Category HIGHWAY ENGINEERING HANDBOOK

There are three basic types of hinge designs. One type, illustrated in Fig. 7.30a, develops a hinge by cutting through all but the back flange. The front flange is connected with a slotted plate known as a friction plate. When the post is struck, the friction plate sepa­rates from the slotted bolt holes as the back flange bends. This type of hinge creates a maintenance problem, since the post is destroyed and must be replaced after each impact. It is also more difficult to predict the resistance of the hinge, which is depen­dent upon the post size and depth of cut.

Another type (Fig. 7.30b) utilizes a rear hinge plate. This plate is similar to the fric­tion plate but does not have slotted bolt holes. With this type of hinge, the sign support is completely cut in two pieces, with the hinge plate bolted on the back and the friction plate on the front. When impacted, the friction plate releases through the slotted bolt holes and the hinge plate bends back. Maintenance after impact is simplified, since the hinge plate can be removed and the upper and lower support pieces reused with a new hinge plate. Proper operation of the friction plate design is dependent upon proper bolt size and torque. If the bolts are too small, or not torqued sufficiently, wind loads will cause the friction plate to become loose and the top of the sign to fall back. If the bolts are too large or torqued too much, the support will not separate properly upon impact [45].

The third hinge type (Fig. 7.30c) utilizes a rear hinge plate and a front hinge plate with a weakened section. When impacted, the section fractures through the plane of the holes, thus permitting the back hinge plate to bend. This design has an advantage over the friction-hinge plate design while remaining easy to repair. The advantage is that the torquing requirements on the friction plate are not critical for proper operation. The front hinge plate in Fig. 7.30c is weakened by drilling holes so that only 33 percent of the plate material remains. Figure 7.31 shows commercially available frangible hinge plates available from Transpo Industries. The three hinge systems presented in Fig. 7.30 are unidirectional and should not be used in areas requiring bidirectional performance. Only the Transpo hinge system offers bidirectional performance.

Inclined slip base designs, commonly used for single sign supports, cause the sign to rise up upon impact and allow the vehicle to pass beneath the sign. In a multiple-sign- support system, each support is fastened to the other supports by the sign panel and any existing sign panel bracing. When an inclined slip base is used on multiple-support assemblies and only one support is struck, the sign panel stops the impacted support from moving upward. As a result, the slip base can become locked, or the sign panel torn from the other supports, causing intrusion of the panel or support into the vehicle. Inclined slip bases should be used only for multiple-support assemblies when all supports are within 6 ft (1800 mm) of each other. The horizontal slip base (Fig. 7.25) and the frangible coupler (Fig. 7.21) are the most frequently used designs for multiple-support systems. The horizontal slip base design, details of which are shown in Fig. 7.26, operates through separation of the top plate from the anchor plate.

Frangible coupling designs, presented in Fig. 7.27, are designed to effect separation from the anchor plate by fracturing the couplings. Figure 7.27a presents a load con­centration design in which the small cross-sectional area, at the necked-down portion of the coupling, breaks at impact. Figure 7.27b presents a frangible aluminum coupling, by Transpo Industries, designed to break upon impact. The couplings are available in

different sizes, designs, and resistance to fracture. Figure 7.28 presents a frangible coupler application for a large sign support. Notice the low profile of this design. The only portions of the sign assembly above ground level are the frangible couplings, so that the possibility of snagging the vehicle undercarriage is practically eliminated.

Horizontal slip base and coupler designs are intended to safely operate by allowing the vehicle to pass under the sign and support assembly upon impact, as presented in Fig. 7.29. This is accomplished by providing a hinge at least 7 ft (2100 mm) from the bottom anchor plate to allow the support to swing away.

There are few single-support systems that can be buried directly and provide accept­able multiple-support performance upon impact. Two such systems are dual 3-lb/ft (4.5-kg/m) U-channel and dual 4-in X 4-in (90-mm X 90-mm) shaped wooden posts. The majority of single-support adaptations to multiple-support assemblies require the use of anchor pieces and breakaway designs. Triple supports consisting of 1.75-in X 1.75-in (45-mm X 45-mm) square perforated tube and triple 2.5-lb/ft (3.7-kg/m) U-channel

NOTE:

RECOMMENDEO TORQUE

ON SUP BASE FLANGED

HEAD BOLT & NUT IS 54 Nm

KEEPER

PLATE

760

kzoo-э!

FIGURE 7.23 Acceptable slip base breakaway device for multiple-square-tube sign assemblies. Dimensions shown as mm. Conversions: 200 mm = 8 in, 760 mm = 30 in,

54 N-m = 40 ft-lb.

are acceptable when installed with an anchor and breakaway design. Manufacturers are developing devices that enable the use of heavier supports for acceptable multiple – support systems. Figure 7.23 presents a slip base breakaway assembly for square-tube supports manufactured by Unistrut Corporation, which is acceptable for three 2.5-in X 2.5-in (64-mm X 64-mm) supports within a 7-ft (2100-mm) path [32]. The bottom subassembly is inserted into a 30-in (760-mm) anchor piece and placed in an 8-in-diameter (200-mm), 30-in-deep (760-mm) concrete foundation.

Multiple supports for large signs are often constructed as slip base designs with galvanized steel wide-flange (W) or American Standard (S) shapes for the sign support. These shapes, depicted in Fig. 7.24, are designated by their depth and unit weight or mass. For example, a W150 X 18 is a wide-flange shape with a depth of 6 in (150 mm) and a unit weight (mass) of 12 lb/ft (18 kg/m).

Multiple-support-sign assemblies that are constructed of W and S shapes are fre­quently designed with frangible or load concentration couplers. The behavior of these designs is similar to slip bases except that, instead of the base slipping from between the bolts, the couplers, which are used in place of the bolts, break at impact.

Multiple-mount sign-support assemblies (Fig. 7.22) are required whenever the surface area or width of the sign is too large to withstand the wind and ice loads. Each state has guidelines, in the form of tables and graphs, that are used to select the size and numbers of supports required to withstand the prevalent environmental loads in different parts of the state. These guidelines should be used to determine the required size and number of supports. The design of multiple-sign-support assemblies requires considerations that

in some instances differ from single-sign-support assemblies. These considerations

include the following:

• Tests have demonstrated that vehicles leaving the roadway at an angle can strike more than one support if supports are spaced closer than 7 ft (2100 mm). If two supports are spaced less than 7 ft (2100 mm) apart, they must pass a crash test as a dual support assembly. Installing two acceptable single sign supports does not guar­antee acceptable multiple-support performance.

• For multiple supports, the sign panel itself is an important part of the sign structure during impact. Depending upon the design, the sign panel must carry the weight of the impacted support and/or provide sufficient rigidity to enable the hinge mechanisms to activate. The sign panel must be made of material of sufficient thickness that it does not break into pieces when a support is impacted.

• Acceptable performance in multiple-support systems requires the sign panel to remain attached to the support(s) that are not impacted. This intended performance can be destroyed by:

The use of bolts to fasten the sign panel that are too small

The absence of washers, which allows the bolt head to pull through the sign panel Sign panel bracing that will twist or break and therefore not transfer the sign weight to the undamaged support(s)

• Slip base mechanisms must be designed with the proper sized bolts and washers. Bolts that are too small may not withstand the wind load forces. Oversized bolts can result in binding or friction forces between the base plates. Washers that are too small can deform into the slots and bind the plates together.

• Large multiple-support signs have a hinge mechanism that allows the support to swing upward upon impact. The hinge height should be at least 7 ft (2100 mm) above

• Hinged multiple sign supports are generally designed to operate safely when impacted from one direction (they are unidirectional). They can be made bidirec­tional by selecting the proper hinge arrangement.

• Two posts within a 7-ft (2100-mm) path should each have a mass that does not exceed 18 lb/ft (27 kg/m).

• Supplemental signs or horizontal members between the supports and below the hinge should not be used.

• Multiple-support systems that are designed with anchor bases should have a maximum of 4 in (100 mm) from the ground to the highest part of the anchor. This will prevent small vehicles from snagging the undercarriage on the anchor.

• Selection of unidirectional, bidirectional, or multidirectional support assemblies should be based on the possible directions from which the signs can be impacted. Unidirectional assemblies will not function correctly unless impacted from the front along the longitudinal axis of the slotted bolt holes. Bidirectional assemblies will function properly when struck from the front or the back. Impacts can be expected to occur from both travel directions in all cases except roadside sign supports located on the right side of divided roadways that have wide medians or positive median barriers. Bidirectional or multidirectional support assemblies should be considered for: Signs placed in the median that are within the clear recovery area of the opposite direction of travel

Signs placed on two-lane roadways or undivided multilane roadways

Signs placed near ramp terminals or intersections where impact could occur from

any approach.

The majority of support types approved for use as single sign supports are approved for multiple installation. The approved usage as multiple supports, however, often requires the use of breakaway designs and a limit on the number of supports allowed within a 7-ft (2100-mm) distance of each other. Dual and triple installation refers to installing two and three supports, respectively, within a 7-ft (2100-mm) radius distance of each other. Acceptance of multiple-sign-support systems is based on the same vehicle deceleration characteristics used for single sign supports except that all of the supports within the 7-ft (2100-mm) path are impacted. Selection of approved multiple sign supports, therefore, requires knowledge of the number of supports required and the associated systems approved for use by FHWA.

Multiple-sign-support assemblies are required for signs with large surface areas but also for wide signs. For example, a guide sign 5 ft X 2 ft (1525 mm X 610 mm) has a small sign area but will need more than one support to prevent the sign assembly from being damaged due to environmental loads. Sign panels that have relatively small surface areas but require multiple supports because of their shape can generally use two smaller- size supports than would be required if they were installed with a single support.

Acceptable single-sign-support performance can be achieved with the use of frangible couplings and load concentration couplers (Fig. 7.21). These couplings are either fabricated from die cast aluminum or extruded from an alloy. The couplers are used as inserts that bolt the support post plate to the anchor piece plate. They present a weak point on the sign-support assembly that fractures upon impact. The majority of applications for frangible couplings are for multiple sign supports. These couplings are discussed more fully in Arts. 7.5.2 and 7.8.2.

7.4.1 Considerations in Design of Slip Bases

Failure of slip base designs to release properly can be due to the bolt torque, the gauge or thickness of the keeper plates, or the weight of the support. The following should be adhered to in the design of slip base supports:

• Horizontal and inclined slip bases can be constructed with wide-flange, standard – shape, and round signposts. Multidirectional designs are usually constructed with round signposts to enable the multidirectional rising action of the lift cone.

• The post should not weigh more than 45 lb/ft (67 kg/m), and the total weight of the support post, hardware, and sign panel should not be more than 600 lb (270 kg).

• The bolts clamping the top and bottom portions of the slip base together should not be tightened more than the specifications. Overtorquing creates high friction between the slip base elements and may prevent the post from releasing properly. The clamping force must be controlled by installing the bolts with a torque wrench, using torque-limiting nuts, or using designs that are not dependent upon specific torque requirements.

• Washers used with the clamping bolts must be of sufficient strength to prevent the washers from deforming into the plate slots when the bolts are tightened to specification.

• The stub height must be no more than 4 in (100 mm) above ground level at the highest point of the slip plate assembly.

• No bolts from the anchor piece should project into the upper support assembly.

• The choice of the proper sign-support type for slip base designs is dependent upon the wind load, sign panel size, and the criterion that the weight of the sign support and sign not exceed 600 lb (270 kg). As a general rule, the maximum sign area presented in Table 7.7 can be used in selecting the appropriate size of wind sign support. Determining the maximum sign size for areas with high wind loads, or for the selection of post sizes other than round shapes, should be performed with reference to state requirements.

The multidirectional slip base design operates on the same principle as the inclined slip base design. The multidirectional design consists of a triangular slip base employing

only three slotted bolt holes, as presented in Fig. 7.20. The bolts are positioned at the flattened corners of the triangular plate. An impact from any direction slides the bolts out of the slots and allows the signpost to separate from the anchor piece. The desired lifting action is obtained by a lift cone located on the bottom plate. The sign support is tubular and beveled at the top triangular slip plate to help the lift cone push the support off the anchor plate during impact. The anchor piece is encased in concrete to prevent

movement. The pipe generally used for multidirectional slip bases ranges from 3 to 5 in (75 to 127 mm) in diameter. The design specification for each size must be checked, since the required bolt size, torque requirements, and lift cone design are dependent upon the size of the sign support.

Unidirectional slip bases for small sign supports consist of inclined slip bases, as shown in Fig. 7.18. The upper support piece is made from rolled-steel shapes, standard pipe, or structural tube. The base of the support assembly is inserted into a concrete footing to prevent movement of the anchor assembly.

The upward thrust obtained from the inclined slip base design is important to the proper action of a single-support sign system. The upward thrust causes the sign panel and support to rise and rotate when vehicle impact separates the mechanism. The sign panel and support stay together as a unit, which passes up and over the vehicle and lands behind it. This action is obtained only when the support is impacted from one direction. An impact from the opposite direction actually pulls the sign support down­ward, causing the support and sign panel to rotate toward the vehicle. Inclined slip bases should not be used where impact from more than one direction is expected. Horizontal slip bases will separate when impacted from the front or the rear but will not provide the uplift capability obtained from inclined-base designs. A typical design for an inclined slip base is provided as Fig. 7.19.

Horizontal slip bases, discussed in Art. 7.5.2, are not recommended for single sign supports. When impact can be expected from more than one direction, a multidirec­tional slip base design should be used.

Slip base designs for small sign supports consist of two components: (1) the anchor assembly up to the bottom of the slip base, and (2) the sign support, containing the top of the slip base on the lower end and the sign panel on the upper end. Small sign slip bases are categorized as unidirectional or multidirectional.

Slip base designs allow the use of stronger sign supports than can safely be achieved by base-bending or fracture designs. The anchor piece of slip base designs is fixed into a foundation and should remain immovable during an impact. The sign sup­port is connected to the anchor piece with bolts through a plate, which are attached to a similar plate on the anchor piece. The holes in the plates are slotted. When a vehicle impacts the sign support, the top plate, which is attached to the sign support, slides along the bottom plate until the bolts slide free of the slots. Inclined slip base designs, or designs with raised center cones, cause the sign support to move upward to allow the impacting vehicle to pass under the sign without being hit on the windshield by the sign during high-speed impact.

When slip base designs were first used, problems were encountered with assemblies that came apart without an impact. This was due to the wind and ice loads vibrating the assembly and causing the bolts to “walk” out of the slots, as in Fig. 7.17. This problem was solved by using a thin (0.04 to 0.02 in or 1.0 to 0.5 mm) keeper plate to ensure that the bolts remain properly located in the slots. During an impact, the bolts tear through the thin keeper plate as they slide free of the slots.

Steel-pipe posts are frequently used in urban areas and have the advantage of being readily available. They require special fastening hardware, and an earth plate when directly embedded, to prevent the post from rotating from its intended position. Standard steel pipe, schedule 40, galvanized, is readily available from plumbing supply wholesalers. The maximum sign panel areas that can be mounted on the 2-in – and 2.5-in­internal-diameter (51-mm and 64-mm) standard steel pipe are listed in Table 7.6.

Round steel supports, made from standard schedule 40 pipe, that have an internal diam­eter (ID) of less than 2 in (50 mm) can be embedded directly into the ground to a depth of at least 42 in (1070 mm) and provide acceptable performance upon impact. A steel earth plate measuring 4 in X 12 in X 0.25 in (100 mm X 310 mm X 6 mm) should be welded or bolted to the pipe to prevent support rotation due to the wind.

Standard schedule 40 pipe, 2-in (50-mm) ID and larger, is no longer approved for direct burial installation and must be installed with a weakening device [31]. A break­away collar assembly is required for standard schedule 40 pipe sizes, equal to or greater than 2-in (50-mm) ID, and also for smaller pipe sizes when the device is likely to be hit. A regular pipe coupling or reducing coupling will provide acceptable breakaway perfor­mance. The use of a pipe coupling will, however, frequently result in damage to the anchor piece. Therefore the reducing coupling is the preferred breakaway device. The anchor assembly consists of a concrete footing, usually 30 in (760 mm) deep by 12 in (300 mm) in diameter and a 24-in-long (610-mm) piece of anchor pipe. The anchor pipe is usually one size larger than the signpost to prevent damage to the anchor and to allow use of the reducing coupling. The possibility of damage to the anchor post can be further reduced by embedding the reducing coupling halfway into the concrete footing.

Round steel tube, in wall thicknesses of 12 gauge or less, can be used with anchor sys­tems instead of standard schedule 40 pipe. These tubes are available from a number of manufacturers. Southwestern Pipe Inc. is one tubing manufacturer that also markets the Poz-Loc anchor system. This system consists of a tubular anchor socket with 2.5-in (64-mm) ID and 27 in (686 mm) long constructed of 12 gauge steel. The socket is pointed to facilitate driving into the ground and accepts a 2-in-ID (50-mm) steel round tube as the sign support. The sign support is held in place by driving a post wedge between the socket

TABLE 7.6 Maximum Sign Area for Standard Steel-Pipe Single-Support Posts

a. Area in U. S. Customary units for 70-mi/h wind, ft2

Internal diameter post size, in Maximum sign area, ft2

2.0 6.5

2.5 11.8

b. Area in SI units for 113-km/h wind, m2

Internal diameter post size, mm Maximum sign area, m2

0.6

1.1

Square steel-tube sign supports are used in many localities. They provide four flat sur­faces for mounting sign panels, facing different directions, without special hardware as required by some support types. Square-tube supports can be purchased from a number of manufacturers and are available with %s-in (11-mm) holes or knockouts at 4-in (25-mm) centers on all sides [26, 27, 28]. The square tubing is available in!4-in (6.4-mm) incre­mental sizes from 1.5 in X 1.5 in (38 mm X 38 mm) to 2.5 in X 2.5 in (64 mm X 64 mm). Maximum sign areas for various square-tube sizes and strengths are illustrated in Table 7.5.

Square tubing can be driven directly into the ground using a drive cap with sledge or power equipment. The performance of the support assembly upon impact, and sub­sequent repair, are enhanced by using an anchor base. Three common methods of installing a single square-tube sign support are presented in Fig. 7.13. Figure 7.13a shows a direct burial installation. Square tube up to a maximum size of 2.25 in X 2.25 in (57 mm X 57 mm) has been approved for installation by direct burial. The perfor­mance of square-tube sign supports upon impact is more predictable, and easier to repair, by the use of an anchor base system [29]. Figure 7.13c shows an anchor base system where a 36-in-long (900-mm) piece of square tube, one size larger than the anchor piece, is driven into the ground. This anchor piece is left protruding 1 to 2 in (25 to 50 mm) above the ground to permit bolting of the signpost. The signpost is inserted 6 to 8 in (150 to 200 mm) into the anchor piece and bolted in place. Figure 7.13b shows a device similar to the anchor base installation except that it uses an outer stiff­ener sleeve one size larger than the 36-in-long (900-mm) anchor base piece. The stiffener sleeve provides a double-walled thickness that reduces damage to the anchor piece. Upon impact, the post yields at the top of the anchor assembly, normally leaving it undamaged as in Fig. 7.14.

Square steel tubes with perforations on all four sides have been found to provide acceptable crash performance for sizes as large as 2.5 in X 2.5 in (64 mm X 64 mm) when embedded directly into the soil. They are acceptable in both strong and weak soil when embedded to a depth of 48 in (1220 mm). Repairing direct-embedment supports,

however, is more difficult than repairing the yielding breakaway system. The V-loc system from Foresight Industries can also be used as an anchor system for square-tube supports.

Figure 7.15 shows anchor systems for square tubing that are manufactured by Unistrut Corporation. Figure 7.15a shows a heavy-duty breakaway anchor for use with 2-in and 2.5-in (50-mm and 64-mm) square tubes. It consists of a /L-in-thick (4.8-mm) wall that eliminates the need for a stiffness sleeve and allows the signpost to break away on impact without damaging the anchor wall. Figure 7.15b shows an anchor post that can be driven directly into extremely hard or rocky soil conditions. It is made

from 1/4-in X 4-in (6.4-mm X 102-mm) steel angle section that can help stabilize the sign assembly in soil conditions that provide poor resistance to lateral and torsional forces. Figure 7.15c shows a stabilization anchor sleeve that helps adjust for inconsistent roadside gradients. The anchor rods help resist the environmental loads that can cause the signpost to lay over or twist in soft or shoulder dropoff conditions. The stabiliza­tion sleeve is installed over an anchor piece and the two rods inserted at a 45° angle to increase stability.

Figure 7.16 presents a soil stabilization anchor manufactured by Xcessories Squared [30]. The stabilizer is attached with a corner bolt, through the lower slots, to an anchor piece of square tube. The tops of the stabilizer piece and anchor are aligned and the

assembly is driven into the ground until only 2 in (50 mm) remains above the ground surface. After the bottom end of the signpost is inserted 8 in (200 mm) into the anchor assembly, it is secured with a corner bolt from the back side, through the stabilizer, anchor, and signpost.