Chapter 4

Slide1: 

You’ll cover the concept of deflection — what it is and how guardrail is designed to provide it — in this chapter. You’ll also cover the basics of guardrail placement in this chapter. The chapter’s topics are: Guardrail Placement Guardrail Stiffening Soil Backing Deflection & Placement

CHAPTER 4 – OBJECTIVES: 

CHAPTER 4 – OBJECTIVES You will be able to perform the following tasks after finishing this chapter: Define deflection Describe proper methods of guardrail placement Give reasons for those placements

PLACEMENT OF GUARDRAIL: 

PLACEMENT OF GUARDRAIL Shy Line Deflection area Guardrail on Slopes Location on shoulder GR Installed at Curb Length of need

SHY LINE: 

SHY LINE Definition: The distance from the edge of the traveled way, beyond which a roadside object will not be perceived as hazardous and result in a motorist’s reducing speed or changing vehicle position on the roadway.

Slide6: 

This picture shows several things. First and most obvious is the transition from Cable GR to a strong post system. As you look at the picture the strong post system is flared at a given ratio according to the Standards for Cable Guardrail terminals. EP Shy Line GR Between the yellow dashed line and the red and orange dashed lines, this is the flare ratio Flare Rate What is the flare ratio?

FLARE RATE: 

Flare the guardrail system away from the traveled way as far and as quickly as possible. Two criteria, however, must be met when designing the flare: The flare rate is limited to the values shown in the above table in order to keep the angle of impact with the guardrail from being too severe. The values are based on vehicle speed and the stiffness of the guardrail system. The guardrail should be flared only if it is on slopes of 10:1 or flatter. FLARE RATE

DEFLECTION DISTANCE: 

DEFLECTION DISTANCE Definition: The distance guardrail will bow when hit.

GUARDRAIL ON SLOPES: 

GUARDRAIL ON SLOPES Cable GR can be placed anywhere on 6:1 or flatter slope W-Beam: Not Placed on Slopes steeper than 6:1 On Slopes between 10:1 and 6:1, must be at least 12’ from edge of shoulder

GUARDRAIL HEIGHT RELATIVETO EDGE OF SHOULDER: 

GUARDRAIL HEIGHT RELATIVE TO EDGE OF SHOULDER 1. When front slope is flat (10:1 or flatter), measure height from groundline. 2. When guardrail is installed on slopes 6:1 or flatter, within 2 feet of the hinge point, measure height from the shoulder slope extended. 3. When front slope is steeper than 6:1, face of guardrail must be no further down the slope than the hinge point. Backslope Hinge point : the point where the roadside cross section changes from one cross-slope to another, such as from the shoulder cross-slope to the frontslope. Frontslope Hinge Point

RECOMMENED GUARDRAIL PLACEMENT FOR OPTIMUM PERFORMANCE: 

RECOMMENED GUARDRAIL PLACEMENT FOR OPTIMUM PERFORMANCE Do not place rigid vertical objects within the deflection distance. Barrier Deflection Distance to be measured from back of post to the rigid object. 3 Feet - for Strong Post (St’d. GR-2) 7 Feet - for Weak Post W-beam (St’d. GR-8) 11 Feet - for Cable (St’d. GR-3)

GUARDRAIL LOCATION & DEFLECTION(STD. GR-INS): 

GUARDRAIL LOCATION & DEFLECTION (STD. GR-INS) Offset multi-directional breakaway devices (as opposed to bi-directional slip bases) beyond the deflection distance whenever possible. Use Std. MB-7F if a 2 foot minimum depth cannot be obtained.

GUARDRAIL LOCATION & DEFLECTION: 

GUARDRAIL LOCATION & DEFLECTION This overhead sign support is located within the 3 foot deflection distance of this GR-2 strong post guardrail. Because the sign support is a fixed object and is located within the 3 foot deflection distance of this GR-2, the system will not properly redirect a vehicle when struck.

GUARDRAIL LOCATION & DEFLECTION: 

GUARDRAIL LOCATION & DEFLECTION Similarly, this sign is within the 7 foot deflection distance of this GR-8 weak post guardrail. This sign may become a hazard to the vehicle and its occupants if this portion of the guardrail is struck. 7 FT.

STIFFENING GUARDRAIL TO DECREASEDEFLECTION (STD. GR-INS): 

STIFFENING GUARDRAIL TO DECREASE DEFLECTION (STD. GR-INS) The guardrail system must be stiffened in front of as well as upstream of the obstacle being protected if the dynamic deflection distance can't be achieved. For two-way traffic, use 8 post spacing design from each end of fixed object. 8 Std. GR-2A Post Spaces before Fixed Object (25'- 0") Roadside Obstruction 25' min. 3' - 11/2" Typical Spacing 6' - 3" @ 6' - 3" DOWNSTREAM – Same as “run-off”. The leave side of a feature or trailing end relative to traffic. UPSTREAM - Same as “run-on”. The approach side of a feature relative to traffic. TRAFFIC Downstream(Run-off side) Upstream (Run-on side) -NEXT-

Slide20: 

Stiffen the guardrail by decreasing post spacing or by double nesting of rail elements. Each stiffening method typically halves the amount of deflection. Begin stiffening 25 feet in advance of the hazard (roadside obstruction), and continue at least to the end of the hazard. The stiffening method may be discontinued 25 feet downstream beyond the obstacle’s beginning when the hazard is a solid obstacle and pocketing is not possible within the obstacle’s length. METHODS USED TO STIFFEN GUARDRAIL TO DECREASE DEFLECTION

SOIL BACKING (STD. GR-INS): 

SOIL BACKING (STD. GR-INS) Weak Post Cable System Shoulder Recoverable Area – 6:1 Slope 1'- 2' The primary purpose of the posts is to hold the cable at the proper height. A guardrail system's redirectional capability is developed through the tension in the cable. The posts can absorb some energy through bending upon impact. Soil backing behind the posts is needed to ensure support. The standards require that the back of the post be 1- 2 feet in front of the hinge point for the recoverable area for new construction. Use your imagination this is cable rail.

SOIL BACKING (STD. GR-INS): 

SOIL BACKING (STD. GR-INS) Strong Post Systems The strength of the strong posts contributes considerably to the guardrail’s redirection capability. It’s necessary, therefore, to develop adequate soil support for the post to prevent its being pushed backwards upon impact. 1- 2 feet of soil should be provided behind a post. Use extra long posts, with lengths of 7 to 8 feet, instead of the standard 6 foot long posts if at least 1 foot of soil support can’t be provided.

SUMMARY“GUARDRAIL LOCATION”: 

SUMMARY “GUARDRAIL LOCATION” Point of measurement for guardrail height:

SUMMARY“GUARDRAIL LOCATION”(cont.): 

SUMMARY “GUARDRAIL LOCATION” (cont.) Dynamic deflection to a vertical rigid object: Dependent on the guardrail system stiffness Multi-directional breakaway devices (such as signs and luminaires) within deflection distance will not significantly affect performance Slip base is not multidirectional

SUMMARY“GUARDRAIL LOCATION”(cont.): 

SUMMARY “GUARDRAIL LOCATION” (cont.) Soil backing: Strong post system (Std. GR-2, 2A) uses soil pressure to resist deflection Desirable to have 2 feet backing behind post If less than 1 foot of backing, use longer post (7 to 8 feet)

3 SELF-CHECK QUESTIONS: 

3 SELF-CHECK QUESTIONS 1. Name a guardrail system characteristic that contributes to the system’s “dynamic deflection.” 2. Hazards and obstacles should be _____ the system’s deflection distance. 3. For strong post systems, _____ to _____ feet of soil support should be provided behind a guardrail system. Use _____ posts if you can’t provide the minimum soil support.

ANSWERS: 

ANSWERS 1. Name a guardrail system characteristic that contributes to the system’s “dynamic deflection.” Characteristics contributing to dynamic deflection include tension in the rails or cable and the strength and spacing of the posts. 2. Hazards and obstacles should be outside the system’s deflection distance. 3. For strong post systems, 1 to 2 feet of soil support should be provided behind a guardrail system. Use longer posts if you can’t provide the minimum soil support.