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
|
FIGURE 7.22 Multiple-mount sign support. |
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.
