Dennis H. O'Brien, P.E.

Manager, Product Planning

Industrial & Construction

Valmont Industries, Inc.

Valley, Nebraska 68064

Dear Mr. O'Brien:

Refer to: HNG-14

Products Division

By your September 16 and 22, 1988, letters to Mr. Thomas 0. Willett, Director,

of Office of Engineering, you requested Federal Highway Administration (FHWA)

acceptance of steel breakaway slip-base luminaire supports for use on

Federal-aid highway projects. .As you know, considerable effort has gone

into evaluating steel slip-base luminaire supports since you made your

request. Your cooperation and assistance in that effort is much appreciated.

Enclosure I summarizes the tests FHWA has evaluated in reaching a decision on

the breakaway acceptability of steel slip-base luminaire supports.

In each of the tests shown in the sumnary the geometry of the slip-base was

nominally the same as California Department of Transportation's (Caltrans)

Type 31 base, which is shown on Enclosure II, except that in one series of

tests the keeper plate thickness was reduced to 0.0149 inches (28 gage). The

pole base plate in the type 31 base Is 1 inch thick, the lower slip plate is

1 I/4 inches thick, and the anchor plate is 1 inch thick. We would also point

out that in all tests two of the slip-base clamp bolts lay in a line parallel

to the direction of traffic and were on the street side of the pole.

from the summarization of tests it can be seen that there is considerable

scatter in the results and that in some tests FHWA's maximum 16-foot-persecond breakaway change in velocity requirement was exceeded and in some

instances the test device was actually stopped. Because of the apparently

unpredictable nature of the slip-base the testing effort was extended and a

theoretical analysis of the slip-base release mechanism was undertaken. As a

result of this work we are now confident that safe slip-base luminaire

supports can be configured that will be within substantial compliance with

FHWA's breakaway requirements. Thus, steel slip-base luminaire supports will

be acceptable for use on Federal-aid highways if proposed by a State highway

agency provided they fall within the limitations setforth below:

Basic Type: Triangular, three-bolt base similar to Caltrans' Type 30 and 31

bases (see Enclosure II).

Minimum Shaft Wall Thickness: 0.1196 inches for diameters up to 10 inches. .G

Bolt Circle Diameter: 14 inches (minimum).

Base Plate Thickness: 1 inch (minimum), 1 l/4 inches (maximum).
2 

Lower Slip Plate Thickness: 1 l/4 inches (minimum), 1 l/2 inches (maximum). 

Anchor Plate Thickness: 1 l/4 inches (maximum). 

Steel Keeper Plate Thickness: 0.0149 inches before coating (28 gage) 

(maximum). 

Height Top of Lower Slip Plate from Ground Line: 4 inches (maximum). 

Clamp Bolt Type: Galvanized ASTM A325 with dry lubricant (Heads and nuts 

shall have heavy hex dimensions). 

Clamp Bolt Size: 7/8-inch to 1 l/4-inch diameter. 

Rectangular Clamp Bolt Washer Size: Length, width, and thickness shall be 

sufficient to prevent significant deflection (bending) when clamp bolt is 

loaded to its tensile capacity. 

Hole in Clamp Bolt Rectangular Washer: Camp bolt diameter plus l/16 inch, 

with edges chamfered to prevent binding with radius under bolt head. 

Clamp Bolt Tension: 8,000 pounds per bolt (maximum). In the absence of a 

more exact method of determining bolt tension the following maximum tightening 

torques shall be used: 

Bolt diameter (inches1 7/8 1 118 1 I/ 

Torque (foot-pounds) 87 :5 104 III4 

Finish: All faying surfaces to be galvanized, free of paint, and smooth and 

free of ridges, scallops, nicks, and burrs. 

Mounting Height: 56 feet, 6 inches measured from bottom of pole base plate to 

centerline of luminaire mounting tenon (maximum). 

Weight: 1,000 pounds (include luminaire, mast arm(s), pole, and base plate) 

(maximum). 

M;;:irm Orientation: Mast arm may be parallel to a flat side of the base 

ed that side faces aDDrOaCh traffic or may pass over a clamp bolt 

Tsee Enclosure III.) 

Placement: The terrain about the pole base shall not inhibit translation of 

the pole and approach topography shall be such that a vehicle leaving the 

roadway at design speed and an angle of up to 25 degrees will not strike the 

pole at a height greater than were the pole located at the edge of the 

pavement. (The approach terrain will not cause an errant vehicle to become 

airborne.) 
3 

While the restrictions listed here are rather extensive and in some instances 

differ from some current practices, for example the clamp bolt tension, keeper 

plate thickness, and mast arm oriention prescribed differ from those in the 

Caltrans standard, one should not infer FHWA is apprehensive about the use of 

slip-base luminaire supports. It is just that our extensive study of these 

structures has given us an insight that leads us to believe they will work 

best and the public will be best served by adhering to the guidance we have 

outlined. 

Sincerely yours, 

d -a. & 

L. A. Staron 

Chief, Federal-Aid and Design Division 

Enclosures 

dab metric and Roadside Design acceptance letter number LS-16. 
Enclosure 1, page 1 of 3 

Agency Hastarm(s) Shaft Mounting Pole Diam. Slip Base Clamp Est. Clamp Keeper Impact Test Veh. Impact Occupant 

Test No. Height (ft) at base (in)Bolt Circle Bolt Bolt Force Plate Angle from TyRe and speed Change in 

Date Length (ft) Length (ft) Total Wall Thick. Diameter Diameter 3 @ (lbs) ea Thickness Roadway Weight (m.p.h.) Velocity 

Weight (i) Weight (li) Weight (#) at base (in) (in) (in) (in) (degrees) (lbs) (f.p.s.) 

*t*************t****************tt*t**********************fL***********************************t*******************t*************~*~***********~*********~*** 

CALTRANS CALIFORNIA TYPE 31 SLIP BASE 

4u4 10.875 14 1 11,400 0.0359 30 '79 HONDA 19.9 8.5 

JUL 26, 84 1;; 6:: 8:: 0.1793 1865 

405 

1:: 6;: 8z 

10.875 14 1 11,200 0.0359 30 '79 HONOA 53.9 12.4 

MAY 23, 85 0.1793 1885 

406 

15; 

35 * 39.25 10 14 1 18,600 0.0359 30 '79 HOWDA 58.8 13.0 

MAY 8,'87 627.4 0.25 l 1850 

407 

123; 

35 l 39.25 

0.;: l 

14 1 12,200 0.0359 30 '79 HONDA 23.7 8.6 

JUNE 23,'87 639.4 1840 

CAPAiILITY VALHORT SLIP BASE 

TESTS - FOIL 

87F033 1.33 AND 16 48.5 55.5 

il.!!93 

14 1 12,500 0.0359 0 FOIL BO8IE 19.8 14.5 

MAR 12,'87 115 AND 112 630 964 1850 

87F034 1.33 AND 16 48.5 55.5 10 14 1 12,500 0.0359 0 FOIL BO8IE 58.7 15.5 

MAR 13,'87 115 AND 112 630 964 0.1793 1850 

THIN WALL 

TESTS - FOIL 

89FO23 1.33 AND 15 46.5833 53.42 14 1 12,500 0.0359 0 FOIL BOGIE 20.7 24.4 

SEP 21,'89 120 AN0 107 415 744 O.::96 1850 

89F024 1.33 AND 15 46.5833 53.42 

0.!96 

14 1 12,500 0.0359 0 '79 RABBIT 19.8 15.8 

SEP 27,'89 120 AND 107 415 744 1850 

8gF025 1.33 AND 15 46.5833 53.42 

0.!:96 

14 1 12,500 0.0359 0 '79 RABBIT 59.2 13.2 

OCT 5,'89 120 AND 107 415 744 1850 

89F026 1.33 AND 15 46.5833 53.42 

0.;1:96 

14 1 12,500 0.0359 0 FOIL 806IE 60.6 13.7 

OCT 24,'89 120 AND 107 415 ,744 1850 
Enclosure I, page 2 of 3 

Agency Mastarm Shaft Mounting Pole Diam. Slip Base 

Height lft) at base (in)Bolt Circle 

Clamp Est. Clamp Keeper Impact Impact Occupant 

Test No. Bolt Bolt Force Plate Angle from 

Test Veh. 

Type and speed Change in 

Date Length (ft) Length (ft) Total Wall Thick. Diameter Diameter 3 @ (lbs) ea Thickness 

Weight (I) Weight (i) Weight (t) at base (in) 

Roadway 

(in) (degrees) 

Weight (m.p.h.) Velocity 

(in) (in) (lbs) (f.p.s.) 

**t**********n*****+*****i**+t*****++ ~** ******************************************************t************+******t*****tt*************************************** 

CLAMP FORCE STUDY - FOIL 

BgF005 None 

APR 12,'89 

89FOO6 * 

APR lg.'89 

89FOO7 I 

APR 20,'89 

8gFOO8 * 

APR 24,'89 

B9FDO9 * 

APR 25,'89 

89FOlO Y 

APR 26,'89 

8gFOll * 

APR 26,'89 

8gFO12 q 

APR 27,'89 

89F014 q 

MAY 19,'89 

8gFO15 * 

MAY 24,'Bg 

8gFOlL I 

MAY 25,'89 

8gFO17 * 

MAY 31,'89 

30.25 

275 

* 

* 

* 

It 

* 

* 

* 

* 

l 

l 

Y 

* 

l 

* 

* 

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I 

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n 

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l 

None 

275 

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(I 

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* 

(I 

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oliZ45 

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14 

l 

* 

* 

1,965 

3,928 

5,891 

7,614 

9,817 

11,780 

9,817 

11,780 

13,743 

7,614 

15,808 

5,891 

0.0359 

* 

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0 

0 

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FOIL BOGIE 

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20.6 6.5 

20.7 5.9 

20.7 8.3 

20.5 .~ 6.4 

20.8 23.2 

20.7 20.6 

20.5 7.7 

20.6 36.9 l * 

20.4 22.7 

20.5 14.8 

20.4 18.2 

20.6 21.4 

* A P-foot high steel tube with 0.25 in. wall thickness was welded to.the bottom of a 33 foot tall pole-which had a wall thickness of 0.1196 in. 

**************t********************************************************~*****************t*********t******t*****************************************~***~* 
. Enclosure I, page 3 of 3 

Agency Mastarm Shaft Mounting Pole Oiam. Slip Base Clamp Est. Clamp Keeper Impact Test'Veh. Impact 

Test No. Height (ft) at base (in)Bolt Circle Bolt Bolt Force Plate 

Occupant 

Diameter 3 @ (lbs) ea Thickness 

Angle from Type and speed 

Date Length (ft) Length (ft) Total Wall Thick. Diameter 

Change in 

Weight (t) Weight (1) Weight (#) at base (in) (in) (in) 

Roadway 

(in) (degrees) 

Weight (m.p.h.) 

(lbs) 

Velocity 

(f.p.s.) 

KEEPER PLATE STUDY - FOIL 

10.0 

0.1793 

* 

Y 

14 

I 

* 

12,500 0.0359 0 FOIL PNOLM 19.9 25.9 

1850 

" NDNE 

l NONE 

* l 

* 

* n 

* 

l l 

I 

q * 

* 

l l 

l 

q I 

* 

I) Y 

* 

I * 

q 

l ” 

I 

* * 

* 

* * 

l 

19.8 

./. 

19.8 

8.8 

13.2 

* NONE 19.9 11.1 

* 0.0149 

0.0149 

0.0149 

0.0149 

0.1049 

0.0149 

0.0149 

0.0149 

20 16.9 

* 19.8 35 ** 

3,600 5.6 

3,600 7 

3,600 6.4 

9,000 

9,000 

8,000 

20 

20 

20 

/ 

20 

20 

20 

11.8 

35.4 l * 

17.7 

YOPO23 None 30.83 None 

4124790 None 486 486 

l 

* 

908024 

4/25/90 

9OPO25 

4/25/90 

9OPO26 

4726790 

YOP027 

4/26/90 

9OPO28 

5/l/90 

9OPO29 

5/2/90 

9OPO32 

5/30/90 

9OPO33 

5/31/90 

. 

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9OPO34 * I 

5/31/90 " * 

9OPo35 * * 

6/05/90 * l 

9OPO36 * * 

6/Ob/gO * " 

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l * Value includes rebound, thus exceeds'inpact speed. 
PLAN - TYPICAL BASE PLATE 

r7 1 

WELDED OPTION 

v&- 

PLAN - BOTTOM PLATES SECTION E-E 

ELEVATION - NON - SLIP BASE ELEVATION-SLIP BASE 
Enclosure II I

Luminaire Slip Base Orientation

Q

Direction

of Adjacent

I

Traffic

u

Direction

of Adjacent

Traffic

Direction

of Adjacent

Traffic

Best Breakaway

Performance

Acceptable

Best Compromise

to avoid undesirable

orientation

Acceptable

Worst Breakaway

Performance

Not Recommended
*

Subject:

From:

To:

Memorandum

U.S. Department

of Transportation

Federal Highway

Administrotbn Washington, D.C. 20590

Slip-Base Luminaire Supports Date: JAN28199l

Reply to

Chief, Federal-Aid and Design Division Attn. of: HNG-14

Regional Federal Highway Administrators

Federal Lands Highway Program Administrator

Our July 6, 1990, memorandum "Breakaway Sign and Luminaire Supports,"

transmitted a sketch titled "Luminaire Slip-Base Orientation." This sketch

illustrated three ways that the triangular slip-base could be welded to the

pole with respect to orientation of the mast arm. These were shown in

decreasing order of preference with respect to crashworthiness. We have had

questions regarding the middle sketch, which showed an acceptable compromise

situation where the far side of the triangular slip-base was parallel to

traffic. Although our memorandum did not address the situation where the

orientation of the triangular slip-base is such that the near side is parallel

to traffic, it is an acceptable, though less desirable, compromise. A revised

copy of that sketch is attached-to show that traffic may

of a slip-base where one face is parallel to the edge of

;;isr;;deither side

.

lY+zdA F

L. A. Staron

Attachment
,~ttac!-ment

Luminaire Slip Base Orientation

.

D-,-L,...-.. 8eSt ureanaway

Performance,

Acceptable

/\ Direction

of Adjacent

Traffic

Preferred Compromise to avoid Less

Undesirable orientation, Acceptable

desirable compromise,

Acceptable

8

II

.Y\

L+( 3

Location of Location of ’ I

Adjacent Traffic Adjacent Traffic ! 1

f” t@l

\ .I

‘4

Direction

of Adjacent

Traffic

Worst Breakaway

Performance,

Not Recommended

,
400 Seventh St., SW. 

Washington. DC. 20590 

Refer to: HNG-14 

SEP 30 1993 

Dennis H. O'Brien, P.E. ~ 

Manager of Product Pl,anning 

Industrial & Construction Products Division. 

Valmont Industries, Inc. 

Valley, Nebraska 68064-0358 

Dear Mr. O'Brien:, 

Thank you for your letter of September 2 requesting clarification of 

acceptable mounting heights for breakaway luminaire supports. In general, 

dimensions suc,has mounting height, pole diameter and mass, bolt size and 

torque, should not exceed those af the tested hardware. We place these 

limitations in our hardware acceptance letters to assure that the hardware 

used in the field is no less forgiving of the errant motorist than the 

hardware used in the crash tests. In our memorandum of June 15, 1989, to the 

Federal Highway Regional Administrator in Portland, Oregon, where we discussed 

luminaire support recommendations, however, we stated: 

c 

. . . the advice is not nearly as forceful on the issue of height as it 

is on the need to limit weigh~t. After considering the likely effect of 

a 60-foot pole in comparison to a 55-foot pole, we conclude there would 

be little difference for an impacting vehicle. Therefore, we would 

recommend allowing 60-foot mounting heights (base of pole to level of 

luminaire.) However, we also believe this should be considered an 

absolute maximum in the absence of further study and testing to 

investigate the effects of pole height and weight." 

We still believe this to be sound advice, even under the 1985 American 

Association of State Highway and Transportation Officials breakaway criteria. 

Therefore, luminaire supports which are considered breakaway by way of our 

Geometric and Roadside Design Acceptance letters LS-16 and LS-25 dated 

June 29, 1990, and October 10, 1991, respectively may use a nominal mounting 

height of up to 18.3 m (60 feet) as requested in your letter. 

You also wished to alert us to the need for increasing the pole wall thickness 

when the height is raised. This causes us some concern, as the crash tested 

hardware found acceptable by way of LS-25 weighed 409 kg (902 pounds) ~This 

approaches the 454 kg (lOOO-pound) maximum mass we have set for breakaway 

luminaire supports to control the risk to vehicle occupants from a pole's 
2

falling on a vehicle's roof. We will not sanction pole plus mast arm plus

luminaire masses in excess of 454 kg (1000 pounds) without automobile crash

tests to confirm their acceptability.

Sincerely yours,

Lawrence A. Staron

Chief, Federal-Aid and Design Division

Supplement to Geometric and Roadside Design Acceptance Letters

LS-16 and LS-25