This month we are going to discuss shoring operations. We’ll take a look at a few different shoring systems and not only break down the construction of them, but also cover the principal behind which all shoring systems are based.
There are probably more than a dozen shoring systems we work with, but the engineering concept behind them all is the same. Yes, that’s right, I said the same. It’s called the double-funnel principal (see Figure 1). The whole concept behind a shoring system is to capture the target load (point A) and transfer it to the ground (point B). It sounds simple, doesn’t it?
While the concept is simple, it takes a lot of training to learn how to construct these systems properly and in a timely fashion. The double-funnel principal is based on transferring a load from point A to point B in a straight line. It all comes down to basic physics. The load is going to go where it wants to go, so it’s our job to direct it where we need it to go, which is the ground.
Let’s start off with the most basic of shoring systems, the crib stack. As simple as it is, there are rules you need to follow.
- The height of the crib stack can be no higher than three times the width of the stack
- The overhang of the lumber should be no more than the diameter of the wood.
- Each layer of the stack needs to line up with its matching layer.
- The stack can be no more than 30 degrees out of plumb.
The weight-supporting capabilities of a crib stack are tremendous. To give you an example, a four-member crib stack built using 4x4 lumber can support 24,000 pounds. The same crib stack constructed from 6x6 lumber can support an impressive 60,000 pounds. Those are some great numbers if you ask me. Cribbing is made from composite plastic/rubber and wood and the minimum length of each piece should be 24 inches. Wood is the material of choice when constructing shoring systems because of its outstanding load-carrying capabilities and the fact that when lumbers exceeds its load-carrying capabilities it will give collapse warning signs, such as creeks, groans and visible crushing on the header and sole plates.
The Double “T” and Vertical Three Post
The Double “T” shore (see Figure 2) has replaced the “T” Spot shore for many rescue teams, purely based on the fact that the load-carrying capabilities are four times that of the “T” Spot (16,000 pounds as opposed to 4,000 pounds at 8 feet high) and it’s more stable than the “T” Spot. Also, the “T” Spot is a temporary shore where as the Double “T” is not.
Let’s now build off of the Double “T” and segue into the Vertical Three-Post shore (see Figure 3). As with all vertical shoring systems, we want to align the uprights (vertical members) with the floor joists above. If you think about it, this makes sense since we’re trying to transfer a load through the shoring system into the ground. One major rule pertaining to these vertical shoring systems is if your shoring operation is being conducted in a wood-frame building, you must not only shore the initial floor your working on, but one floor below. That’s two shoring systems perfectly in line with each other. If your building is concrete, you will need to shore two floors below.
The rectangular pieces of wood nailed to both the vertical and uprights of the shoring system are called gusset plates. These are used to connect the uprights to the sole plate and header. Specific nail patters have been developed by engineers taking into account the shear strength of the nails being used, the psi rating (pounds per square inch) of the lumber, as well as the psi used to nail the plates to the lumber (120 psi).