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CEE 4606 - Capstone II Structural Engineering

CEE 4606 - Capstone II Structural Engineering. Lecture 3 Seismology, Earthquakes, and Roof Design. Earthquake Loads.

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CEE 4606 - Capstone II Structural Engineering

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  1. CEE 4606 - Capstone IIStructural Engineering Lecture 3 Seismology, Earthquakes, and Roof Design

  2. Earthquake Loads “Earthquakes systematically bring out the mistakes made in design and construction - even the most minute mistakes; it is this aspect of earthquake engineering that makes it an educational value far beyond its immediate objectives.”-Newmark and Rosenbluth

  3. Ductile vs. Non-ductile Concrete Construction • Note the obvious differences of capability of concrete columns to take load after earthquake damage. • The spirally reinforced column (ductile reinforcing) has an obvious capacity to carry much more load than the tied corner column (non-ductile reinforcing). San Fernando, 1971

  4. Ductile vs. Non-ductile Concrete Construction • This photo was taken while looking at the exterior of a damaged classroom building • The column suffered a shear failure. • Note that the column did not fail at the top (where anticipated) due to combined shear and bending Peru, 1974

  5. Photo of Column from the Inside • Note that at the top of the column ductile reinforcing was used (ties very close together). • The failure occurred where the spacing of ties was expanded. • Ductile reinforcing of concrete is a necessity. • Follow the IBC and ACI codes for seismic detailing requirements Peru, 1974

  6. Earthquake Design • Of course the degree of importance of an earthquake loading in any given location is related to the seismicity of the region: • Likelihood of occurrence • Probable intensity of the earthquake

  7. How is seismicity determined? • Historical records • China 3000 years • Middle East 2000 years • Latin America ??? • In the 1960’s the US developed the World Wide Standardized Network • 120 stations in 60 different countries

  8. Seismographs • Instrument that records the earth’s motion • North-South • East-West • Vertical • Pen-Plotter • Digital

  9. What causes earthquakes? The lithosphere is broken into rigid plates that move.

  10. Arabian Plate

  11. Seismic Waves • When the earth shakes it releases seismic waves • Body waves pass through the “body” of the planet (fastest waves and can be refracted and reflected) • Surface waves stay near the surface • There are many different types of waves

  12. Types of Seismic Waves P Wave Body S Wave Love Wave Surface Rayleigh Wave

  13. Body Waves

  14. Primary Waves • P-waves (body waves) • Are the fastest; consequently, they reach the recording station first. • Move in a push-pull fashion, alternating pulses of compression and tension • Can travel in any medium • Arrival at your site may be accompanied with thunder-like noises and rattling windows (similar to a sonic boom)

  15. Like a slinky

  16. Secondary Waves • S waves (body waves) • The second wave to reach the recording station • Transverse waves that propagate by shearing or shaking particles in their path at right angles to the path of advance • Travel only through solids • The wave motion that is most damaging to structures

  17. Snapping a piece of rope

  18. Love Waves • Surface waves • Motion is essentially an S wave that has no vertical displacement • Moves the ground from side to side 90 degrees to the direction of propagation • Can be very damaging to structures

  19. Love Waves

  20. Rayleigh Waves • Most common surface wave • Similar to water wave except they have a backwards rotation • Cause horizontal and vertical movement • Slower than Love waves • Pass through ground and water • Long periods and travel a long way (once they get started)

  21. Being on a ship

  22. Waves

  23. Waves P waves travel approximately 1.7 times faster than S waves

  24. Locating the Source • The epicenter can be located using the lengths of time the various seismic waves take to reach a seismograph • P waves travel approximately 1.7 times faster than S waves; therefore, the larger the difference in arrival time, the farther away from the epicenter you are • This gives you distance • What about direction?

  25. Use Multiple Seismographs

  26. Example Problem

  27. Example Problem continued

  28. We use that procedure for all earthquakes

  29. Magnitudes of Earthquakes • The magnitude is an estimate of the relative size (amplitude) of an earthquake measured from a seismogram

  30. Richter Scale • 1935, Charles Richter of CIT defined the magnitude of an earthquake Magnitude - the logarithm to the base ten of the maximum seismic wave amplitude (in thousandths of a millimeter) recorded on a standard seismograph at a distance of 100 kilometers from the earthquake center • For every tenfold increase in amplitude on the seismogram, the Richter Number increases by 1.0

  31. Magnitudes • Earthquakes of magnitude < 5.0 are not expected to cause structural damage • Earthquakes > 5.0 are potentially very damaging

  32. S - P = 24 sec Max height = 23 mm Connect points with a straight line Read intersection Magnitude = 5.0 Procedure for Measuring Magnitude

  33. Earthquake Intensity • Intensity is the severity of the ground motion at any point • The measuring scale is the Modified Mercalli (MM) • Scale of I to XII • I - Nothing to XII - Total Destruction

  34. Relating Richter to Mercalli

  35. El Salvador • 7.6 magnitude quake • January 2001 • Centered off the Salvadoran coast about 65 miles southwest of San Miguel • There were pockets of destruction, with destroyed towns next to areas that were completely unscathed

  36. Tremors were felt at our site

  37. Earthquakes Over 5.0 Richter in Honduras, 1900 - 1980 Structural damage would be expected in structures designed in accordance with US codes

  38. Earthquake Design Section 1613 –Definitions Section 1614 –General • Exceptions • Additions and/or alterations • Change of use

  39. Earthquake Design • Considerationsare similar to wind • site characteristics • occupancy • structural configuration and system • height and weight • zoning (wind speed vs. ground acceleration) • Section 1615 – Site Ground Motion Figures 1615 (1) through (10)

  40. Contours Acceleration in % of gravity Linear interpolation Specific time of response (.2 sec) Assumption of 5% damping Site Class Figure 1615(1) – East Coast

  41. Section 1616 – EQ Load Criteria Selection • Seismic Design Criteria • Lateral resisting systems • Continuous path • Seismic Use Group and Importance Factors (I, II, or III) • Table 1604.5 • Same as for wind design

  42. Section 1616.3 – Seismic Design Category • Design Categories A – F (used to be Zones 1 – 4) impacts: • Structural system • Height and plan limitations • Components design • Types of analysis

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