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Sorry to bombard you with questions recently, but we are working on a statewide “worst-case” foundation design for cable barrier end anchors that will be used for each proprietary system regardless of the soil conditions. We are planning to develop both a block and shaft design, and when determining the maximum tension load, we want to assume a TL-3 impact when the cables are -10F. Determining the temperature induced load is straightforward, as the manufacturers provide tables with the tension per cable at each temperature. But I am trying to determine what we would us for the momentary vehicle impact load. We found this 2010 report on Foundation Design for High Tension Cable Guardrails posted on MwRSF’s website, and I am struggling to decipher what impact load was assumed in their calculations. They used 40 kips total to account for both, and state that the temperature and impact loads are approximately equal, but they state that the temperature induced load is 28 kips earlier in the report...

When I looked in AASHTO LRFD 10^th edition, table A13.2-1 shows design forces for each test level for beam and post railings. Can we assume that these loads would also apply when analyzing HTCB, a flexible barrier system? If yes, then could I assume the 18 kips for the longitudinal force generated by a TL-3 impact (I assume they are using the pickup truck to come up with TL-3 loads…)? Is the 54-kip transverse load relevant in respect to the anchor terminal foundation design or would that be more applicable when looking at the line post foundations?

I was thinking that you all may have looked at this when sizing the foundations for the terminal(s) used in the development of Midwest’s generic HTCB system, and it appears you guys use a 2’x10’ reinforced shaft each time. I found that each test on Midwest’s generic system has included a tension chart and I summarized the results across various pick-up truck tests below. It looks like on average there is about 28.5 kips induced by the impact when the cables are tensioned to 100F. Therefore, should I use ~30 kips for my impact load even though we are assuming a low temperature of -10F….would the tension experienced in a cold-weather impact be different than the MASH test impact? If anything, I would expect it to be less since the cables would likely deflect less, so I am thinking 30 kips may be reasonable and conservative?

Any thoughts or guidance on this is greatly appreciated.

Good questions there, but a lot to unpack.

I will take a stab at things and we can discuss more.

First, you are correct that cable systems have an inherent preload that varies as the temperature changes. The standard preload is usually designed for a cable system based on the tension at its highest expected operating temperature. The cable system should then retain the minimum tension required for the system to function as it gets colder. Often the tension specification will provide a chart for tension based on the ambient temp at the time of installation.

Because of this, the tension in the cables can increase significantly as temperature decreases. Below is an example table for a NYDOT cable system. you can see that the tension in each cable can increase 2.5 times at the coldest temperature.

This tension is inherent in the cable system without an impact event. An impact event then adds to the tension in the system. The 40 kips noted in the report below was referring to the lateral load on the cable. The tensile load may be different than the lateral load.

A better measure of the design loads for the anchor would be the measured tensile loads from testing. You show those in the table you provided. I highlighted that column below. Those loads include the cable preload and the maximum tension developed in all of the cables during impact. The anchor needs to account for both cable preload and impact load as both are applied during an impact. These loads are also system dependent to some degree. Systems other that the pooled fund cable systems may have higher preloads. Additionally, systems with lower deflection will tend to develop higher tensile loads due to the impact. This is because lower deflection requires increased lateral loading. Similarly, development of increased lateral load generally increases the cable tension. In a simple sense, lateral load is developed based on the tension in the cable and the angle developed by the cable as shown below. Flateral is equal to 2TsinΘ. Thus, lower deflections develop lower cable angles and require higher tensions in the cable to develop lateral load. This can also be affect by post strength. The angle that can be developed in the cable is based on post capacity, so stronger posts can help develop higher angles and lateral force. Lots of variables acting together.

In short, design of an anchorage should likely consider both impact load and potential preload concurrently. The impact loads above are a reasonable starting point, but they may vary for other barrier systems. Using those loads, the anchors would need accommodate roughly 30 kips due to impact loading plus whatever preload the systems have. For a worst-case system, one would want to consider a preload based on a the coldest temperature to be conservative.

One upside on the anchor design is that anchor capacity should increase when extreme cold temperatures are prevalent as the soil around the anchor will be frozen and stiffer than normal soil conditions.

Aside from that, there is the question of anchor creep. We have heard reports of lower capacity anchors moving over time due to the preload in the cables alone. Thus, anchorages may need to be designed to limit that behavior as well.

Not sure if I answered all your questions but take a look at these items and I am happy to discuss them further.

Thanks!