COLLAPSE SEARCH AND RESCUE OPERATIONS: TACTICS AND PROCEDURES
Part 11: EXterior Rake Shoring
Exterior rake shores are erected in emergency situations primarily to stabilize and resupport existing bearing or nonbearing exterior walls. These walls may be cracked, damaged, leaning, bulged, or in some way not properly supporting their loads. Erecting a series of fixed rake shores properly anchored and braced together will stop the wall from moving any further. Rake shores always must be installed in series, and at least two must be erected in any given situation. By connecting all the shores together, the rakes are stabler and can more safely support the extensive load applied to them.
You generally will encounter two styles of rake shores—the friction shore and the fixed rake shore. Friction shores are used primarily in the construction industry, while the majority of rake shores your team will construct and encounter in rescue situations will be of the fixed type.
THE FRICTION SHORE
A friction rake shore is one whose stability relies on the compression force applied to the rake itself. The rake shore generally consists of the rake and some wedges or blocking at cither the top or the base or sometimes in both positions. The rake generally is installed against the object or wall to be supported and then wedged tightly into position. The only way this type of rake usually can stay in position is by being kept under constant compression. In a rescue situation where fire department personnel will be operating, this style of rake generally is not recommended. If any movement, shifting, or secondary collapse occurs, this type of rake may loosen, slip, or fail entirely, placing the rescuers in jeopardy.
THE FIXED RAKE SHORE
In the fixed rake shore, all of the structural elements arc tied together, making the shore one integral unit. Thus the shore itself is stable and will be able to stand up to unexpected forces applied to it. whether they are from a secondary collapse or vibrations from earthquake-related aftershocks. Because of its ability to stay together after additional unexpected stresses are applied to it. this style of shoring is recommended for rescue situations. The shore’s extra strength and stability add an additional degree of safety to rescue operations, hopefully preventing any unforeseen problems.
Emergency service personnel normally will deal with two types of fixed rake shore—the solid sole plate type and the split sole plate type. Each type has several variations. Some are adjustable; a few can be prcerected, then moved into place; some utilize more lumber than others, which is not necessarily an advantage. The solid-sole type and the split-sole type both can be used either on solid surfaces such as asphalt or concrete or bare ground. In general, the solid-sole rake is utilized in urban environments. where concrete and asphalt commonly cover the ground, while the split-sole rake shore is used in suburban areas, where open ground is prevalent.
It is up to your team to determine which type of rake shore it will use and which variation is most practical for your department. Only after surveying the structures in your response area and the type of ground stability you will typically encounter will your team be able to decide which type of rake shore is best for your department’s use. This is just one more reason preplanning and presurveying your response area are so important.
LOAD TRANSFER
Most brick-and-joist buildings are constructed so that the interior weight is carried by the floor beams and transferred to the bearing walls, which in the majority of these buildings are exterior walls. As the impact of additional weight is transferred to these exterior bearing walls, it can cause deflection and instability of these walls. The purpose of the exterior rake shore is to help the bearing wall carry this additional weight and transfer it to the ground, where it can be safely accepted. As the weight is applied to the rake shore, the rake itself will come under compression, causing it to slide upward on the wall plate or away from the wall on the sole plate.
For this reason, cleats, also called thrust blocks, are placed against the rake to stop that element from moving. These cleats must be a minimum of two feet in length and properly nailed using the “five-nail pattern” method. If done properly, the rake will not slide in either direction, and the load applied to the rake will be fully absorbed by the rake and then safely transferred to the ground.
SUPPORT POINT
In both wood-frame and brick-and-joist constructions, the floors generally are designed to support the building’s main loads, transferring them to the exterior bearing walls, through which they are then directed to stable ground. The main support point at which the rake should intercept the building’s load is the center of the floor joist of the floor you want to stabilize. In general, if you come within one foot of this point, preferably lower than the joist’s center, you will get full efficiency of the shore. With this in mind, you can round off your rake height to the nearest foot, making it easier to measure and cut. If your rake shore point is placed above the recommended area, your shore will be less effective and may not be able to support the building load or prevent a secondary collapse from occurring.
DETERMINING YOUR RAKE SHORE ANGLE
There has been an ongoing discussion of which angle is “best” for exterior rake shores. Generally speaking, any angle between 30 and 60 degrees will work effectively: however, the closer the angle is to 45 degrees, the more efficient the rake will be. The further above or below a 45-degree angle the shore is erected, the more the lateral force will be applied to the rake, putting it less and less under compression and lessening its load-bearing capability, which may possibly create a problem later in your operation.
At times, however, the support point for your rake shore will be higher than usual, and your shoring lumber will not be long enough to utilize a 45-degree angle. In such cases, you can use up to a 60-degree angle, if necessary. Bear in mind, however, that a 60-degree angle is the maximum recommended angle you can use and still safely erect rescue shoring.
Several methods for determining the angle and lengths of your rake shores are available to your rescue team. For further information, see “Collapse Search and Rescue Operations: Tactics and Procedures Part 7: The Steel Square,” in the November 1993 issue of Fire Engineering.
TEMPLATES
Surveying your collapse rescue response area is one of the easiest ways to help determine which rake shore angles will be the most appropriate for your team’s rescue operations. By examining the types of structures in your district and their possible collapse potential, you will get a better idea of the size of shoring you may be called on to erect.
Check the average floor height of the buildings, if possible. This will give you an idea of the height at which the rake shores may be placed and thus the possible rake lengths and angles that are best suited for the operations you may encounter. Of course, each collapse situation will be different. But having a general idea ahead of time of the sizes and types of shoring you will need, as well as the method you will use to erect it, will help greatly in an actual rescue situation. Decisions have to be made quickly during these operations: if some of your options are laid out beforehand, these critical choices will go more smoothly.
EXTERIOR SHORING SIZE-UP
Several factors, including type of construction, extent of damage, type and stability of the ground your shores will bear on, secondary collapse potential, reason for building failure, and height of the wall to be stabilized, all should be considered in your exterior rake shore size-up. These factors are discussed in detail below.
Type of construction. Construction type will help you determine what size material to use and possibly how close to install the rakes. Buildings of lightweight construction, such as wood-frame structures, private homes, and row houses, usually do not generate a large amount of heavy collapse debris. These types of buildings can be easily supported by smaller-size lumber, such as 4 x 4s. As buildings get larger, their walls are constructed of heavier materials, such as the brick or concrete block found in many commercial structures, and the weight of the walls greatly increases. To handle this additional weight, you will need larger lumber for your rakes—4 x 6s and 6 x 6s may be necessary.
Deciding which size to use can only be done on scene and after taking into consideration several other factors. In concrete-constructed buildings and some of the much larger masonry-constructed buildings, you may need even larger sizes of lumber. The larger-sized lumber, such as 8 x 8s or 12 x 12s, is not always readily accessible and is more labor intensive to work with. For ease of operation, consider the use of construction equipment to lift and place material of this size.
Extent of damage. How extensive the damage is to the structure will dictate if in fact you will do any shoring at all. As you size up the structure, you will have to determine if the area is safe enough for rescue personnel to operate in. After this area has been addressed, the next size-up consideration will be to survey the damaged wall areas you want to stabilize. Check the structural integrity of the walls in question. Do they have cracks or bulges? Is the wall out of plumb? How much of the remaining structure is relying on this wall for support? The answers to these questions will determine if you need to erect shoring.
In many collapses, especially those involving unreinforced masonry walls, such as brick or concrete block, the additional forces applied to the wall force the mortar to separate from the block. If these cracks are numerous and extensive, the wall could lose its entire structural integrity. In such a case, trying to resupport these heavily damaged walls may be fruitless. These are some of the factors that the incident commander must evaluate before any attempt is made to erect rescue shoring.
Secondary collapse potential. The potential for secondary collapse is another factor that may influence exterior shoring placement. lor instance, if you are installing a series of rakes along an exterior bearing wall and a section of the area above has a great deal of debris or heavy machinery, you may need to erect additional shoring. You might use a series of rakes placed closer together than usual, or you may increase the size of the lumber you use for the rake shores.
In some situations, you may have to shift the support point of the rake to a position at which the majority of the weight is being applied. These decisions can only be made on the scene, as each collapse situation will be unique. Only after carefully sizing up conditions can the shoring officer and the incident commander make these decisions.
Reason for failure. Another part of the size-up that needs consideration is the cause of the collapse. If the structure collapsed due to forces of nature, such as high winds or earthquake vibrations, operations will have to be conducted under the strictest safety conditions. It is possible that these conditions may still prevail while rescue operations are being undertaken. This is especially true in earthquake situations, where aftershocks can occur weeks after the initial event.
In view of the massive potential for secondary collapse, rescue personnel must take special safety precautions. Other factors to consider are the age and condition of the structure. If a section of a building collapses due to old age or disrepair (such as a vacant building), the entire stability of the remaining structure is suspect, and operations will have to be adjusted accordingly.
Ground stability and type. Several factors are involved in determining which type of rake shore to erect and how to stabilize it. Generally speaking, it you are responding to a building collapse in an urban environment, chances are the rake will be erected on concrete or asphalt. On this type of surface, the solid-sole rake type would be the most advantageous. The shore can easily be anchored down to hard surfaces using any number of methods.
In suburban areas where you are likely to encounter bare ground adjacent to the damaged structures, the split-sole rake shore may be the easiest to use. However, if the ground is stable and solid, the solid-sole rake also can be used. Since each collapse is unique, these versatile anchoring options are a definite plus for your rescue team.
Height of rake. Determining the height at which the rake shore will intersect the wall will tell you which angle will work the best and, thus, whether your available lumber will be long and strong enough to do the job.
