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Compression Design Characteristics

Structural bearing pad design procedures are usually based on service loads, excluding impact. The design procedures for elastomeric bearings contained in AASHTO Standard Specifications for Highway Bridges(2) presented here also reflect the latest research contained in the October, 1987 NCHRP Report 298(3) and recent work conducted by Roeder and Stanton(7).
The maximum compressive stress, sc, allowed by AASHTO, is determined from the equation
sc £ GS/b (1)
 Where: sc = Average compressive
 stress caused by dead and live
loads, excluding impact
G = Shear Modulus at 73°F, psi
S = Shape Factor
b = Modifying factor having
a value of 1.0 for internal
layers of reinforced bearings,
1.4 for cover layers and 1.8 for
plain pads
This equation is based on limiting the amount of bulge in compressed elastomer material. Excessive bulging caused by high sear forces in compressed pads leads to pad cracking and ultimate failure. Shape factor, S, is a nondimensional relationship associated with the bulging caused by compressing a bearing pad. It is an important consideration in plain and reinforced pad design and is defined as the area of the pad divided by the area of the pad circumference:

The data shown in Fig. 2 for SORBTEX shows that the SORBTEX pads having shape factor from 0.3 to 5.8 can easily support maximum stresses far above the 58 to 483 psi limits established by AASHTO for Cholorprene. In fact, SORBTEX pads have been designed for 2000 psi uniform stress levels for over 20 years, as discussed in the PCI Design Handbook(5).
Based upon the results of over 35 years of experience with SORBTEX pads in bridge and building applications and the results from the 1991 test program, an allowable maximum uniform compressive stress of 2500 psi is considered safe. This stress level will produce measured compressive strains of 7.5 to 13.3 percent for the wide range for tested shape factors as shown in Fig. 2. This ability to sustain higher stresses than steel reinforced Chloroprene pads is clearly shown in Fig. 3 where a 60 durometer reinforced Chloroprene pad with a shape factor of 5 at a stress of 800 psi produces a compressive strain of 5 percent, while the SORBTEX pad with a shape factor of about 5 will sustain 1500 psi at the identical 5 percent strain. At a shape factor of 4, the comparison is even more beneficial with the SORBTEX pad able to sustain 2300 psi at a strain of 7.3 percent, while the reinforced AASHTO Chloroprene pad can only sustain a stress of about 800 psi for the same 7.3 percent strain.
Since Chloroprene bulges significantly with durometers of 50, 60 and 70, as explained and shown in NCHRP Report 298(3), steel plates are used to reinforce and laminate thinner layers of Chloroprene to reduce the bulging problem. Stress/strain curves for 5 nominal 50 to 55 durometer differently shaped steel - plate - reinforced Chloroprene pads having shape factors of about 5 are shown in Fig. 4. These same stress/strain curves are shown in Fig. 5 with the SORBTEX stress/strain curve for the same nominal shape factor of 2. These data further illustrate the greater load carrying capacity of SORBTEX when compared to steel plate - reinforced AASHTO Chloroprene pads. A review of the data in Fig. 5 for steel plate - reinforced Chloroprene pads working at a stress of 1000 psi indicates that the SORBTEX pad can sustain from 1300 psi to 3000 psi while allowing the same compressive strains of about 7 to 12 percent.Uniform compression tests were conducted on typical SORBTEX expansion bearing pads laminated to PTFE, polymer with PTEE and steel with PTEE. The purpose of these tests was to determine if the compression stress/strain behavior was influenced by the low-friction laminate. The test results are shown in Figs. 6 to 8. These tests on SORBTEX pads with the shape factors of 0.7 to 2.0 indicate that the various low-friction SORBTEX pads can also be designed for 2500 psi uniform compressive stress. For these tests on triplicate specimens of 8 different laminated SORBTEX pads, the compressive strain at 2500 psi ranged from 12.5 to 16.2 percent with an average compressive strain of 13.7 percent. This average strain for 13.7 percent compares quite favorably with the 12.5 percent average strain measured on the conventional SORBTEX pads having the same range in shape factor while also at 2500 psi. Therefore, the low-friction SORBTEX pads have very similar stress/strain curves when compared to the conventional SORBTEX pads.
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Where: L or L1 = loaded length of
pad, in.
W or W1 = loaded width of
pad, in.
t = thickness of pad or
layers, in.
AASHTO further limits the maximum uniform compressive stress to 1000 psi for steel reinforced pads or 800 psi for fabric reinforced bearings or plain pads. An increase of 10 percent is allowed where shear translation is prevented.
While these limitations are valid for normal elastomeric bearings made of AASHTO-grade Chloroprene or Isoprene, testing indicates that SORBTEX pads have limited bulging at these AASHTO maximum stress levels and do not suffer from cracking or delamination until much higher compressive stress levels are applied.
The compressive stress/ strain curves for the uniformly loaded SORBTEX pads from the 1991 test program are shown in Fig. 2. These average curves, from triplicate specimens, are from pads with shape factors ranging from 0.3 to 5.8.
Figure 3 shows SORBTEXT stress / strain curves for shape factors from 0.3 to 5.8 plotted for comparison purposes with 60 durometer AASHTO-Grade Neoprene© (Chloroprene) stress/ strain curves(3) for shape factors from 3 to 20. These comparisons demonstrate a much lower sensitivity to bulging for SORBTEX when compared to Chloroprene which is very sensitive to bulging. Since SORBTEX is much less sensitive to the detrimental effects of bulging, the use of the AASHTO equation(1) and stress limits are not valid. For instance, the AASHTO equation for plain or fiber-reinforced pads would restrict the allowable stress for various tested shape factors as follows:
Fig. 8