Shoring Systems for Structural Collapse - Part 1

Mike Donahue takes a look at a few different shoring systems and not only break down the construction of them, but also covers the principal behind which all shoring systems are based.


Take another look now at the bottom of the uprights or vertical members. You should see pairs of wedges placed between the vertical (upright) and the soul plate. The principal behind these wedges is rather simple. When we build our shoring system, we don’t build it to fit the placement space exactly. It’s rare that you have something perfectly level to shore, which would allow you to build a system exactly to spec. So, we leave off 1 1/2 inches from our measurement to allow us to place the wedges under each upright and tap them together until the system as a whole is firmly in place. Once we’re satisfied, we’ll take a 16D nail and toenail it behind each set of wedges (both sides) to prevent any possible slippage. When taking measurements for the verticals, you must subtract seven inches if using 4x4 lumber and 11 inches for 6x6 lumber from the total height measurement to allow room for the sole plate and the header.

Remember, all of our shoring must be perfectly plumb once they’re set in place and we need a plumb vertical path for our load to be transferred through. Here’s an easy way to understand this. Take a piece of cribbing and place it vertical, perfectly straight up and down. Now place your palm on top of the ruler and apply a good amount of pressure. The cribbing should not move. Now perform the same exercise, but place the cribbing out of plumb, the cribbing should kick out under the pressure due to the fact that there was no straight path of travel for our load force to follow. It’s a simple demonstration, but it really does a great job explaining this concept.

Lace Post

If you take a close look at a Lace Post shore (see Figure 4), it’s actually two vertical shores laced together. Lacing can be constructed by using 2x4 lumber or 2x6 lumber. The vertical lacing will help to counteract forces that could cause the system to rack (collapse forward on itself). The cross bracing you see helps counteract any torsional loads/forces that are placed on the system. The cross bracing should be installed so that whichever side of the shoring system you look through it forms an “X”. By doing this, you give the system equal torsional resistance on all four sides. Lace Post shoring systems are one of, if not the, strongest shoring systems you can build and are often used to support heavy loads such as a floor in a parking deck. Of course, based on the size load you’re supporting, you may need to build a few to cover the desired area.

Throughout this article you’ve read about gusset plates, vertical bracing and cross bracing. Installing these features is not as simple as throwing some nails in the wood and calling it a day. Everything we nail has a pattern assigned to it and for good reason. Engineers did testing on shear load ratings on different size nails, and strength ratings on the wood based on type, psi, and its ability to retain its integrity when nails were placed in it. Through that testing, these nail patterns were developed. Here are a few:

 

 

 

 

  • 2x4 lumber – three 16D nails @ 120 psi
  • 2x6 lumber – five 16D nails @ 120 psi
  • ¾-inch plywood gusset plate 6? x 12? – four 8D nails @ 120 psi
  • 12x24 top gusset plate used in a Double “T” shore – 17 8D nails horizontally and five 8D nails vertically @ 120 psi
  • 12x24 midpoint gusset plate used in a Double “T” shore – eight 8D nails on each upright @ 120 psi

 

All the shores will have subtle changes as the height increases, such as brace placement or the dimension of lumber used. Engineers have developed what is called an LD Formula. This formula tells us how long a piece of lumber can be and still accept the load safely. A great example of how this works on a small scale is to take two wooden rulers and cut one at the six-inch mark and leave the other a foot long. Place the foot-long ruler vertical and with your hand apply force to the top. You’ll see the ruler bow and flex. The load you’re applying clearly exceeds the load-carrying capabilities of the ruler. Now perform the same experiment on the six-inch piece. That piece should be able to carry the load you’re applying. On a small scale, one conforms to the LD Formula one does not. Here is a breakdown of the LD Formula.

Engineers work with two numbers ¬– 25 and 50, with 50 being the high end of the scale, the number that lives on the edge and the number that will maximize the allowable vertical height and still operate safely. For this example, we’ll use 25. That will give us the length of a 4x4 at its peak load-carrying capacity.

25 x 3.5 (dimension of a 4x4) = 87.5

 

87.5 / 12 = 7.291... Let’s just say eight feet.

 

A 4x4 at eight feet in length has a capacity of 8,000 pounds. That same 4x4 with a length of 12 feet (max. length) has a capacity of 3,500 pounds. If your verticals need to be 12 feet in height, you are better off using 6x6 lumber. A 12-foot piece of 6x6 lumber has a vertical capacity of 20,000 pounds. That’s a big difference.