Logged in as: Public User

MGS questions

Question
State IL
Description Text My name is Mark Bell and I am a guardrail contractor in Illinois.
The question came up relating to the wood block outs that are positioned between the steel post and the W Beam / Thrie beam rail elements.
The nature of wood is that it is not milled to exact tolerances as is the case with fabricated steel products. Certainly the wood mills can get close to specified sizes.

In reviewing your website I came across two pdfs that may assist in answering my questions.
Referance 1: MGS Standard drawing page 3 of 7 dated 7/20/11 illustrates a 6" wide x 12" deep x 14" tall wood block with a double nail driven through the flange of the post to hold the block from spinning. (the double nail suggests that the wood spacer / block out is un-routed)
Referance 2: FPL_MGS_SP_RD drawings for Southern Yellow Pine details both wood posts and block outs and page 9 of 9 provides specifications for the wood that include tolerances for Timber Spacers (block outs)
It states the following: The material may be rough sawn or surfaced, full size, hit or miss, with a tolerance of 6 mm (1/4 inch) for all dimensions.

My questions are as follows:
What is the acceptable tolerance PLUS/MINUS for any dimension that wood blocks and posts are allowed before it becomes un-usable ?
Will the acceptable tolerance also apply to wood block outs that have a routed edge to prevent it from spinning on the post ?

I have a meeting on February 3rd, 2014 and would like to be prepared to answer these questions.
I greatly appreciate your reviewing this and getting back to me as soon as it is possible.
Keywords
  • Guardrail
Other Keywords none
Date January 27, 2014


Response
Response

MwRSF has successfully crash tested the MGS in blocked (12” deep) and non-blocked (no offset block but with backup plates) applications. In these testing programs, improved performance was observed when using the offset blocks. TTI later successfully crash tested the MGS with an 8” offset block. As a result of these programs, I would not get too concerned with minor deviations in the block dimensions when comparing full-sawn, rough sawn, and dressed blocks. As a matter of fact, some fabricators start the process with larger wood sizes so that the dressed block actually measures 6”x12” or 6”x8”.

 

Previously, manufacturers and other State DOTs also inquired into the MGS block tolerances. Since similar inquiries have been made and responses have already been complied, I am forwarding to you information from the 2007 discussions with the Pooled Fund members states. Hopefully, my response above and the attached information will sufficiently answer your question.

 

Generally speaking, The states set the block tolerances they will accept based on their preferences. They may have additional considerations for the blockouts outside those we use in our design and testing procedures. In addition, any guidance we could provide is not “acceptance” in particular. Our comments would be based on our design and testing and our best engineering judgment, but would not provide acceptance or compliance with any type of standard.

 

Thanks 

Date January 27, 2014
Attachment 2301.pdf
Attachment PDB11ab.pdf
Attachment Response Summary - MGS Implementation - Discussion Topic No. 1 - Standard Half Quarter Post Spacing!.zip


Response
Response

Thank you for your response. I found it very helpful.

 

Based on your comment below, the MGS that has a block thicknesses between 8" and 12" will successfully pass the test. 

To clarify one point...your comments apply to guardrail that is utilizing a 'steel' posts...correct ?

One additional question: the MGS details illustrate nailing the block with a one double headed 16d nail through a 1/4" hole through the web of a steel post...to prevent the block from spinning. If a wood block manufacturer provides the block with a routed surface while maintaining thicknesses from 8" to 12" , is the depth of the routing considered a critical dimension to meet adherance to the MGS system as tested ?


I'll look forward to hearing back from you.

 

Thank you

 

Mark Bell
Date January 27, 2014


Response
Response

See my comments below in red. 

Thank you for your response. I found it very helpful.

 

Based on your comment below, the MGS that has a block thicknesses between 8" and 12" will successfully pass the test.

 

This is true. The MGS has also been tested successfully without blockouts. Thus, shorter blocks would likely work too. However, we do not recommend shorter blockouts unless roadway width is an issue.

 

To clarify one point...your comments apply to guardrail that is utilizing a 'steel' posts...correct ?

 

The MGS has only been tested with reduced blockout depths on steel posts. We have tested several wood post versions of the system, but they have all had the 12” blockouts. Shorter blockout depths may work, but would likely need further investigation.

 

One additional question: the MGS details illustrate nailing the block with a one double headed 16d nail through a 1/4" hole through the web of a steel post...to prevent the block from spinning. If a wood block manufacturer provides the block with a routed surface while maintaining thicknesses from 8" to 12" , is the depth of the routing considered a critical dimension to meet adherance to the MGS system as tested ?

 

We have tested the blockouts nailed and with no nailing. For testing purposes, it makes little difference. Spinning of the blockout during a vehicle impact is not an issue. The nails and the routing are more of a in-service feature to prevent blockout rotation over time, as you noted. Thus, depth of the routing would have little effect on safety performance. However, if the routing was not sufficient to prevent block rotation over time on field installations, it would become an issue. I don’t have information on what routing depths are effective in the field. My suggestion would be to work with the state DOT’s to determine the depth that they prefer for the routing based on field experience.

 

Date March 7, 2014


Contact Us:
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
(402) 472-0965
Email: mwrsf@unl.edu
Disclaimer:
The information contained on the Midwest Roadside Safety Facility (MwRSF) website is subject to change without prior notice. The University of Nebraska and the MwRSF is not responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the use or misuse of or reliance upon any such content, goods, or services available on this site.