Yellow Jacket Basket Litter

FORWARD

This report presents findings of a test undertaken on 27 November 1992, in the Applied Dynamics Laboratory of McMaster University. It follows an earlier test carried out in November of 1991 that is described as the '91 Test'.

An International Stretcher Systems Yellow Jacket Basket Litter with accompanying spine board was provided with standard nylon lifting straps made for use with the litter. The basket was thus hung from overhead steel beams by the straps provided at the four locations designated as lift points. A schematic of the test arrangement is shown as figure 1.

It will be seen in the figure, that load was applied by 4 (four) cross beams, each of which sustained the same amount of load. Therefore, for a total jack loading if force P, each load point transferred P/4 onto the spine board. By having the 4 (four) cross beam members equally spaced along the spine board resulted in simulating uniform loading very accurately (within a few percent for the case of bending moment) in the mid span area.

In addition, a spring-type LVDT (linear voltage displacement transducer) was positioned below the aluminum basket to ascertain deflection at given levels.

LOADING PROCEDURE

Load was applied with a centrally placed hydraulic jack operated with a hand pump, and monitored by a load cell indicator. The load cell was calibrated for accuracy before and after testing.
The procedure for testing was two fold:

1. To bring the load up to a level of 2,500 lb (1136 kg) and maintain it at that level for 15 minutes.
2. To gradually load the basket litter to failure.
3.  

TEST RESULTS

As mentioned earlier, a McMaster University test was performed in November 1991 on a similar basket litter loaded by iron ingots evenly distributed in excess of 2,500 lb (1136 kg). That test was conducted using the methods laid down in the U.S. Federal Specification for testing Litter, Rigid, Stokes. At that time the litter was examined visually. No permanent strain of deformity was detected after the '91 Test'.

The November 92 Test, just concluded, was tested in accordance with the loading procedure described above. A description of the results an observation is presented below.
 

PART 1 - LOAD SUSTAINED AT 2,500 LB (1136 KG)

Initial readings of load and displacement were made prior to application of jack loading. Due to the dead load of the spreader beams, jack, load cell and cross beam members, the initial load was estimated at 247 lb, (112 kg). Table 1 gives the values of load and centralized deflection readings as obtained from the read-out indicators referred to earlier. Note that at 2,500 lb (1136 kg), ss53 lb (1024 kg) was live load, (applied via the jack) and 247 lb (112 kg) was a dead load. At the 2,500 lb (1136 kg) level, there was a small amount of load relaxation due to creep-either due to the nylon straps or the wooden spine board.

On three occasions, the load was adjusted to reach the 2,500 lb (1136 kg) level during the 15-minute test. The Yellow Jacket Basket Litter framing itself did not suffer any permanent deformation during this phase of testing.

PART 2 - LOADING TO FAILURE

As noted in Table 1, the central load was increased gradually with central displacement readings of the aluminum frame take at periodic intervals. Loading was stopped after 6" (15 cm) deflection because of the jack stroke reaching its limit. After restarting, a maximum jack load of 38 kN (8500 lb) was reached, associated with weld failure at the joint of one of the curved circumferential stiffeners with the top exterior perimeter member. When the dead load is addend, this translates as a failure load of about 8,750 lb, (3969 kg).

DISCUSSION AND CONCLUSIONS

At failure, there was substantial deformation of the Yellow Jacket Basket Litter. It was apparent that extensive ductility would be evident prior to failure-indicative of the significant warning and significant reserve strength. As noted in Table 1, the deflection was of the order of 10", (25 cm) at failure. Such an extensive state of deformation was partly due to stretching of the straps, but was primarily die to the bending of the aluminum frame itself. It is my opinion that the Yellow Jacket Basket Litter is a good design, in that it has excellent deformation and ductility properties before failure, and possesses a much-higher load resistance than would normally be required. For example, for a 250 lb person, the 8,750 lb (3969 kg) capacity translates as a force of 36 g's, which far exceeds what a person would survive in any situation.

In the context of requirements by the appropriate jurisdiction, the 2,500 lb (1136 kg) requirement for 15 minutes was easily met-both in the November '91 and November '92 tests, albeit with slightly different distributions of loading.

In addition, the failure load of 8,750 lb (3969 kg) appears to be more than adequate for a static test to estimate a litter frame's failure, taking into account the g-forces that would be present in a rescue type operation of a person who is very much overweight.

TABLE 1 FRAME LOADING INFORMATION FOR NOVEMBER '92 TEST

NOTE: 4045Kn = 100 lb (45 kg) force. For full load, add 247 lb (1112 kg). (0.056 kN). Thus 38.0 kN = 8,500 lb (3855 kg) force (live load only)