USA Banner
  U.S. flag
              An official website of the United States government Here's how you knowOfficial US Government Icon
              **Official websites use .gov**A **.gov** website belongs to an official government organization in the United States.
            Secure Site Icon
              **Secure .gov websites use HTTPS**
                  A **lock** ( A locked padlock ) or **https://** means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites. 
 U.S. Department of Transportation Icon United States Department of Transportation
 Search DOT
            Search
                    About FHWAFHWA Organization
                Headquarters Offices
                Field Offices
                Strategic Plan
                Business Opportunities
                Staff Directories
                Careers
                Highway History
                Contact UsProgramsAcquisition Management
                Civil Rights
                Federal Lands Highway
                Infrastructure
                Innovative Program Delivery
                Operations
                Planning, Environment, and Realty
                Policy
                Research, Development, and Technology
                Safety
                Browse by TopicsResourcesCore Highway Topics
                Federal Aid Essentials
                Automation
                Complete Streets
                Laws and Regulations
                Policy & Guidance Center
                Section 508
                Publications and Statistics
                Professional Development
                National Highway Institute
                Resource Center
                Accessibility Resource Library
                FHWA Research LibraryNewsroomPress Releases
                Speeches & Testimony
                Photos
                Videos
                Media ContactsMenu FHWA Highway Safety Programs Home
                        Safety
                        Roadway Departure Safety
                        Reduce Crash Severity
                        Hardware Eligibility Letters


            
            In This Section
        Roadway Departure Safety
                Strategic Approach
                                Strategic Plan
                Crash Emphasis Brochures
                Focused Approach
                Noteworthy Practices
                Partner Resources
                Keep Vehicles on the Road
                                Pavement Friction
                Horizontal Curve Safety
                Rumble Strips and Rumble Stripes
                Nighttime Visibility
                Provide for a Safe Recovery
                                SafetyEdge
                Clear Zones
                Reduce Crash Severity
                                Overview
                Hardware Eligibility Letters
                Focus on Reducing Rural Roadway Departures (FoRRRwD)
                                Overview
                All Public Roads
                Proven Countermeasures
                Systemic Approach
                Safety Action Plans
                FoRRRwD Resources
                
                Safety Eligibility Letter SS-27
  
  **Hardware Type:**
            Sign Supports, Mailboxes, and Delineator Posts
                          **Code:**
            SS-27
                          **Date:**
            05/15/1992
                          **Testing Criteria:**
            NCHRP 350
                          **Manufacturer:**
            Montana D.O.T.
                          **Device Description:**
            Round wood post supports
                          **View PDF:**
            ss27.pdf (83.19 KB)
    
    United States Department of Transportation - Federal Highway Administration

May 15, 1992

Refer to: HNG-14

David S. Johnson, P.E.
Preconstruction Engineer
Montana Department of Transportation
2701 Prospect Avenue
Helena, Montana 59620-9726

Dear Mr. Johnson:

Thank you for your letter of March 6 requesting Federal Highway Administration's (FHWA) acceptance of round wood posts for use where breakaway supports are needed. The question of accepting round sections based upon testing of dimensioned timber posts was discussed informally at both meetings of the Small and Large Sign Support Testing Study oversight representatives. It is our recollection that the test of a 127-mm (5-inch) round wood post (as included in the crash test matrix) was expected before extrapolating dimensioned posts crashworthiness to round wood posts. However, because it seems that there will not be enough pooled-funds to test that system and your letter requested acceptance of posts larger than 127-mm in diameter, we have reconsidered our position.

From what little testing has been done on large wood sign supports, we believe that these posts fail in bending as well as in shear. Therefore, the section modulus of the posts ought to be considered along with the cross-section area. Since the test results for the 140-mm x 190-mm (nominal 6-inch x 8-inch) post were near the upper limit of acceptability, we are unwilling to accept a round post of an equivalent section modulus without testing. However, we are willing to consider acceptable those round wood posts, which have a section modulus no greater than 80 percent of that for the 140-mm x 190-mm rectangular post. The enclosed table shows that the post sizes listed in our letter are acceptable up to the 190-mm (7.5-inch) diameter with a 70-mm (2.75-inch) hole under this criterion. The crashworthiness of larger diameter posts should be proven through testing. Also, the specifications for posts close to the margin of acceptability should contain a limit on the maximum diameter at 100 mm (4 inches) above the groundline.

Sincerely yours,

L.A. Staron, Chief
Federal-Aid and Design Division

Enclosure

Geometric and Roadside Design Acceptance Letter Number SS-27
 

Rectangular Wood PostsNominal

			Size
			Width

			(in)
			Depth

			(in)
			Hole

			Diameter

			(in)
			Area

			(in2)
			Section Modulus

			(in2)4 x 4
			3.5
			3.5
			0.0
			12.25
			7.154 x 6
			3.5
			5.5
			1.5
			14.00
			17.296 x 6
			5.5
			5.5
			2.0
			19.25
			26.406 x 8*
			5.5
			7.5
			3.0
			24.75
			48.2648.26 x 80% = 38.61

Round Wood PostsDiameter

			(in)
			Hole

			Size**

			(in)
			Area

			(in2)
			Section

			Modulus

			(in2)
			Within

			80%?4.0
			0.0
			12.57
			6.28
			Yes4.5
			0.0
			15.90
			8.95
			Yes5.0
			0.0
			19.63
			12.27
			Yes6.0
			0.75
			23.79
			21.14
			Yes6.5
			1.25
			25.11
			26.64
			Yes7.0
			2.00
			24.68
			32.37
			Yes7.5
			2.75
			24.03
			37.32
			Yes8.0
			3.25
			25.00
			44.83
			No ****6" x 8" weakened post was successfully tested in weak soil

**Holes are centered at approximately 100 mm (4 inches) and 460 mm (18 inches) above the ground, with their axes horizontal and in a plane parallel to the sign face.

***Unacceptable without confirmation testing.

SI conversion table:

In = 25.4 mm

In = 645.16 mm2

In = 16 387.064 mm3

FHWA Safety Home| FHWA Home | Feedback

FHWA

United States Department of Transportation - Federal Highway Administration U.S. DEPARTMENT OF TRANSPORTATION

**Federal Highway Administration**

1200 NEW JERSEY AVENUE, SE

WASHINGTON, DC 20590

202-366-4000Subscribe to email updates
                    About
  

        
              About FHWA
                Careers
                Org Chart
                Staff Directories
                Work with UsPrograms
  

        
              Acquisition Management
                Civil Rights
                Federal Lands Highway
                Infrastructure
                Innovative Program Delivery
                Operations
                Planning, Environment, and Realty
                Policy
                Research, Development, and Technology
                SafetyPolicies, Rights, Legal
  

        
              About DOT
                Accessibility
                Budget and Performance
                Civil Rights
                FOIA
                Information Quality
                No FEAR Act
                Office of Inspector General
                Privacy Policy
                USA.gov
                Web Policies and Notices
                Web StandardsNews & Events
  

        
              Newsroom
                Press Releases
                Speeches & Testimony
                Media Contacts
                Connect with Us