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CHAPTER 4: ENGINEERING PROPERTIES OF ISOLATORS
4.3 ENGINEERING PROPERTIES OF HIGH DAMPING RUBBER ISOLATORS
100
Figure 4.10: HDR Shear Modulus and Damping
4.3.2 Damping
Although the majority of the damping provided by HDR bearings is hysteretic in nature there is also a viscous component which is frequency dependent. These viscous effects may increase the total damping by up to 20% and, if quantified, can be used in design.
Viscous damping is difficult to measure across a full range of displacements as the power requirements increase as the displacements increase for a constant loading frequency. For this reason, viscous damping effects are usually quantified up to moderate displacement levels and the results used to develop a formula to extrapolate to higher displacements.
Figure 4.11 shows the equivalent viscous damping for a load frequency of 0.1 hz, a slow loading rate at which viscous effects can be assumed to be negligible. For strains up to 100%, the tests used to develop these results were also performed at a loading rate of 0.4 hz (period 2.5 seconds), an average frequency at which an isolation system is designed to operate.
The damping at 0.4 hz was higher than that at 0.1 hz by a factor which increased with strain.
At 25% the factor was 1.05 and at 100% the factor was 1.23. The frequency dependency indicates the presence of viscous (velocity dependent) damping in the elastomer. The velocity increases proportionately to the frequency and so the high frequency test gives rise to higher viscous damping forces. The tests at various strain levels are performed at the same frequencies and so the velocity increases with strain. The velocities are four times as high at 100% strain as at 25%. This is why the factor between the 0.4 hz and 0.1 hz damping increases.
The added viscous damping adds approximately 20% to the total damping for strains of 100%
or greater, for this compound increasing the damping from 8% to 9.6%.
0 2 4 6 8 10 12 14
0 50 100 150 200 250 300 350
Shear Strain %
Equivalent Viscous Damping %
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Shear Modulus MPa
Damping Shear Modulus
102
Figure 4.11: Viscous Damping Effects in HDR
4.3.3 Cyclic Change in Properties
The properties of a HDR bearing will change under the first few cycles of loading because of a process known as “scragging”. When a HDR bearing is subjected to one or more cycles of large amplitude displacement the molecular structure is changed. This results in more stable hysteresis curves at strain levels lower to that at which the elastomer was scragged. Partial recovery of unscragged properties is likely. The extent of this recovery is dependent on the compound.
Figure 4.12: Cyclic Change in Properties for Scragged HDR
When HDR bearings are specified the specifications should required one to three scragging cycles at a displacement equal to the maximum test displacement. You should request information from each manufacturer as to scragging effects on a particular compound to enable you to decide on just how many scragging cycles are needed.
Once a HDR bearing has been scragged the properties are very stable with increased number of cycles, as shown in Figure 4.12.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 20 40 60 80 100 120 140 160 180 200
Shear Strain %
Equivalent Viscous Damping %
0.1 hz Damping 0.4 hz Damping 1.2 x 0.10 hz Damping
Extrapolated Data
VARIATION WITH REPEATED CYCLES STRAIN = 100%
0 0.1 0.2 0.3 0.4 0.5 0.6
1 2 3 4 5 6 7 8 9 10
LOADING CYCLE NUMBER
SHEAR MODULUS (psi)
0 2 4 6 8 10 12
EQUIVALENT DAMPING %
Shear Modulus Equivalent Damping
4.3.4 Age Change in Properties
Although most HDR compounds have a more limited service record than other natural rubber formulations the same additives to resist environmental degradation are used as for other elastomers and there is no reason to suspect that they will have a shorter design life.
However, as the compounds are so specific to particular manufacturers you should request data from potential suppliers. The specifications will require the same accelerated (heat) aging tests as for lead rubber bearings.
4.3.5 Design Compressive Stress
HDR bearings are generally designed using the same formulas as for LRBs and so the comments in the sections on LRBs also apply.
4.3.6 Maximum Shear Strain
The maximum shear strains for LRBs usually have an empirical limit which may restrict the shear strain to a lesser value than permitted by the design formulas. These limits are related to performance of the lead core and so do not apply to HDR bearings. The maximum shear strain is based on the limiting strain formulas and may approach 300% for MCE loads, compared to a 250% limit for LRBs.
The higher shear strain limits for HDR bearings may result in a smaller plan size and lower profile than a LRB, for a smaller total volume. However, this also depends on the levels of damping as the displacements may differ between the two systems.
4.3.7 Bond Strength
The bond strength requirements are the same as for LRBs previously.
4.3.8 Vertical Deflections
The vertical stiffness, and so deflections under vertical loads, is governed by the same formulas as for LRBs and so will provide similar deflections for similar construction although the specific elastomer properties may cause more differences.
Long Term Vertical Deflections
HDR bearings are cured differently from LRBs and have higher creep displacements. The compression set (after 22 hours at 158F) may be as high as 50%, compared to less than 20%
for low damping rubber compounds. This may cause an increase in long term deflections and you should seek advice from the supplier on this design aspect.
104 4.3.9 Wind Displacements
HDR bearings generally rely on the initially high shear modulus to resist wind loads and do not require a supplemental wind restraint. The wind displacement can be calculated using compound-specific plots of shear modulus versus shear strain. This may require an iterative procedure to solve for a particular lateral wind load. There have been no reported instances of undue wind movements in buildings isolated with HDR bearings.