Fire Service Ladders and Their Use—Part X
The Use and Abuse of the Big Stick
Editor’s Note: This, the tenth in the series of chapters dealing with fire department ladders, ladder evolutions, ladder use and preventive maintenance, as its title suggests, is devoted to operation, testing and maintenance of the aerial ladder, termed in fire department nomenclature “the big stick.”
This text and the illustrations are taken from the illustrated address under the same caption given by Hubert Walker, Manager, MFA Sales Engineering Department, American-LaFrance-Foamite Corporation, Elmira, N. Y., before the twenty-third annual Fire Department Instructors’ Conference in
Memphis, Tenn., on Thursday, January 11, 1951. Mr. Walker’s instructive and helpful lecture was so timely and fitted so nicely into the series of installments on ladders, ladder use and care, written
by Roi B. Woolley and being published serially in this journal, that requests to include Mr. Walker’s message in this series as a part of the record, were made of Richard E. Vernor, Chairman, Fire Department Instructors’ Conference, and Mr. Walker himself.
The editors gratefully acknowledge the cooperation of these gentlemen in granting permission to so include this F. D. I. C. paper as a part of FIRE ENGINEERING’S treatise.
Readers may desire to review this chapter in the light of two previous installments devoted to aerial ladders, namely. Chapters VII and VIII published in the November and December, 1950, issues under the title “Technique of Testing Fire Department Ladders (Aerial).”
Mr. Walker is one of the nation’s foremost engineers of fire apparatus and his unprejudiced, unselfish observations we believe to be a distinct contribution to the fundamental literature of the fire service.
Because of its length and the publisher’s desire not to omit any text or pictures of Mr. Walker’s educational address it is necessary to publish it in two installments, of which this. Chapter X is the first. The second part will follow in March.
FIRE DEPARTMENT instructors charged with the responsibility of training firemen in the operation and use of aerial ladders have had little factual data to guide them. The situation can be likened to the condition existing in the early days of aircraft use when the pilot flew without instruments, “By the seat of his pants.” This means the pilot used his best judgment and, in the fire service, instructors and operators have also had to mainly rely on personal judgment for the safe operation of the aerial ladder.
Even though aerial ladders have been used by the fire service for a period of over eighty years, no standards of load rating or operation have been established. Each manufacturer has built aerial ladders to his own design. A fire department buys an aerial ladder of 65′, 75′, 85′ or 100′ length, but it has not been given complete data on the load that can be carried under all conditions of operation. Pumpers are bought to a performance standard of capacity rating in G.P.M., plus a specification for the pressure at which this capacity must be obtained. Aerial ladders should be rated on some comparable basis. Fortunately, the fire service has operated the aerial ladders within the load limits in most cases. It is the instances of overloading and abuse that prompted the request for more data.
Before any discussion of the use of the aerial ladder can be undertaken, let us briefly review some of the fundamental factors that must be considered and recognized as limiting the use or loading of an aerial ladder.
No Stronger Than Its Trussing
First: The main or lower rail of any aerial ladder, 55 feet or longer, will not support a load, and in most cases will not support its own weight, without trussing. This applies to the butt section of all wood aerial ladders, and to all sections of metal aerial ladders.
This can be re-stated in a shorter phrase that you should never forget: an aerial ladder is no stronger than its trussing.
On a wood aerial ladder, the trussing is designed to carry its load and has a maximum of strength when the trussing is in tension. It has been good practice for 80 years to place the end of the aerial ladder about 6″ to 12″ from the part of the building on which it is to rest. The weight of firemen or hose lines will cause ladder deflections to rest the end on a window sill or roof, which load and position places the trussing in tension for maximum support and strength.
When the wood aerial ladder is placed against a building before men ascend the ladder, the weight of the men places the main beams in bending and tension, the trussing in compression. The trussing has very little or no load carrying ability under these conditions and the safe load capacity of the ladder is sharply reduced. Its load capacity depends entirely on the angle of the ladder and the length extended.
Metal aerial ladders have almost equal strength with the trussing in tension or compression, so the metal aerial can be lowered to rest on a window sill or roof with perfect safety.
Second: The aerial ladder operates on a hinge pin or shaft, which permits only vertical movement. To enable ladder movement for use in other positions than in line with the truck, a turntable is provided with the lower end of the ladder supported by this turntable. The larger the diameter of this turntable, the greater the ladder stability and lesser load on the turntable bearing surfaces. Turntable diameters vary greatly with different manufacturers, one being 36″ diameter, one 42″ diameter, one 56″ diameter, and one 65″ diameter.
When a ladder is resting on a building, there is little load on the turntable except static weight of the ladder and operating mechanism.
In a cantilever position, that is, with the upper end of the ladder unsupported, the loading on the turntable is a maximum. Most fire department aerial ladder use is under cantilever conditions.
One fire department recently visited was asked how the aerial performed. The reply was interesting. “We have had it over a year, but have not used it on a fire. We train one day a week; take cats out of trees; put up all the city’s Christmas season decorations, and will take them down; put ropes in the schools’ flag poles, and hang the Community Chest banners.”
While these activities are perhaps good public relations, the important job is still fire fighting.
Supports Must Be Stable
The increasing use of the aerial ladder as a water tower involves the use of the aerial ladder in a cantilever position. This brings us to the third factor in aerial ladder operation that must always be the concern of the men in charge of operation—stability of the supporting structure.
The ladder is securely attached to the turntable, which in turn is attached to the chassis frame. When aerial ladders weighing from 900 pounds to 2,200 pounds are moved in an arc to a position at right angles to the truck, we immediately add load to the chassis springs on the side of ladder movement and the increased weight also adds a load on the tires to cause additional tire deflection. As you learned in high school physics—“For every action, there is an equal and opposite reaction,” and we have an equal lightening of the load on the opposite springs and tires which increases the roll or torque action on the chassis. Any vertical movement of the ladder under these conditions produces a whipping action in the ladder that may be dangerous.
The required stability is provided by ground jacks whose function is to support the ladder weight without transfer of load to either springs or tires.
It is obvious that as the spread between the jacks increases, the greater is the stability, and less will be the unit pressure under the jacks.
Some different designs of jack arrangements are provided the fire service.
Types of Ground Jacks
On 4-wheel aerials, two types are available. One hydraulic with jacks spaced 84″ apart as measured across the chassis. A second is the use of mechanical type jacks with spread up to 145″.
On tractor drawn aerial trucks there aye three designs. One with four mechanical jacks for complete ladder support; a second design provides one pair of ground jacks used in combination with jackknifing the tractor to give maximum ladder stability and load capacity. This takes part of the load moment through the fifth wheel which connects the tractor to the trailer chassis. Axle jacks are provided between the tractoy rear axle and the tractor chassis frame to eliminate rear spring deflection.
The third design utilizes no ground jacks, but requires jackknifing the tractor and the use of axle jacks on the tractor.
Manufacturers have tested their aerial ladders and they will safely cayry the maximum recommended load at angles and extended lengths under the recommended conditions of ground jack use.
The fire department instructor knows all this—or does he? All manuals I have seen would indicate full data to be top secret information known only to the manufacturers’ engineers.
One manufacturer does place an automatic indicator in fyont of the operator that registers recommended maximum load at all angles, and any extension for three conditions of service, that is—top of the ladder supported, top unsupported, and as a water tower.
Wood aerial ladders have a load limit as established by the ladder design. In most cases the ground jacks will support greater load than can be carried safely by the ladder.
In a cantilever position, the wood aerial ladder will carry one man at the outer end, full fly extension, at an angle of 30° or more. The ladder must be in good condition, however, to be safe for this load. Full test data on both wood and metal aerials has been given in a series of articles published this past year in FIRE ENGINEERING, If you have any doubts about youy aerial ladder, we suggest that you review this series of articles or write the manufacturer.
Figure 1 shows a wood aerial ladder being tested to destruction. Note the butt ladder stays straight due to the trussing, all the deflection under load is in the fly ladder.
Figure 2 shows a wood aerial laddey under test by a fire department.
The load carrying capacity of the all metal aerial exceeds the load capacity of the wood aerial ladder. The strength increase depending on the manufacturers design and material used.
Figure 3 shows the maximum safe extension of one make of metal aerial at various angles and under two service conditions; one, as a cantilever only, and second, for water tower use. What you are not told is how many men can be on the ladder. The load is five men in cantilever position, two on the top section and three men approximately spaced equal for the rest of the ladder.
When the ladder is being used as a water tower, the load is one man at the top. plus a ladder pipe (1,100 g.p.m. capacity) and a charged 3″ hose line.
Figure 4 shows another make of 100-foot all metal ladder. The load rating on this ladder is for three men—one man on each section of the ladder. Note the difference in ladder extension permissible, depending on whether the tractor is jackknifed or straight ahead.
Figure 5 shows the tractor position required when the maximum load capacity of the aerial ladder is required. Street conditions do not always permit the jackknifing and it is then of utmost importance that the ladder operator know the ladder load limitations. Several fire departments have leayned the hard way that basic stability is a factor in safe aerial ladder usage.
If the tractor is not jackknifed truck should be so located that ladder will reach building when horizontal and not extended. It will then be safe to extend to any length to reach upper floors, provided upper floors are not set back.
Ladder may be extended to 60 feet horizontal reach (with tractor jackknifed only) in emergency, but it will be necessary to retract ladder before hoisting.
When used as a water tower, the tractor should be jackknifed, it greater extension or reach than shown in diagram is required.
More Details on Testing
The testing of aerial ladders is an important requirement, First, by the manufacturer to be sure it will meet his idea of fire service requirements; second, by the fire department to be sure the ladder is in safe condition for service use.
Figure 6 shows an all metal 100-foot aerial ladder being tested with progressively increased load until failure occurs. The test load in this photo was 3,100 pounds plus the weight of a 3″ charged line, or a total of 3,300 pounds. Note how square and straight the ladder remains under load.
Figure 7 shows the same ladder under 3,600 pounds total load.
And Figure 8 shows the same ladder at the moment of failure under added load.
Figure 9 shows another 100-foot ladder being tested under conditions of supporting the top end of the ladder. The strength of the ladder is greater in this position than in the cantilever position.
This ladder is recommended for safe maximum load of eight men when the top is supported, as compared to five men for thp same angle at full extension in cantilever position.
The lateral or side loading strength of an aerial ladder is not as great as top loading. Generally the greatest chance for ladder damage from improper handling or use is side or lateral overloading.
Figuyes 10, 11, 12 and 13 show progressive ladder deflection under load applied at the tip from nozzle reaction as the pressure is increased. These views are from a series of tests conducted several years ago when changes were made to give increased lateral strength.