PPP Notes - Seismic Design

SEISMIC FOUNDATION ISSUES

 (These notes are compiled from AGS)

SEISMIC DESIGN

 

• According to the theory of plate tectonics, the Earth’s crust is divided into constantly moving plates.
• Earthquakes occur when, as a result of slowly accumulating pressure, the ground slips abruptly along a geological fault plane or on near a plate boundary.
• The resulting waves of vibration within the Earth create ground motions at the surface, which, in turn, induce movement within buildings.
• The frequency, magnitude, and duration of the ground motion; physical characteristics of the building; and geology of a site determine how these forces affect a building.

 

DESIGN JUDGMENT

 

• During a seismic event, buildings designed to the minimum levels required by model codes often sustain damage, even significant structural damage.
• Early discussions with an owner should explore the need to limit properly loss in an earthquake, and the desirability of attempting to ensure continued building operation immediately afterward.
• To achieve these results, it may be necessary to make design decisions that are more carefully tuned to the seismic conditions of a site than the code requires.
• The relationship between the period of ground motion and the period of building motion is of great importance to building design.
• Fundamental periods of motion in structures range from 0.1 seconds for a one-story building to 4.0 seconds or more for a high-rise building.
• Ground generally vibrates for a period of between 0.5 and 1.0 second.  If the period of ground motion and the natural period of motion in a building coincide, the building may resonate, and the loads will be increased.
• Theoretically, one part of the seismic design solution is to “tune” the building so that its own period of motion falls outside the estimated range of ground motion frequency.
• In practice, this tuning is very seldom carried out.
• Rather, design professionals rely on increased load effects required by the applicable code to take care of the problem.

 

SEISMIC CODES

 

• The building code adopted in most jurisdictions in the United States is International Building Code (IBC).
• There are some significant changes to the seismic forces determined by this code compared to seismic forces determined by previous building codes.
• The IBC 2006 code seismic provisions are designed around a level of earthquake that is expected to be exceeded only 2 percent of the time in the next 50 years.
• The level of seismic design for most structures, per the IBC, is based on a “collapse protection” strategy (commonly referred to as a “life safety” level), which assumes that there may be significant damage to the structure up to the point of collapse but that the structure does not collapse.
• The structural engineer will design a lateral force-resisting structural assembly to resist a design-level earthquake. 
• These designs are developed from detailed maps that indicate the ground spectral accelerations of buildings, which are based upon known past seismic events, in combination with probability studies.
• These maps typically include known fault locations, which help to determine the distance of the building from any known fault.
• The ground accelerations can typically be found down to the county level in the United States. 
• The geotechnical engineer works with the design team to develop the site coefficient, which is dependent on the local soil layers and depths.
• The following information is based on the requirements in the IBC 2006 Building Code, which in turn is based on the 2000 National Earthquake Hazards Reduction Program (NEHRP). 
• Detached one- and two-family dwellings are exempt from seismic regulations in areas other than those with high seismicity.

 

TERMS

 

• The seismic community has an extensive set of terms that describe common conditions in the field.
• Here is a short list of these terms and their definitions:
• Base Shear (static analysis): Calculated total shear force acting at the base of a structure, used in codes as a static representation of lateral earthquake forces.  Also referred to as equivalent lateral loads.
• Design Earthquake: Earthquake ground motion for which a building is designed.  This is typically about two-thirds of the maximum considered earthquake (MCE) when designing per the IBC codes.
• Drift and Story Drift: Lateral deflection of a building or structure.  Story drift is the relative movement between adjacent floors.
• Ductility: The ability of a structural frame to bend, but not break.  Ductility is a major factor in establishing the ability of a building to withstand large earthquakes.  Ductile materials (steel, in particular) fail only after permanent deformation has taken place.  Good ductility requires special detailing of the joints.
• Dynamic Analysis: A structural analysis based on the vibration motion of a building.  Dynamic analysis is time-consuming, and normally reserved for complex project.
• Forces, In-Plane: Forces exerted parallel to a wall or frame.
• Forces, Out-of-Plane: Forces exerted perpendicular to a wall or frame.
• Maximum Considered Earthquake (MCE): The greatest ground-shaking expected to occur during an earthquake at a site.  These values are somewhat higher than those of the design earthquake, particularly in areas where seismic events are very infrequent.  The code maps are based on earthquakes of this magnitude.
• Reentrant Corner: The inside building corner of an L-, H-, X-, or T-shaped building plan.

 

ESTABLISHING SEISMIC FORCES

 

• The equivalent lateral force procedure is the most common method used to determine seismic design forces.
• In the ELFP, the seismic load, V (base shear), is determined by multiplying the weight of the building by a factor of Cs (V=CsW). 
• The value of Cs depends on the size of the design earthquake, the type of soil, the period of the building, the importance of the building, and the response-modification factor (a variable that accounts for different levels of ductility for different types of later force-resisting systems used).
• This force is applied at the base of the structure then is distributed vertically throughout the building according to the stiffness of the lateral elements of the structure (for a “rigid” diaphragm), or according to tributary width of lateral elements of the structure (for a “flexible” diaphragm).

 

DESIGN AND RESISTING SEISMIC FORCES/FOUNDATIONS ISSUES

 

• A design that resists seismic forces for a structure makes use of the lateral system’' ductility.
• Ductile lateral systems are designed to deflect more under seismic loading than what would be expected from something such as wind loading.
• This allows for the use of smaller effective seismic design forces and more reasonably sized members.
• It’s important, however, that the overall design still be capable of handling the expected deflections.
• Story drifts that are too large can result in secondary forces and stresses for which the structure was not designed, as well as increase the damage to the interior and exterior building components, and hinder the means of egress from the building.
• Typical means of resisting these forces include the use of moment frames, shear walls, and braced frames.
• Each of these types of lateral systems can be made up of one of the main structural materials (such as steel or reinforced concrete moment frames; masonry, wood, or reinforced concrete shear walls; or steel or reinforced concrete-braced frames). 
• The building configuration and design parameters will have a major effect on which system to choose and, subsequently, the lateral system chosen will have a major impact on the foundations required to resist the loads.
• Moment frames typically are distributed more evenly over the building footprint and have little or no uplift.
• Moment frames also generally have large base moments that can be difficult to resist. 
• In addition, moment frames will tend to have greater lateral deflections than other stiffer systems (such as shear walls or braced frames).
• Concrete shear walls and steel-braced frames are more localized, not only concentrating lateral shear at the base but also having a high potential for net uplift forces to be resisted.
• These forces are difficult to resist with some foundation systems and should be reviewed extensively before selecting the lateral load-resisting system.
• Tall, narrow structures tend to have overturning issues before they will face sliding issues.
• Conversely, short structures face sliding problems rather than overturning problems.
• Seismic motion rocks the building, increasing overturning loads, and can act in any direction.
• Thus, resistance to overturning is best achieved at a building’s perimeter, rather than at its core.
• Building foundations must be designed to resist the lateral forces transmitted through the earth and the forces transmitted from the lateral load-resisting system to the earth.
• In general, softer soils amplify seismic motion.

 

FOUNDATION STRATEGIES FOR SEISMIC DESIGN

 

UPLIFT

 

• Braced buildings typically end up with high-tension loads at the foundations. 
• Shallow foundations are difficult to design with high-tension loads.
• Some strategies are available to resist these uplift forces:
• Increase the dead load by removing adjacent columns, increasing the tributary area.
• Deepen the footing, increasing the soil load.
• Increase the footing dimensions, to increase the soil and concrete loads.
• Decrease column spacing, to decrease the brace forces.
• Change the foundation type (typically, to a deep foundation that can resist the uplift more effectively).

 

SHEAR

 

• Braces and shear walls tend to collect the lateral forces and concentrate the loads in a few locations.
• Shallow and deep foundations have limited lateral load-resisting capability.
• By combining several foundations together, it is possible to effectively increase the lateral load resistance.
• The concrete tie beams are typically designed to distribute the lateral loads through tension or compression of the beam.