Earthquakes are one of the sources of lateral loads in buildings. In certain regions of the world that are located in seismic zones such as Indonesia, India, USA, Japan etc., earthquakes are regular occurrence and designs against earthquake forces should be incorporated in buildings. Light buildings such as low-rise buildings not more that 3-storeys above grade or level and wood framed homes are known to have performed well in major seismic events owing to their;
- Light-weight and resilient construction,
- The strength provided by nonstructural systems such as interior walls, and
- Load distribution capabilities
These advantages could still present a great threat to human life if good judgment and wrong application of earthquake forces to buildings during design is the case. Hence it is very important that one is acquainted on how to properly apply earthquake forces to buildings.
The lateral forces associated with seismic ground motion are based on fundamental Newtonian mechanics [F (force) = mass (m) x acceleration (a)] expressed in terms of an equivalent static load. The total lateral force at the base of a building is called seismic base shear. The lateral force experienced at a particular story level is called the story shear. The story shear is greatest in the ground story and least in the top story. Seismic base shear and story shear (V) are determined in accordance with the following equation:
Where,
R = the response modification factor (dimensionless)
W = the total weight of the building or supported by the story under consideration (lb); 20 percent of the roof snow load is also included where the ground snow load exceeds 30 psf.
1.2 = factor to increase the seismic shear load based on the belief that the simplified method may result in greater uncertainty in the estimated seismic load
SDS = the design spectral response acceleration in the short-period range determined by expression:
Where,
Ss = mapped short-period spectral response acceleration which can be obtained from a seismic map for the purpose.
Fa = soil site amplification factor
When determining story shear for a given story, the designer attributes to that story one-half of the dead load of the walls on the story under consideration and the dead load supported by the story. For housing, the interior partition wall dead load is reasonably accounted for by the use of a 6 psf load distributed uniformly over the floor area. When applicable, the snow load may be also being included. The inclusion of any snow load, however, is based on the assumption that the snow is always frozen solid and adhered to the building such that it is part of the building mass during the entire seismic event. The design spectral response acceleration for short-period ground motion SDS is typically used because light-frame buildings such as houses are believed to have a short period of vibration in response to seismic ground motion (i.e., high natural frequency).
The value of Ss ranges from practically zero in low-risk areas to 3g in the highest-risk regions of the United States. A typical value in high seismic areas is 1.5g. In general, wind loads control the design of the lateral force-resisting system of light-frame houses when Ss is less than about 1g. The 2/3 coefficient in expression above is used to adjust to a design seismic ground motion value from that represented by the mapped Ss values (i.e., the mapped values are based on a “maximum considered earthquake” generally representative of a 2,475-year return period, with the design basis intended to represent a 475-year return period event).
Fa associated with a standard “firm” soil condition used for the design of residential buildings. Fa decreases with increasing ground motion because the soil begins to dampen the ground motion as shaking intensifies. Therefore, the soil can have a moderating effect on the seismic shear loads experienced by buildings in high seismic risk regions. Dampening also occurs between a building foundation and the soil and thus has a moderating effect. If a site is located on fill soils or “soft” ground, a different value of Fa should be considered, though, soft soils do not necessarily affect the performance of the above-ground house structure as much as they affect the site and foundations (e.g., settlement, fissuring, liquefaction, etc.).
The response modification factor, R-factor is incorporated in seismic design in recognition that buildings can effectively dissipate energy from seismic ground motions through ductile damage. The R-factor was conceived to adjust the shear forces from that which would be experienced if a building could exhibit perfectly elastic behavior without some form of ductile energy dissipation. It also incorporates differences in dampening that are believed to occur for various structural systems. The concept has served a major role in standardizing the seismic design of buildings. Thus, structural building systems that are able to withstand greater ductile damage and deformation without substantial loss of strength are assigned a higher value for R. Table 2 shows typical R-factors.
Notes:
1The R-factors may vary for a given structural system type depending on wall configuration, material selection, and connection detailing, but these considerations are necessarily matters of designer judgment
2The wall is reinforced in accordance with concrete design requirements in ACI-318 or ACI-530. Nominally reinforced concrete or masonry that has conventional amounts of vertical reinforcement such as one #5 rebar at openings and at 4 feet on center may use the value for reinforced walls provided the construction is no more than two stories above grade.
