By Mike Donahue
For this month’s article, I wanted to take a look at sloped floor shoring systems. Shoring systems can look complicated purely because of the number of components and steps involved in their construction. Remember the simple concept called the double funnel principle? The double funnel simply represents transferring the captured load from point A to point B, with point B representing the ground. Just because one shoring system looks different from another doesn’t mean it’s performing a different job. The theory in constructing a sloped floor shoring system isn’t any different from constructing a raker shore; it’s based purely on taking a load from point A to point B. If you can understand this concept, you’re already halfway there in understanding the sloped floor shoring system. Let’s take a look at a type 2 and type 3 sloped floor shoring systems.
There are two only minor differences between type 2 and type 3. The first is the load they’re designed to support. The class 2 system is designed to support a floor that has a load that is not only pushing down on the floor but also pushing it horizontally. The horizontal load, in this case, would be debris from the collapse.
The second difference is the position of the uprights. Figure 1 shows the uprights at an angle as opposed to being “plumb” (straight up and down). This is because the shoring system is seeing a horizontal load from the debris. If the uprights were plumb, the horizontal load force would attack the uprights, pushing them forward and causing the sole plate connections to fail followed by the top plate connections.
Now, you’ve always been taught that when it comes to constructing shoring systems, “plumb” is the word to live by. Why? The system needs to be plumb to transfer the captured load into the ground. As a refresher, take a look at Figure 2, which shows how the supported load would travel if an upright was out of plumb. If you are looking at the picture of the class 3 sloped shore and saying, “Well, those uprights certainly aren’t plumb,” you are correct. However, in relation to the load and force directions, they’re positioned to provide a straight path to the ground. All of the other components, much like any other system, work together to resist applied force and act as a guide to steer the load down into the ground.
For instance, when a load is traveling, it’s following the path of least resistance. Find a wall that ends prior to an accent wall and stand next to it, pressing your weight into it. Now, walk forward, keeping the pressure applied. When you pass the end of the wall, the applied weight will find its path and thrust forward. Think of the wall as the header, the point where the wall ended the upright. The force you’re generating is being transferred to the wall and is constantly looking for a place to go. The end of the wall is a new path for the load.
In Figure 3, you can see multiple brace points as well as tie-in points. In shoring, you’ll use two means to connect one piece of lumber to another. One is a gusset plate, and the other is a cleat (Figure 4).
Like any other shoring system, you need to protect it from lateral force (side to side)—better known as racking—and torsional forces, which is a twisting force. All shoring systems have been engineered to meet what is referred to as “earthquake standards.” An earthquake can easily apply lateral, torsional, and additional compressive forces. Because the shoring system is under a load and cannot move and because of the resistance it provides, these forces can be detrimental.
The cleats and gusset plates provide a strong connection between the header and sole plates. The lateral and cross bracing provide resistance against lateral and torsional forces. If you look on top of the sole plate just behind the upright, you’ll see another cleat called the bottom cleat. This cleat prevents the upright from moving backwards and receives the load and transfers it into the sole plate. As I described earlier, the load force wants to find a path with no resistance. When it hits this cleat, its path changes because of the resistance and travels in a downward direction.
Last, we have a thrust block (Figure 5). Thrust blocks are needed in shoring systems that have a horizontal force pushing against them, similar to the raker shore. Something needs to act as the class 3 slope shore to create a dead end for the force to find and be directed into the ground. Remember the double funnel principle, which transfers the load from point A to point B (the ground). The thrust block is generally a six- × six-inch piece of lumber that is “pinned” (drilling holes into it and driving the rebar through the holes and into the ground). Notice the placement of the rebar; there is one on either side of the sole plate. Doing this creates a strong target point for the load to hit. It will also help protect the thrust block from lateral and torsional forces.
Now, construct this system on soft ground; you’ll need to create something solid for the sole plate in which to transfer the load (Figure 6). Reason being let’s just say your system is supporting 5,000 pounds without something for the sole plate to sit on and spread the load out you’ll be concentrating 5,000 pounds on a footprint of 5½ inches (six- × six-inch lumber). The sole plate will be crushed into the ground and, based on how the earth conforms to the sole plate, will bend because of the way the load has settled it into the earth. Simply place three or four two- × six- × 18-inch cleats next to each other perpendicular to the sole plate. This will spread the load out over a footprint of 16½ inches to 22 inches. Both ground pads will be located directly underneath the uprights.
Now, look at the second slop floor shore; the type 3 you see shows very little difference. This type is used when there isn’t any load (debris) resting on it that can move upward or downward. You should notice the uprights are plumb and we’ve done away with the cleats located on top of the sole plate. The reason for this is that the load force is now being transmitted plumb, vertical, and into the ground. Slightly different shore, same double funnel principle.
When all is said and done, all loads will travel vertically thanks to gravity. Your job is to transfer these loads into the ground following the path gravity provides. The force of gravity is unavoidable and unbeatable. You can, however, join forces with gravity and allow it to take out supported load where it needs to be…the ground.
Until next month stay safe…stay progressive.
Mike Donahue has 17 years of fire service experience and has been a career firefighter in the city of Elizabeth, New Jersey, for the last 13 years, working out of Rescue Company 1 for the past 10 years. Mike teaches a Middlesex County College as an adjunct professor and acts as the fire service program coordinator. Mike is the owner of Progressive Rescue and can be reached at [email protected].