Structural collapse: How to integrate timber and mechanical raker systems
Building and combining various types of raker systems during structural collapse incidents
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Structural collapse events involving racked or outwardly leaning walls require crews to erect raker systems. Many technical rescue companies across the country are developing capabilities to confront this challenge through training, equipping and developing a response strategy with their local task force or strike team.
Raker systems are not only complex but elaborate, too. The wall system often requires several pairs of raker systems to be placed.
One of the easiest solutions to develop a rapid and simplified raker capability is the acquisition of mechanical rakers. These systems, although highly efficient, are expensive compared to the timber commonly used to build a raker. This translates to a limited number of mechanical systems that usually need to be integrated with timber systems.
The focus of this article is to provide a snapshot of the various raker systems and factors to consider when combining mechanical and timber rakers. When referencing mechanical rakers, I will be referring to using Paratech Rescue Systems rakers.
3 types of rakers
So, what is a raker? There are three basic types of rakers (Fig. 1):
- Flying (friction) raker: In the timber version, this is a temporary shore that is utilized when debris is piled up at the base of the wall and rescuers need to make quick access or provide quick support. It is temporary because it carries a much lower load capacity than other variations of timber rakers. Mechanical rakers, however, have the same load capacity as their other variations and can be braced in the same manner, providing the same load capacity and utilizing less materials or parts. This makes the mechanical flying raker the preferred application for mechanical assemblies.
- Solid sole (full triangle) raker: This is the most desirable timber raker because of its load capacity and stability. This raker requires a clear and ideally grade ground path to the wall in both the mechanical and timber version. In the mechanical version, there is no real advantage over the flying raker application, and it is heavier, more cumbersome to put into position, and utilizes more materials or parts.
- Split sole raker: This raker is the most appropriate timber selection when soil adjacent to the wall may have uneven grade or a small amount of debris at the base of the wall. Both the solid sole and split sole have the same load capacity.
Building a quality raker system
How are raker systems constructed? Before you begin, analyze the building and calculate the dimensions and configuration for the raker system.
Step 1: Identify the insertion point. The insertion point is a 2-foot zone on the wall where the horizontal flooring or roof system ties into the wall. It may also be the identified generically as the upper target point on a wall where support is required. The top of the zone is at the connection between the wall and the upper floor and the bottom of the zone is 2 feet below that.
When choosing your insertion point, there are many factors to consider, such as obstructions on the wall, available material lengths, wall presentation and integration with mechanicals. As a general rule, though, any point selected within the 2-foot zone is acceptable. We stick to whole numbers in 1-foot increments for the insertion point to keep things as simple as possible. To illustrate this, if the height on the leaning wall of a two-story structure where the second floor ties in is 10 feet, then an insertion point of 8 to 10 feet is acceptable.
Step 2: Identify the configuration. Analyze the wall and the area around the wall to determine which type of raker is optimal: flying, split or solid.
Step 3: Identify the angle. Rakers are most commonly built or erected as either 60-degree or 45-degree systems. However, any angle from 30 degrees to 60 degrees is acceptable. The lower the angle, the more efficient the raker is at resisting the wall forces. The tradeoff to this is lower-angle rakers require more material or parts because they are larger and take up more ground space.
Step 4: Identify the capture point on the ground (Fig. 2). This requires an understanding of basic slope formulas, or rise and run. A given rise (insertion point) has a given run based on the degree of raker selected. For simplicity, we will use the 45-degree and 60-degree formulas as an example. In a 45-degree system, the rise/run ratio is 12/12. This means that for every 12 inches of rise, there are 12 inches of run. Therefore, if the insertion point on the wall is nine feet, then the collection point on the ground is also 9 feet. In the 60-degree example, the rise/run is 12/7. This means that for every 12 inches of rise there is 7 inches of run. If the insertion point is 9 feet, then the collection point is 63 inches (derived from 9 x 7). The collection point is the next step because we need to determine where to establish our waler system to anchor the rakers to the ground.
Step 5: Establish the wale system (Fig. 3). Wale systems vary depending on the type of raker selected. As a general rule, however, they are anchoring points on the ground designed to provide a load point for the rakers to push against and resist the wall force. These are commonly made up of 6 x 6 material that runs parallel to the wall and is pinned to the ground using a minimum of 36 inches x 1-inch steel pickets driven at 12 inches spacing through or behind the wale (6 x 6).
The maximum spacing between each raker system is eight feet. This would mean the pins would have to be driven to accommodate the designated spacing. This can get tricky when integrating mechanical raker systems. There are rules about how many pickets are used and how the wale is established. Additionally, the placement location of the wale will change depending on the type of raker. In a timber system, the collection points on the raker may be a 4 x 4 or 6 x 6 soleplate that extends at least 24 inches past the collection point to accommodate a cleat, as well as any deflection of the raker. On a split sole raker, the collection point for the raker is comprised of a “shoe” that is constructed with an integrated cleat that extends 18 inches beyond the collection point.
When locating the waler systems, you must know where your raker system is going to end in relation to distance from the wall and allow for a pressurizing gap, which will be filled with 4 x 4 or 2 x 4 wedge packs. We typically calculate 3½ inches for 4 x 4 wedge packs and 1½ inches for 2 x 4 wedge packs. For example, if a 60-degree split sole raker is selected with a 9-foot insertion point, then the wale would be placed with the inside edge at 84½ inches. This was derived from the following:
- Collection point = 63 inches
- Additional shoe/cleat spacing = 18 inches
- Additional 4 x 4 wedge pack = 3½ inches
- Total = 84½ inches
Step 6: Calculate the raker and prefabricate the system (Fig. 4). While one crew is working on the wale system, the other crew can construct or assemble the raker system. We already have the insertion point and collection point calculated, next is to calculate the raker length. The actual raker is the mechanical strut or timber segment that connects the wall plate to the soleplate and provides the resistance to the wall forces. The raker will run at the angle we have predetermined.
There are two basic coefficients for the 45- and 60-degree rakers. To calculate the length of the 45-degree raker, multiply 17 x insertion point. For a 60-degree raker, multiply 14 x insertion point. For a 9-foot insertion point, the 45-degree raker length would be 153 inches, and the 60-degree raker would be 126 inches. From this, it’s clear that higher-angle raker systems use more material.
The prefabrication phase can now begin. Measurements are sent to the cut table, all of the required pieces are cut or assembled, and the hammers begin to fly. There are many more specific tips and requirements for building each variation of raker, so it’s important advice to strictly adhere to the U.S. Army Corps of Engineers shoring guides and follow the directions.
Step 7: Erect the raker system (Fig 5). Once the raker system is constructed, it can be walked into position and pressurized utilizing wedge packs, or collars in the mechanical application. The wall plates will also have to be pinned to the wall to create sheer resistance. To complete the system, the rakers have to be tied together through bracing. There are requirements that must be followed regarding the bracing for both midpoints and lateral force resistance, which can be found in the shoring guides.
Building a small raker system
Now, let’s talk about important variables and integration of mechanical systems. First and foremost, mechanical raker systems are much faster, stronger and more efficient. However, you will not have an unlimited quantity of them. The best advice is to focus on placing the mechanical systems first for quick access in such a way that they can eventually be removed and recycled into another location or application. This is accomplished by seeing the “big picture” perspective and building slightly smaller mechanical systems than the projected timber systems.
To walk through this, let’s go back to step one – identifying the insertion point. If the zone for the insertion point is 8 to 10 feet, then always utilize the top or middle of the zone for the timber target. This means an insertion point of 9 or 10 feet will be chosen. If mechanicals are going to be used for the first pair of rakers, this allows room to build slightly smaller, and still be in the zone.
To explain building smaller, as well as illustrate the efficiency of the Paratech mechanical system, we need to walk through both the design and assembly for the system. We will use a Paratech 60-degree flying raker for the example.
Step 1: Use a rail junction: The wall plate has holes in the side that receive a “rail junction.” This is simply a connection piece for the raker to connect to the wall plate. The bottom and top holes are 6 inches in from the edges and the remaining holes are set at 1-foot increments. This allows us to simply count holes without ever pulling a measurement. The wall plates are 6 feet long and can be spliced together to form longer segments. Next, place a rail junction into the top hole of the wall plate.
Step 2: Calculate raker length. If the selected insertion point was 9 feet, use 8 feet for the mechanical, and then calculate the actual raker length: 8 x 7 = 56 inches. Mechanical systems use struts and extensions that can be assembled to varying lengths and are highly adjustable. Simply piece together a strut and extension if necessary that can reach 56 inches around the middle of its adjustability range. Connect this assembly to the rail junction.
Step 3: Establish the brace and control the angle. The connecting element that goes from the lower portion of the wall plate to the raker strut must be a brace. This is a different component than the strut, as it resists both tension and compression. In this example, simply select a two to 3-foot brace with another rail junction on one end and a “clamp and clevis” on the other end. A clamp and clevis is a connecting piece that can be attached to any point on the raker strut assembly.
When implementing the brace, simply count the holes from the top rail junction down and use the 12/7 rise/run ratio to maintain a 60-degree configuration. This is accomplished by coming down five holes, which equates to 5 feet. Place the lower rail junction in that hole. Put the brace into the rail junction, attach the clamp and clevis to the other end of the brace and then bring the brace into a perpendicular position to the wall plate. Stroke the total brace assembly out until it is approximately 35 feet from the rail junction to the center of the clamp assembly.
This is typically just visualized by dialing out the brace approximately 6 inches. Be sure to maintain an approximate 90-degree orientation to the wall plate, and pivot the raker toward the clamp and clevis assembly and attach it. This will maintain the proper degree of raker, regardless of raker length.
Step 4: Place and pressurize (Fig 6). Place the mechanical raker into position. Remember that these will be built in pairs at a minimum. Wale systems are not necessary with mechanical raker systems because the baseplates at the collection point on the ground have pre-engineered holes to allow pins to be driven directly through them. Additionally, the wall plates have wall surface holes to pin directly into the wall for sheer resistance. Pressurization is achieved by simply turning the locking collars. The pair of rakers, once in place and pressurized, can then be connected using timber bracing or mechanical bracing.
Keys for integrating timber raker systems with mechanical raker systems
Place the initial mechanicals at 6-foot spacing as well as slightly lower insertion points. This allows permanent timber raker systems to be built while they remain pressurized and in place. Once the timber systems are completed, the mechanical systems can be disassembled and recycled to other locations or applications.
If the mechanical systems are going to stay in place and additional timber systems added laterally, the slightly lower insertion point allows nail pads to be added to the mechanicals, which scabs out or bumps up the position for placement. This creates a more contiguous line for integrated timber bracing when connecting timber systems to mechanical systems.
Breaking down the complexities
I hope this simplified some of the complexities of raker systems and provided a snapshot of the options. Structural collapse requires monumental amounts of practice and research.
Stay the course and train hard!