Structural DeSign design issues for structural engineers
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ontinuing on the foundation established in the last article (STRUCTURE, August 2016), let’s now look at two fatigue design methodologies: AISC and Damage Tolerance. AISC is based on the safe life philosophy – if the engineer keeps the stresses low enough, the structure will perform adequately. It also assumes cracking occurs at the end of the structure’s life. Damage Tolerance approaches the problem from the opposite perspective. It assumes the structure inherently has discontinuities in critical locations from the first day it is in use. These discontinuities are below the inspection threshold, but will grow as time goes on. The engineer designs toughness, redundancy, and inspection into the structure. This is done in a closed loop system, receiving feedback at critical stages in the structure’s life.
AISC Fatigue Design General Concepts
AISC and Damage Tolerance Approaches Part 2
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By Paul W. McMullin, S.E., Ph.D. • • • Paul McMullin is a Founding Partner at Ingenium Design in Salt Lake City. He is an Adjunct Professor and the lead editor of the Architect’s Guidebooks to Structures series. Paul can be reached at Paulm@ingeniumdesign.us.
AISC fatigue design methodology is very similar to that found in AASHTO and AREMA. Key concepts of AISC fatigue design include: Fatigue design is not required if the structure will see less than 20,000 cycles, or when the stress range is below the threshold FTH. Use service loads (allowable stress load combinations). The AISC provisions assume suitable corrosion protection. Calculating the number of cycles can at best be a guess. Talk to the operator and be conservative.
AASHTO – American Association of State Highway Transportation Officials AISC – American Institute of Steel Construction AREMA – American Railway Engineering and Maintenance-of-Way Association • Ignore the stress concentration (the table values take this into account). Stress range is calculated considering only the fluctuating stresses, not total stresses. Permanent stresses, such as dead loads, do not contribute to the fatigue stress range. For example, if there is a 5 ksi cyclic load in combination with a 15 ksi dead load (Figure 1a), the stress range is only 5 ksi. It is possible to make the mistake that the stress range is 20 ksi, which would lead to a substantially heavier design. Looking at another condition, if a 10 ksi fullyreversing stress exists but no permanent loads are present (Figure 1b), the stress range is 20 ksi. This is because we are adding peak-to-peak stresses. If we took the stress from zero to peak, we would underpredict our stress range by a factor of two. Allowable Stress Range Once the engineer has accurately calculated the stress range, they need to compare it to the allowable stress range. There are two ways to do this: calculate the stress range based on the number of cycles, or limit the stress to the threshold. A description of both methods follows. Using an estimate of the number of cycles, the allowable stress range, FSR , can be calculate based on the following equation: FSR =
( nC ) f
SR
0.333
≥ FTH
When calculating stresses, the following need to be considered: • Use an elastic stress analysis. • Include prying effects in bolts. • Include the effect of eccentricities.
Where Cf = factor from AISC tables nSR = number of cycles in design life FTH = fatigue threshold stress range If the design of the structure is based on the fatigue threshold stress – which may be prudent for structures that may be in service well beyond their service
(a)
(b)
Stress Calculation
Figure 1. Stress range examples for (a) high permanent stress, and (b) fully reversing stresses.
10 November 2016
