your time to consider the question and possible provide guidance.  

In terms of spacing between holes in a guardrail post, we would be primarily concerned about bearing or shear tear out failure across the limited area adjacent to the holes. This would be similar failure modes to those that we would be concerned about relative to the edge of the post flange as well.

Currently, we do not observe many failures of the post flange due to post bolt loading. Typically, the bolt will fail prior to the flange. We do observe bolt tear out in some cases when the edge distance is at the lower end of the potential range. That said, it has not been observed to be an issue in testing as the bolt does not release until well after the guardrail has released from the post, and the post has been significantly loaded and deflected.

In your proposed case the vertical hole spacing is reduced for typical. Thus, one would be concerned with tear out, bearing failure or failure of the minimum tensile area. The tensile area between and adjacent to the holes is sufficient to develop the shear load of the 5/8” diameter bolt. Thus, we would only be concerned with shear tear out bearing failure. For shearing tear out, structural practice is generally to have a thickness of 1.5 D between the edge of the hole and the free edge. As you noted, this would likely be violated for current flange edge distance for a single bolt. However, the hole distance you note has 1 1/16” of steel between holes. The 1.5 D recommendation for tear out would be 0.9375” which would be consistent with standard structural recommendations. Bearing stress would likely be over the design limit. That value is typically calculated as the yield stress is equal to the force divided by the thickness of the plate times the bolt diameter. In this case the applied force can’t be greater than the shear strength of the A307 bolt which would be 15.56 kips. This would give you a bearing stress that exceeds capacity.

My general thought is that the vertical hole spacing is not an issue. It may exceed the bearing stress calculation, but that is not likely to be an issue as bearing failure would not occur until significant post deflection had occurred and the rail had already released from the post.