When it comes to designing residential buildings, seismic load analysis is an important consideration that shouldn’t be overlooked. Structural engineers, architects, and structural designers all need to factor it into their designs in order to ensure the safety of a building during an earthquake.

In general, earthquakes can cause damage to residential buildings and other types of structures, including collapse, which can result in injury or death to the occupants. To ensure the safety of residential structures during an earthquake, engineers and architects use seismic load analysis methods that determine the strength and stability of the building and its components.

Figure 1: A damaged building in downtown Napa, California, after the 6.0 magnitude earthquake on August 24th, 2014 (Reference).

This article discusses the procedure for calculating seismic loads in residential buildings using the linear static approach in Chapters 11 and 12 of the ASCE 7-16.

Seismic Load Analysis in Brief

Seismic load analysis is a procedure used to determine the forces a building or structure will experience during an earthquake and evaluate the building's ability to withstand those forces.

The procedure typically involves conducting a structural analysis of the building, including modeling the building's response to ground motion and determining the forces that will be applied to the building's various components.

This analysis is then used to design the building's structural systems and components to resist seismic forces and ensure the building's safety and integrity during an earthquake.

The Minimum Design Loads and Associated Criteria for Buildings and Other Structures, commonly referred to as ASCE 7, is a standard published by the American Society of Civil Engineers (ASCE).

The ASCE 7 standards provide guidelines for the design of structures to resist various types of loads, including wind, snow, rain, and seismic loads. The first edition of the ASCE 7 standard was published in 1988. The standard is typically updated on regular bases every three to six years, with each edition incorporating the latest research and advancements in the field of structural engineering.

Currently, the ASCE 7-16 is widely used in the United States and is adopted by many states and local building codes. It is also used as a reference in other countries.

Seismic Load Calculation Using the ASCE 7-16 Provisions

Identifying Site Class

The procedure to perform a seismic load calculation for a residential building using the method discussed in Chapters 11 and 12 of the ASCE7-16 starts by identifying the site class (commonly done by doing a geotechnical site survey).

The importance of correctly identifying the site class is the impact of the soil type on the seismic loads coming to the building. The mapped risk-targeted maximum considered earthquake (MCER) spectral response acceleration parameters at short periods (Ss) and at a period of one second (S1) as well as the long-period transition period (TL) based on the building location.

These spectral parameters can be obtained from the ASCE 7 Hazard Tool, using the building's longitude and latitude.

Figure 2: ASCE 7 online hazard tool used to obtain the spectral parameters (Reference).

Defining the Seismic Ground Motion Values

Once the spectral parameters for the building's site are obtained, the site coefficients (Fₐ and Fᵥ) are defined using the data provided in Table 1.

Table 1: Site coefficients at short and one-second periods

Thereafter, the design response spectrum is developed using Eqs. 1 to 4 as well as Figure 3.

where Sₘₛ and Sₘ₁ are the MCER, 5% damped, spectral response acceleration parameter at a short period and period of 1s, respectively; SDS and SD1 are the design, 5% damped, spectral response acceleration parameter at a short period and period of 1s, respectively.

Figure 3: Design response spectrum defined in the ASCE 7-16 (Reference).

Obtaining the Seismic Force Resisting System (SFRS) Properties

Once the design spectrum is established, the risk category, response modification coefficient (R) based on the type of seismic resisting system and the material used, importance factor (Ie), and approximate period parameters Ct and x are obtained from Tables 2, 3, 4, and 5, respectively.

Table 2: Risk category of buildings and other structures under various load type

Table 3: An example of the design coefficients and factors for various seismic force-resisting systems

Table 4: Importance factor for various load types

Table 5: Values of approximate period parameters Ct and x

Calculating the Effective Seismic Weight on the Building

Once the parameters are obtained and set, the effective seismic weight of each story in the residential building is calculated based on the self-weight of the structural and non-structural elements and the partial contribution of the live load in the building.

Moreover, the structure's natural period (T) in the direction of concern is computed according to the method explained in section 12.8.2 of the ASCE 7-16. Thereafter, the seismic response coefficient is calculated using Eq. 5 and checked against the conditions in Eqs. 6, 7, and 8.

The value of Cs shall not exceed, for T ≤ Tʟ:

for T > Tʟ:

The value of Cs shall not be less than

In addition, for structures located where S1 is equal to or greater than 0.6g, the value of Cs shall not be less than

Calculating Seismic Base Shear

Finally, the seismic base shear of the structure in the direction of concern is calculated using Eq. 9 and distributed to each story using Eq. 10.

where W is the effective seismic weight of the structure

where C_vx is the vertical distribution factor calculated as

w_x and w_i are the portion of the total effective seismic weight of the structure located or assigned to levels x and i, respectively; h_x and h_i are the height from the base to levels x and I, respectively; k is an exponent related to the structure's period where k = 1 for a period of 0.5s or less, k = 2 for a period of 2.5s or more, and k is linearly interpolated between 1 and 2 for periods between 0.5 and 2.5s.

Conclusion

In order to comply with ASCE 7-16 requirements for analyzing seismic loads in residential buildings, engineers must follow the procedure outlined in Chapters 11 and 12. To find out how ClearCalcs integrates these calculations for seismic load analysis, check out our Seismic Analysis Calculator to ASCE 7-16.

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References

• Aksoylu, C., Mobark, A., Hakan Arslan, M., & Hakkı Erkan, İ. (2020). A comparative study on ASCE 7-16, TBEC-2018 and TEC-2007 for reinforced concrete buildings. Revista de la Construcción, 19(2), 282-305.
• American Society of Civil Engineers. (2017). Minimum design loads and associated criteria for buildings and other structures. American Society of Civil Engineers.
• AL-OBAIDI, A. L. I. (2018). Comparison between the American code ASCE7-16 and the Australian code AS1170. 4 against the seismic design
• Fajfar, P. (2018). Analysis in seismic provisions for buildings: past, present and future. In European Conference on Earthquake Engineering Thessaloniki, Greece (pp. 1-49). Springer, Cham.