Technical Manual

Inc.

p 293

Chapter 4

Miscellaneous Structural Support Products

CHAPTER 4

MISCELLANEOUS STRUCTURAL SUPPORT PRODUCTS

material, thus resulting in a crack. If the top and

bottom of the wall are considered laterally fixed

and the soil loads are modeled using equivalent

fluid pressure, a designer could calculate how

much lateral force needs to be applied from the

interior side of the wall at the elevation of the

crack to reverse the bending in the wall. This

mathematically results in compressive stress

on the interior face and tensile stress on the

exterior face. With compressive stress on the

interior face, the horizontal crack would be

expected to close. In practice, however, if this

calculated resistance force were to be applied

per the described analysis, the crack will likely

remain. Even if this applied resistance force

exceeded the calculated value by several orders

of magnitude, the crack will likely remain. What is

actually happening is the anchor forces applied

at the interior are tending to impose translational

displacement of the wall and the soil. The soil

is resisting movement, and the force it exerts

on the exterior of the wall (passive resistance)

increases to match the forces applied on the

interior face of the wall. The equivalent fluid

pressure assumption has therefore led us to an

erroneous solution.

There are other phenomena that are difficult to

explain with an equivalent fluid pressure model.

One would expect the aforementioned wall crack

to appear at the point of maximum bending. The

crack often appears at a higher elevation than the

equivalent fluid pressure model would predict.

In fact, it can be very close to the elevation of

the exterior grade. In this case, frost is often the

culprit. Although a true water table is most often

not present, there can be significant soil moisture

near the surface. When frozen, this can exert very

large forces on the wall resulting in a horizontal

crack much higher on the wall than an equivalent

fluid pressure model would predict. These forces

are not only large, but difficult to quantify.

The second item that is often misrepresented

in analysis is the load in the anchors. Many

designers will attempt to treat the anchors as

reaction points in their analysis. They treat

the wall as being laterally fixed at the top and

bottom and with a third “support” at the anchor

location. They then model a gradient load to

represent equivalent fluid pressure. This is now a

statically indeterminate structure which requires

a more complex analysis. Unfortunately, this

effort is wasted for a couple of reasons. The first

reason is that, as has already been discussed,

the equivalent fluid pressure model will yield

erroneous results. The second reason is that

the load in the anchor should already be known

because it is actually an applied force and not a

calculated reaction. Anchors are supplied with

a threaded rod and nut with wax typically used

as a lubricant. The nuts are tightened with a

torque wrench to a specific torque value. This

installed torque relates directly to tension in the

rod. There is no need to calculate the load in

the anchor since it is already known with the

application of torque.

Some designers recognize the wall anchors as

applied forces but still find it necessary to treat

them as calculated reactions. They are aware

that the forces that are applied at the anchors

are far in excess of those that will be calculated

as reactions. The designer may then make a

comparative rationalization demonstrating that,

for example, the force applied at the anchor is

twice as much as a middle support would offer

and therefore, the anchor system should be more

than adequate. Once again, the soil pressure

distribution exerted on the wall after the anchors

are installed will bear little resemblance to an

equivalent fluid pressure gradient so applying

twice as much force than a fictional calculated

reaction value really serves no purpose.