cee 4606 - capstone ii structural engineering

Villanova UniversityDept. of Civil & Environmental Engineering

CEE 4606 - Capstone IIStructural Engineering

Lecture 3 Seismology, Earthquakes, and Roof Design

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

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

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

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

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

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

Seismographs• Instrument that

records the earth’s motion

• North-South • East-West• Vertical• Pen-Plotter• Digital

What causes earthquakes?

The lithosphere is broken into rigid plates that move.

Arabian Plate

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

Types of Seismic WavesP Wave

S Wave

Love Wave

Rayleigh Wave

Surface

Body Waves

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)

Like a slinky

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

Snapping a piece of rope

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

Love Waves

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)

Being on a ship

P waves travel approximately 1.7 times faster than S waves

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?

Use Multiple Seismographs

Example Problem

Example Problem continued

We use that procedure for all earthquakes

Magnitudes of Earthquakes• The magnitude is an estimate of the

relative size (amplitude) of an earthquake measured from a seismogram

Richter Scale• 1935, Charles Richter of CIT defined the

magnitude of an earthquakeMagnitude - 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

Magnitudes• Earthquakes of

magnitude < 5.0 are not expected to cause structural damage

• Earthquakes > 5.0 are potentially very damaging

Procedure for Measuring Magnitude

• S - P = 24 sec• Max height = 23

mm• Connect points

with a straight line

• Read intersection• Magnitude = 5.0

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

Relating Richter to Mercalli

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

Tremors were felt at our site

Earthquakes Over 5.0 Richter in Honduras, 1900 - 1980

Structural damage would be expected in structures designed in accordance with US codes

Earthquake Design

Section 1613 –DefinitionsSection 1614 –General

– Exceptions– Additions and/or alterations– Change of use

Earthquake Design• Considerations are 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)

Figure 1615(1) – East Coast• Contours• Acceleration in

% of gravity• Linear

interpolation• Specific time of

response (.2 sec)

• Assumption of 5% damping

• Site Class

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

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

• Section 1617 – EQ Loads – Minimum Design Lateral Force and Related Effects– 1617.4 Equivalent Lateral Force Procedure

• Equation 16-34 • Base Shear, V = CsW

– 1617.5 “Simplified” Procedure• Equation 16-49 • Base Shear, V = 1.2SDSW/R

Types of Analysis

• Based on calculating the base shear, V, of the structure and distributing the load vertically to the different story heights

Static Procedures

1618 - Dynamic Analysis Procedures

• Modal Response Spectrum Analysis• Linear Time-History Analyses• Nonlinear Time-History Analyses• Alternative Advanced Analyses• Generally, the simple analyses

provide more conservative designs

Other Sections• 1619 – Soil Structure Interaction

Effects• 1620 – Design, Detailing

Requirements and Structural Component Load Effects

• 1621 – Architectural, Mechanical, and Electrical Component Requirements

• 1622 – Nonbuilding Structures

Some Information Sources

• www.usgs.gov• www.nhc.noaa.gov• www.eeri.org• Compendix - Database of

engineering journals (thru VU library)

Chapter 15 Roof Assemblies and Rooftop Structures

• 1501 – General• 1502 – Definitions• 1503 – Weather

Protection• 1504 –

Performance Requirements

• 1505 – Fire Classification

• 1506 – Materials• 1507 – Requirements

for Roof Coverings (largest section)

• 1508 – Roof Insulation• 1509 – Rooftop

Structures• 1510 - Reroofing

Roof Design

• Constraints– Material availability– No lifting equipment– Typical construction practice

• Structural Layout• Critical Loads

Truss Questions• Are the trusses supported by the

columns on the porch?– Trusses should be supported by columns

• Do the walls extend to the roof or to the base of the truss?– Trusses will be exposed on the interior

of the structure• Truss information?

– Constructed from 2 - 5”x2”x1/16” channels welded together

Truss Construction

Trusses

Connection Detail

Homework

• Continue research on wind and seismicity of Honduras– Determine a design wind speed– Determine the ground acceleration

• Determine the loads for your roof design

Next Lecture

• Load paths• Construction considerations• Design guides• Review of Progress Report #1

Requirements

cee 4606 - capstone ii structural engineering

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