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We are questioning our own special provisions

Are the below highlighted values reasonable?

This is for the high tension median guardrail being constructed down the interstate. 

The supplier has asked questions regarding the loading design, stating that the system they propose to use is designed to release once a dynamic load is applied.  They want to know if they can design the anchors to a static load (temp) of 9 kips.  They do not specifically indicate that their design has each cable terminated at its own anchor, but they imply it – so, for that case, by our specs the anchors should be designed to resist 20 kips combined static and dynamic load.

So my question is, did we miss something in our spec?  Is holding the contractor to the spec unreasonable? Should we have just specified anchors be designed to resist a static load, and if so, what should that load be?

The contractor is waiting on response.

HIGH TENSION CABLE GUARDRAIL

This project is not considered “Level Terrain” and manufacturers only tested to level terrain

conditions are not eligible for use on this project.

Subsection 902.02 in the Standard Specifications is amended to include the following:

High Tension Cable Guard Rail and Fittings

a. The cable, fittings, anchor rods, plates, turnbuckles, clevises, and tension

spring assemblies shall be hardware as tested to MASH TL-3 by the

manufacturer. A list of approved manufacturers is on the Departments

Qualified Material Vendors List.

b. The anchors should withstand a dynamic and static load (due to

temperature variation) of 31 kips if all cables are connected to a single

anchor or 20 kips if the cables are individually anchored. The anchor

should not be damaged, and anchor movement should be less than 2 in.

under this load.

c. Fitting hardware used to connect the cable ends to the end terminal

and/or end anchors should be properly installed to avoid cable pull-out

from the connector during impact.

d. When a concrete footing is used for the posts, the footing should be

designed to withstand a load higher than the plastic capacity of the post

(i.e., the post would bend before the post footing breaks or moves

significantly).

e. High tension cable systems shall be furnished with cable

splices/turnbuckles as a means of adjusting the tension of individual

ropes at a maximum interval of 1000 feet and at each anchor.

f. Nebraska Licensed Professional Engineer will design all end anchorage

assemblies and intermediate line post foundations in accordance with the

current version and all interims of AASHTO LRFD Bridge Design

Specifications. The Plan will contain all the necessary details and design

information, including required embedment depth, to construct end

anchorage assemblies, line posts, and line post foundations. The soil

parameters used in the design will be based on the borings taken at the

end anchorage assembly locations during the site investigation except

that the soil strengths used in the design will not exceed a friction angle of

35° for the end anchorage assemblies and 30° for the intermediate line

posts for cohesion-less soils or 1000 pounds per cubic foot for cohesive

soils for the end anchorage assemblies and intermediate line posts.

Support or resistance provided by the top 3 feet of soil shall not be

included in the design of end anchorage assemblies.

I am not sure I can give you a direct answer to your question, but I have thoughts and information.

First, I assume that this is a proprietary system. As I don’t know the details of the system in question, it is difficult to determine what the requirements for the end anchorage should be. However, the statement below that the anchors are designed to release under dynamic load would not be true. They may release when impacted on the end of the system as part of a cable end terminal, but the anchors would need to develop significant loads in order to redirect vehicles impacting along the barrier length of need (LON).

The anchor capacity for a cable system would depend on both static preload and dynamic impact loading.

High tension cable systems rely on preload applied to the cable to compensate for temperature swings. As such, they typically have higher static anchor load requirements as comparted to older low-tension systems with spring compensators. For example, if a system utilized 4 cables with a nominal 3,000 lb preload, the anchors would need to be designed for at least 12,000 lbs plus some factor of safety. Additionally, one would need to consider lower temperatures increasing the preload. If the nominal preload was 3,000 lbs at 70 degrees F, one could expect the preload to increase significantly if temperatures dropped well below freezing. This would increase the anchor demand. Potentially more than the 9 kips indicated below.

You would also want the anchors to be designed to minimize the anchor creep due to the preload. We have heard of HT cable systems that have issue with anchor creep if inadequate end anchorage was used.

In terms of the dynamic loading, it would be somewhat system dependent. We have run several instrumented cable barrier tests here at MwRSF. In those tests we have seen dynamic cable tensions exceed 40 kips. Thus, HT cable anchors may need to be designed for anchorage loads at or above 40 kips. We previously did a preliminary study with guidance on end anchor configurations for HT cable barriers. This explains the temperature/preload loading and dynamic impact considerations and provides some anchor designs. See report at the link below.

https://mwrsf.unl.edu/researchhub/files/Report48/TRP-03-236-10%20(MATC-UNL-101).pdf

If this is a proprietary system, the manufacturer should be able to provide guidance relative to the anchorage design for their system as they have details on loading and associated anchor requirements from their designs. However, these may be different for different systems and different from the non-proprietary designs we have worked on at MwRSF.

Based on the above, we would expect anchorage loading requirements to be relatively high. We have designed our anchors for non-proprietary cable systems to comply with these types of loads. However, your best bet would be to contact the manufacturer to determine what their requirements are for the specific barrier system you are using.

Let me know if you have further questions.