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-28
  
  **Hardware Type:**
            Sign Supports, Mailboxes, and Delineator Posts
                          **Code:**
            SS-28
                          **Date:**
            05/26/1992
                          **Testing Criteria:**
            NCHRP 350
                                  **Device Description:**
            3 1⁄2" Diam. Thin Walled Aluminum Tube
Single Spliced (6" c-c) Marion Steel or Franklin Steel 4 ppf U-channel post
                          **View PDF:**
            ss28.pdf (493.79 KB)
    
    United States Department of Transportation - Federal Highway Administration

May 26, 1992

Refer to: HNG-14/SS-28

Mr. Leon N. Larson
Regional Federal Highway Administrator (HSY-04)
Atlanta, Georgia

This is in reply to your memorandum of April 9 requesting guidance on the use of 89-mm (3.5 inch) diameter think walled aluminum tube and 6-kg/m (4-pound-per-foot) U-channel small sign supports. You attached a draft report, Evaluation of the Crash Worthiness of the Florida Think Walled Aluminum Tube and Steel U-Channel Sign Supports, dated February 1992 and a video of the testing. The report contained results of 20 full-scale crash tests conducted by the Texas Transportation Institute. Sketches of the aluminum tube and spliced U-channel test installations are attached. A summary of the test results is presented below:

Test

			Number
			Post*
			Soil

			Type
			Impact

			Speed

			Km/h
			Occupant

			Impact

			M/s
			FHWA

			Assessment1
			100 mm Tube
			Weak
			31.2
			5.46
			Fail (note 1)2
			100 mm Tube
			Weak
			96.9
			6.77
			Fail (note 2)19
			89 mm Tube
			Weak
			30.3
			3.05
			Fail (note 3)20
			89 mm Tube
			Weak
			98.3
			3.32
			Pass3
			Marion Rev.
			Weak
			29.6
			0.27
			Pass4
			Marion Rev.
			Weak
			99.5
			(no contact)
			Pass5
			Marion Rev.
			Strong
			31.1
			(no contact)
			Pass6
			Marion Rev.
			Strong
			98.3
			(no contact)
			Pass11
			Marion
			Weak
			30.9
			0.85
			Pass12
			Marion
			Weak
			99.6
			(no contact)
			Pass13
			Marion
			Strong
			32.3
			(no contact)
			Pass14
			Marion
			Strong
			99.5
			(no contact)
			Pass7
			Franklin Rev.
			Weak
			32.7
			1.92
			Pass8
			Franklin Rev.
			Weak
			99.0
			2.32
			Pass9
			Franklin Rev.
			Strong
			27.4
			4.79
			Pass10
			Franklin Rev.
			Strong
			97.8
			2.80
			Pass (note 4)15
			Franklin
			Strong
			31.2
			1.77
			Pass16
			Franklin
			Strong
			100.4
			1.74
			Pass17
			Franklin
			Weak
			32.2
			2.16
			Pass18
			Franklin
			Weak
			100.4
			1.62
			Pass (note 4)Low speed test target velocity is 32.2 km/h (20 mph)

High speed test target velocity is 96.6 km/h (60 mph)

Maximum allowable occupant impact speed is 4.88 m/s (16 fps)

The designation "Rev." means that the signpost was mounted in front of the stub post, which is the reverse of the configuration previously recommended by FHWA. Based upon this testing, FHWA continues to prefer the "behind-the-stub" orientation over the other. However, the results are not sufficient to insist on one over the other.Note 1: This test failed due to excessive velocity change.

Note 2: This test failed due to excessive velocity change and vehicle instability (the vehicle rolled over).

Note 3: The vehicle pushed the sign support over and rode up it until it came to rest of the support. The researchers ruled this test a failure because the support "failed to yield to the vehicle." FHWA does not concur in that assessment and believes that the low occupant impact velocity (at the "desirable" level) permits our acceptance of this support. However, the performance of this support should be considered marginal because it raised the front end of the test vehicle 600 to 750 mm (2 to 2.5 feet) in the low speed test, suggesting the support is close to causing instability in light-weight impacting vehicles.

Note 4: In this test the sign support and panel impacted the roofline. The researchers ruled that the resulting dent was significant and unacceptable passenger compartment intrusion. In prior tests of other supports FHWA judged similar amounts of passenger compartment intrusion acceptable. The relatively low velocity change occurring during the test weighs very much in favor of passing the support. Review of the high-speed film of the test show that the dummy did not contact the damaged vehicle interior. Therefore, FHWA had judged the intrusion acceptable. Nevertheless, the performance must be considered marginal.

Thin-Walled Aluminum Tube

Testing of the 89-mm (3.5 –inch) thin-walled aluminum tube [4.8-mm (3/16-inch) wall thickness, alloy 6061-T6] in weak soil showed its performance to be acceptable, though marginal because of lifting of the vehicle in the low speed test. The thin-walled aluminum tube was previously accepted (Memorandum to Region 1 dated 12/12/89 up to 100 mm (4 inches) in diameter when installed in strong soil. The present Florida testing shows that this larger support fails when installed in weak soils. Therefore, the 89-mm aluminum tube is the largest diameter that should be used in weak soil. FHWA has announced an intention to institute a policy of only accepting those small sign supports that are found crashworthy in both soil types. This will likely occur when FHWA adopts the updated crash test guidelines (revised NCHRP Report 230.) This was discussed in our memorandum of July 6, 1990, "Breakaway Sign and Luminaire Supports".

6-kg/m (4-pound-per-foot) U-Channel

Splice details: The Florida U-channel splice differs from the breakaway splice derived from Arizona DOT testing that we previously found acceptable for strong soils only. In the Florida splice the overlap is 200 mm (8 inches) and the splice is secured with A307, 9.5-mm x 50-mm (3/8-inche by 2-inch) bolts spaced at 150-mm (6 inches) center-to-center. The stub in the Florida testing was driven to a minimum embedment of 914 mm (36 inches). The Arizona splice has an overlap of 150 mm (6 inches) and uses 7.9-mm (5/16-inch) Grade 9 bolts spaced 100 mm (4 inches) center-to-center. In both cases, a 16-mm (5.8-inch) spacer is used to separate the webs (Arizona’s experience is that the spacer must be structurally adequate to transfer the load between the webs of the signpost and the stub.) In both designs the U-channel stubs are driven until they extend no more than 100 mm above the groundline. Acceptance of a breakaway U-channel post configuration in this memorandum refers only to a post with a 150-mm center-to-center-of-bolts Florida splice, in either the "normal" or "reversed" position.

In the Florida splice the diameters of the splice holes and the splice bolts are the same, meaning that the splice holes must be reamed in order to insert the bolts. This destroys the corrosion protection in the hole. Thus, if effective countermeasures are not taken, one should expect accelerated loss of the zinc corrosion protection of the splice bolts as it sacrifices to protect the post, resulting in early loss of splice strength. Perhaps coating the splice holes and bolts with zinc-rich paint paste would be an effective countermeasure. No matter what countermeasures are taken, the strength of the supports depends on the integrity of the splice bolts. Therefore, we would strongly recommend a program to ensure that the integrity of the splice bolts is not lost.

Materials Information: The "Marion" supports are 6-kg/m (4-pound-per-foot) posts of high-carbon billet steel having a nominal yield stress of 552 Mpa (80 ksi). These "Rib-Bak" posts are produced by the Marion Steel Company. The "Franklin" supports are 6-kg/m, re-rolled rail steel posts having a nominal yield stress of 414 Mpa (60 ksi). These posts are produced by Franklin Steel.

We have received questions about the interchangeability of Franklin and Marion posts and stubs. No crash tests have been done on mixed stub and signposts; FHWA recommends against interchanging them without testing to confirm acceptability. A highway agency that desires to qualify posts of one manufacturer on the stubs of another is advised that careful attention be given to the details of the splice orientation and spacer hardware. The variation in cross-sections between the two products is sufficient to cause problems in nesting under some splice orientations.

In summary, we concur with the Florida DOT that the 89-mm (3.5 inch) diameter aluminum tube support and the 6-kg/m (4-pound-per-foot), 414-Mpa (Franklin, 60-ksi) or 552-Mpa (Marion, 80-ksi) U-channel posts with 150-mm (6-inch) center-to-center-of-bolts lapped breakaway splices are acceptable for use on Federal-aid highway projects in all soil types, when requested by a state. This acceptance is limited to single supports within a 2100-mm (7 foot) path. Test results of both the thin-wall aluminum tube and the Franklin U-channel supports were marginally acceptable. Care should be taken to ensure that supports are installed in the same manner as those tested with respect to signpost or stub embedment depth, minimum sign height, splice details, etc.

L.A. Staron

3 Attachments

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