Adams, Aerials, Apparatus, Pumpers, Rescues

Apparatus Purchasing: Compartment Floor Ratings

Issue 3 and Volume 21.

Apparatus manufacturers (OEMs) want their proprietary manufacturing specifications (specs) promulgated by prospective purchasers.

Buyer and seller proponents claim a level of quality is being established. That can be a valid claim. Opponents who advocate competitive bidding complain of favoritism and denounce the practice. That claim also has merit. OEMs protesting because another OEM’s spec was published are probably whining because they were beat before they got to the bid opening-a harsh but true statement. Once the authority having jurisdiction (AHJ) publishes an OEM’s proprietary specification as its legal purchasing document, it owns it. It is the AHJ’s to justify and to defend if challenged. The merits of publishing proprietary specifications are not in question. The subject of this article is when those specs include an OEM’s compartment floor rating-an ill-defined, confusing, and misunderstood topic.

My intent was to illustrate how OEMs establish a compartment floor rating for a typical full-height body compartment approximately 40 inches wide by 60 inches high by 24 inches deep (40 × 60 × 24) located ahead of a pumper’s rear wheels. Some OEMs declined to comment. Others gave ambiguous responses. Others were extremely detailed. I researched manufacturers’ Web sites, literature, brochures, and technical specifications.

1 Riveted to the web on the frame rail of this antique Mack pumper are rigid stamped steel outriggers. Similar supports today are mostly removable aluminum or steel structural angle iron, channel, or tubular configurations. Each manufacturer has its own unique method of fabricating compartment support systems. (Photos by author unless otherwise noted
1 Riveted to the web on the frame rail of this antique Mack pumper are rigid stamped steel outriggers. Similar supports today are mostly removable aluminum or steel structural angle iron, channel, or tubular configurations. Each manufacturer has its own unique method of fabricating compartment support systems. (Photos by author unless otherwise noted.)

OEMs do not speak the same language when addressing the subject. What some call (compartment) floor ratings others call weight capacities, load capacities, weight ratings, and storage ratings. All are measured in pounds. I collectively refer to all as weight ratings.

Whether OEMs specify them to establish a level of quality, to hold specific equipment, or to beat their competition is immaterial. OEMs can say, claim, or infer anything they want in their literature and suggested specifications. They’re seldom asked to substantiate advertising, especially by prospective purchasers who favor their products. In a regulated bidding environment, if a purchaser adopts an OEM’s weight rating, accurately defining it is very important. Purchaser is synonymous with the fire department, AHJ, apparatus purchasing committee (APC), or whoever signs the check.

Accountability

When purchasers specify compartment weight ratings-which they have the right to do-the burden is on bidders to comply or take exception. In political subdivisions, purchasers have a legal responsibility to impartially evaluate bidders’ claims of compliance. That may not be an easy task. Differences between a 1,000-gallon and a 750-gallon booster tank and between a 300-hp and a 500-hp motor are obvious and quantifiable. Differences in weight ratings are not.

2 3 A Ferrara extruded aluminum body. Regularly displayed on the show circuit, the Ferrara unfinished body shows every minute detail of the company’s structural support system. Photo 3 shows a typical heavy-duty three-inch by four-inch double I-beam cross member that runs side to side across the chassis frame rails. Upper and lower gussets and weldments are readily visible for prospective purchasers to see, evaluate, and compare
2 A Ferrara extruded aluminum body. Regularly displayed on the show circuit, the Ferrara unfinished body shows every minute detail of the company’s structural support system.

Spec writers can inadvertently place themselves in an uncomfortable position when specifying a compartment weight rating. Remember that regardless of who wrote the specs, the fire department owns them. It may have to justify internally to whoever signs the check how and why a particular rating was chosen. It may have to provide legal justification for accepting or rejecting bidders’ claims of compliance. No fire department wants to defend its technical purchasing specs in open forums such as prebid conferences and bid openings or, worse yet, in City Hall if publicly challenged by unsuccessful bidders. It could be embarrassing if the APC can’t describe what a compartment weight rating is or why it was specified in its document. Stating the APC copied a preferred manufacturer’s proprietary specification may not hold water legally or in the court of public opinion. It’s difficult to defend a requirement with multiple meanings. It’s harder to accept or reject bidders’ claims of compliance to such a requirement.

NFPA 1901

National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus (2016 ed.), only requires a minimum cubic footage of compartmentation for various types of apparatus. It does not require minimum weight ratings for individual compartments. Chapter 12 (chassis and vehicle components) says OEMs must provide a minimum weight allowance for miscellaneous equipment not including ground ladders, suction hose, and hose loads. The amount must meet Table 12.1.2 or a purchaser-provided list of equipment to be carried-whichever is greater. The weight allowance pertains only to the gross vehicle weight ratings (GVWR) of the chassis and its components.

Additionally, ancillary equipment does not have to be compartmented. Sentence 15.3.1 says: “Equipment holders or compartments shall be provided for all tools, equipment, and other items that are on the fire apparatus.” Purchasers are free to mount equipment wherever they choose. Whether equipment is stored inside a compartment, on top of or underneath the apparatus, or towed behind it is a local decision.

2 A Ferrara extruded aluminum body. Regularly displayed on the show circuit, the Ferrara unfinished body shows every minute detail of the company’s structural support system.
3 A Ferrara extruded aluminum body. Regularly displayed on the show circuit, the Ferrara unfinished body shows every minute detail of the company’s structural support system.

NFPA 1901’s Appendix, sentence A.12.1. (large equipment capacity), addresses average equipment loading in compartments. It says a compartment with an average loading has an equipment weight of approximately eight pounds per cubic foot; a lightly loaded compartment has an average weight of four pounds per cubic foot; and a heavily loaded compartment can reach 12 pounds per cubic foot. Using NFPA’s figures, the 40 × 60 × 24 compartment containing 331⁄3 cubic feet could hold about 133, 266, or 400 pounds of equipment. That is far less than the compartment weight ratings claimed by those OEMs that do so. NFPA statistics make for an interesting read. However, they may be irrelevant to a spec writer.

I am not disputing NFPA 1901’s emphasis on not overloading or underloading an apparatus. Braking and handling characteristics, the safe operation of the apparatus, and the safety of its crew are paramount and not in question. Dictating the amount of compartmentation, the carrying capacity for ancillary equipment, and how tightly equipment is packed is-in my opinion-up to the purchaser.

FAMA

The Fire Apparatus Manufacturers’ Association (FAMA) has an online weight and dimension chart. Wyatt Compton, inside sales coordinator/application engineer for Spartan, says, “NFPA’s Annex C references www.fama.org for an equipment spreadsheet that can be downloaded, a rig’s inventory entered, and the projected equipment loading estimated. NFPA 1901 defines what equipment load is to be accounted for in Chapter 12. The fire department should make certain that it’s within the recommendations. If a fire department finds its weight requirements are higher than normal, this should be a primary topic of discussion with any potential apparatus manufacturer, as it could have heavy influence on the overall design.”

FAMA’s chart is an excellent tool. However, diligence is advised. Don’t become self-enamored with it. Understanding it and being proficient using it do not qualify one as a degreed apparatus design engineer. As an example, the chart indicates a five-gallon pail of foam is 14.5 inches by 11.25 inches by 11.25 inches, weighing about 45 pounds (1,835 cubic inches or 2½ cubic feet). The 40 × 60 × 24 compartment may figuratively hold six pails on the floor and six on each of three shelves for a total of 1,080 pounds. Although the raw dimensional format indicates it can hold 24 pails of foam (120 gallons), it’s unlikely it really can. Published compartment dimensions usually do not consider factors such as door jambs, compartment headers, door pan or door roll interference, shelving tracks and the “lip” heights, and actual usable interior dimensions of trays and shelves. Don’t self-design a nonfunctional compartment. Realistically, who can reach up into the rear of the top shelf and lift up a foam pail whose handle is more than six feet off the ground? Use FAMA’s chart as a guideline-not as gospel. It is a resource that shouldn’t be abused but could be.

4 The typical tubular stainless steel under compartment subframe provided by CustomFIRE on its pumper compartments located ahead of the rear wheels. Visible on each end of the cross piece running parallel to the frame rail are torsional isolators provided to mount the body and isolate and provide flex between the chassis frame and the body. Several methods of attachment provide flex between the chassis and body. Compression mounting with U-bolts to the top flange of frame rails is another common method. (Photo courtesy of CustomFIRE
4 The typical tubular stainless steel under compartment subframe provided by CustomFIRE on its pumper compartments located ahead of the rear wheels. Visible on each end of the cross piece running parallel to the frame rail are torsional isolators provided to mount the body and isolate and provide flex between the chassis frame and the body. Several methods of attachment provide flex between the chassis and body. Compression mounting with U-bolts to the top flange of frame rails is another common method. (Photo courtesy of CustomFIRE.)

Doug Kelley, product manager at KME, cautions purchasers on chassis overloading. “One thing that is critical to mention, however, is that just because a compartment can structurally hold a certain amount of weight, the department still has to make sure that everything fits within the weight ratings of the total vehicle,” he says. “For most bodies, the structural rating of the compartment is much higher than what is normally carried in the space available. However, it is certainly possible that if each compartment was loaded to its maximum capacity, then the vehicle would be overloaded on the axles. Therefore, a department has to look at two things-not overloading the individual compartment and also making sure that the sum of all the weight carried does not overload the vehicle and the axles.”

Adding to Kelley’s comments, purchasers should keep in mind that NFPA 1901 says the fire department is responsible to maintain the proper loading of the vehicle after its delivery.

The Real World

I question if specifying minimum compartment weight ratings is advantageous when purchasing a new rig. If for whatever reason purchasers deem it appropriate to do so, they should define exactly what the ratings are to include. Why? Several OEMs’ published weight ratings for compartments ahead of the rear wheels vary from 500 to 1,500 pounds. Besides using different terminology to describe weight ratings, OEMs also have different definitions for them! One says it is just for the equipment resting on the floor of the compartment. Another says it includes all the equipment in the compartment and on the shelves. Another’s rating also includes the actual weight of the compartment itself. Some say there will be no permanent floor deflection. Another says there will be no deflection. Several mention that ratings include both dynamic and static loads. Others don’t differentiate. How can an APC evaluate bidders’ claims of compliance to a vague requirement with multiple meanings?

Factors that could affect compartment weight ratings include how OEMs support their compartments and how compartments are integrated into their total body construction. I take no side in how compartments are supported, how they are fabricated, or what they are constructed of. Let the sales and marketing folks argue that. Purchasers should compare apples to apples.

Purchasers are cautioned the terminology and definitions for subframe, substructure, and superstructure also vary from manufacturer to manufacturer. I define a subframe as any structural support attached to the side of the chassis frame rail that extends underneath a compartment floor. A substructure is any structural support resting on top of the frame rails that compartments are attached to. My definition of superstructure is any structural member located above the chassis frame rails that frames and supports compartments and the body. Some OEMs use subframes only. Some use substructures only. Others combine subframes and substructures into a single fabrication.

5 6 Each manufacturer has a preferred method to determine compartment floor ratings including finite element analysis and strain gauge data testing and computer modeling. These photos show actual physical testing conducted by CustomFIRE on the rear portion of the subframe for the rear side compartments, the rear compartment, and work platform (rear step). This test was conducted on the jig table used to fabricate the subframe; hence, the weight had to be counterbalanced on the front to prevent it from tipping over. (Photos courtesy of CustomFIRE
5 Each manufacturer has a preferred method to determine compartment floor ratings including finite element analysis and strain gauge data testing and computer modeling. These photos show actual physical testing conducted by CustomFIRE on the rear portion of the subframe for the rear side compartments, the rear compartment, and work platform (rear step). This test was conducted on the jig table used to fabricate the subframe; hence, the weight had to be counterbalanced on the front to prevent it from tipping over. (Photos courtesy of CustomFIRE.)

Many compartments are directly supported from underneath the floor by structural members (photos 1, 4, 7, and 8). Others are attached to structural members that are not necessarily underneath the compartment. They are sometimes unfairly and derogatorily called “hung” compartments. Underneath structural supports are self-explanatory. The latter method is best described by paraphrasing a former manufacturer’s specification: “Above and perpendicular to the frame rails, the integral body and substructure are full-body-width cross members that compartment side walls are attached to. The ends of each intermediate cross member supporting the booster tank are attached to the rear walls of the compartments. Floors, ceilings, and back walls are reinforced with hat-type sections to prevent bowing or stress on any surfaces.” I make no reference to, nor show preference for, welding, bolting, or bonding compartments to structural members. Talk to your local vendor.

Another method is described by Spartan’s Compton. “One type of construction offered by Spartan is a space frame body,” he says. “The space frame bodies are constructed with a self-supporting tube structure that has sheet metal components added for compartmentation and features.” It is an integral combination substructure and superstructure that frames each compartment and the entire body. Extruded aluminum bodies also feature this type of framework (photos 2 and 3).

A term called “drop” is the distance from the top of a chassis frame rail to the floor of a compartment. Depending on the type of chassis and its components, drop is between 15 and 20 inches. In fairness to OEMs who attach compartments to substructures at frame rail level, some extend their structural framework to below the compartment floor level.

5 6 Each manufacturer has a preferred method to determine compartment floor ratings including finite element analysis and strain gauge data testing and computer modeling. These photos show actual physical testing conducted by CustomFIRE on the rear portion of the subframe for the rear side compartments, the rear compartment, and work platform (rear step). This test was conducted on the jig table used to fabricate the subframe; hence, the weight had to be counterbalanced on the front to prevent it from tipping over. (Photos courtesy of CustomFIRE
6 Each manufacturer has a preferred method to determine compartment floor ratings including finite element analysis and strain gauge data testing and computer modeling. These photos show actual physical testing conducted by CustomFIRE on the rear portion of the subframe for the rear side compartments, the rear compartment, and work platform (rear step). This test was conducted on the jig table used to fabricate the subframe; hence, the weight had to be counterbalanced on the front to prevent it from tipping over. (Photos courtesy of CustomFIRE.)

Compton continues, “Spartan also offers a modular formed body that relies on the subframe with floor supports in specific preengineered locations.” Jim Kirvida, president of CustomFIRE, describes CustomFIRE’s compartment support system. “Our compartment, and in fact our entire body structure, is supported 100 percent by the subframes. It rests on them (photo 4). Our method of construction does not require using full-body-width transverse beams above the chassis frame rails to hold the structure together.”

Photos 2 and 3 show a Ferrara extruded aluminum pumper body. Ferrara is one manufacturer that regularly displays unfinished bodies at trade shows. Nothing is left to the imagination, individual interpretation, or deciphering technical specifications. “Here it is; this is how we build it; compare it to the other guys.’” Ferrara’s salespeople justifiably stand tall next to it. Its Web site has a brochure that shows a view of its body support systems from underneath the rig. Likewise, Rosenbauer’s online brochure describing its FX body has some excellent views of its FX body subframe.

Alexis has a brochure available on its Web site that is very descriptive, short, and to the point describing its body subframe. It states, “Our underbody outrigger system provides total compartment support from the bottom up to withstand the load-carrying requirements of today’s apparatus.” Kelley says, “KME’s typical compartment design will support more than 1,000 pounds of point loading applied directly to the bottom of the compartment without any failure of the floor sheet.” He explains, “Point loading just means all of the load at a single point-like all the load balanced on the head of a pin. It doesn’t really happen in real life, but it’s the worst-case scenario from an engineering perspective.” Again, no preference is shown for any one method of construction.

Defining and Testing

If a fire department is going to put itself in a potentially vulnerable position by specifying an OEM’s compartment weight rating, it should do so wisely. Spec writers should know why they are requiring a compartment weight rating. To be fair and equitable to potential bidders, they should define exactly what the rating includes. Establish benchmarks that will be used to compare and evaluate proposals received. One could be by a measurable test method.

Kelley states, “KME’s weight ratings are determined by finite element analysis (FEA) of the compartment assembly. They are typically based on the compartment floor alone. Shelves and trays have their own weight ratings independent from the floor structure. KME uses a mixture of real world testing/experience and FEA to validate the strength of our compartments. However, the actual load rating will be determined by a combination of factors, including NFPA allowance, the vehicle weight ratings, the size of the compartment, and the expected use.”

7 Two rear views of a finished CustomFIRE subframe mounted on a chassis showing structural supports for the rear compartment and the rear side compartments. Visible in photos 8 and 4 are cross members that, on CustomFIRE’s rigs, provide support only for the booster tank cradle assembly. Apparatus manufacturers are mandated to follow tank manufacturers’ minimum requirements for tank supports, however some exceed the minimums. (Photos courtesy of CustomFIRE
7 Two rear views of a finished CustomFIRE subframe mounted on a chassis showing structural supports for the rear compartment and the rear side compartments. Visible in photos 8 and 4 are cross members that, on CustomFIRE’s rigs, provide support only for the booster tank cradle assembly. Apparatus manufacturers are mandated to follow tank manufacturers’ minimum requirements for tank supports, however some exceed the minimums. (Photos courtesy of CustomFIRE.)

Compton adds, “Spartan’s computer model FEA data is bolstered by strain gauge data. It is important that any analysis data be contrasted with real-world results.” CustomFIRE provided photos 5 and 6 showing a 4,350-pound dead load testing on CustomFIRE’s standard cantilevered rear platform, which serves as a subframe to the rear compartment floor and to the rear tailboard. Kirvida says purchasers should consider similar testing, including comparable load testing for subframes under exterior compartments ahead of the rear wheels. He says, “Ask to witness it at the factory.”

Body and Chassis Interface

Apparatus bodies and compartments originally sat on L-shaped outriggers attached to the chassis frame rails (photo 1). This method was widely used well into the 1970s when compression mounting to the top flange of chassis frame rails with U-bolts became a popular option and is still used today. Others still use brackets attached to the sides (webs) of chassis frame rails to attach bodies-regardless of whether a subframe, substructure, superstructure, or any combination thereof is used. Again, I have no preference for either method of chassis interfacing.

When asked to compare bolting body support members to the web of a chassis frame rail vs. compression mounting to the top flange with U-bolts, Kirvida declined to comment. He says, “The torsional isolators CustomFIRE uses are similar to motor mounts. Chassis manufacturers bolt their motor mounts to the webs of frame rails as they do cab mounts, chassis suspension mounts, radiator mounts, steering gear, cross members, battery boxes, and tow hooks. Ask the other apparatus manufacturers why they mount their bodies the way they do.” Body and chassis interface should address chassis flex, frame twist, torque, torsional resistance, and reducing stress on bodies-an important topic we’ll address in the future.

8 Two rear views of a finished CustomFIRE subframe mounted on a chassis showing structural supports for the rear compartment and the rear side compartments. Visible in photos 8 and 4 are cross members that, on CustomFIRE’s rigs, provide support only for the booster tank cradle assembly. Apparatus manufacturers are mandated to follow tank manufacturers’ minimum requirements for tank supports, however some exceed the minimums. (Photos courtesy of CustomFIRE
8 Two rear views of a finished CustomFIRE subframe mounted on a chassis showing structural supports for the rear compartment and the rear side compartments. Visible in photos 8 and 4 are cross members that, on CustomFIRE’s rigs, provide support only for the booster tank cradle assembly. Apparatus manufacturers are mandated to follow tank manufacturers’ minimum requirements for tank supports, however some exceed the minimums. (Photos courtesy of CustomFIRE.)

There are alternatives to specifying compartment floor ratings. Purchasers regularly specify the gauges and types of materials to be used throughout an apparatus. Included are thicknesses; physical properties; dimensions and configurations of materials used for subframes, substructures, superstructures, and bodies; and the compartment material itself. It has been a common and accepted practice for decades. Also, it may be advantageous to physically describe the compartment support system and method of body and chassis interface as a level of quality rather than a weight rating that may be difficult to explain and justify. If something is not in writing, it does not exist. If it is in writing, it should be well defined because it may have to be justified and defended. Write carefully.

BILL ADAMS is a member of the Fire Apparatus & Emergency Equipment editorial advisory board, a former fire apparatus salesman, and a past chief of the East Rochester (NY) Fire Department. He has 50 years of experience in the volunteer fire service.