[Part 1] Step by Step Analysis Procedure of Seismic Loads Based on IBC2012/ASCE7-10
First of all, I would like to thank all our followers and engineers who were motivating us with the good feedback of our content. After a long detailing of the earthquakes theory and its affection on the buildings, I thought to write an article where we dig a little bit in the standard requirements and steps to follow. Hoping to be useful for all engineering students and every fresh graduate looking forward to step into the structural engineering and modeling.
I highly recommend for all engineers, whether they still students or already graduate, NOT to go directly forward the modeling software. I believe that it’s very important for every engineer to understand the science of engineering, formulas, standard requirements and the soul of engineering. Unfortunately, we see most of engineers get deeply inside the software modeling and get lost in the small details of modeling without having a clear comprehension of the purpose of analyzing or how to adequately use these software. I think every engineer can model a building, bridge, road, etc but not every model is correct. As most of models need iterations and modifications to meet the standard or reference requirements. We will go through the art of modeling and analyzing , mainly for CSI software but at a later stage after we provide good theoretical information.
In case you have missed our previous articles related to this field, It may be useful for you to check it before continue reading this article:
- How Earthquakes are caused ?
- What are the Seismic Effects on Structures?
- How Architectural Features Affect Buildings During Earthquakes?
- How Buildings Twist During Earthquakes?
- How to Make Buildings Ductile for Good Seismic Performance?
- How Flexibility of Buildings Affects their Earthquake Response?
- How do Earthquakes Affect Reinforced Concrete Buildings ?
- How do Beams in RC Buildings Resist Earthquake ?
- How do Columns in RC Buildings Resist Earthquakes?
- How do Beam-Column Joints in RC Buildings Resist Earthquake?
- Why are Open-Ground Story Buildings Vulnerable in Earthquakes?
- Why are Buildings with Shear Walls Preferred in Seismic Regions?
- How to Reduce Earthquake Effects on Buildings?
Alright, so let’s begin with the purpose of this article. With reference to IBC2012 section 1613.1:
“Every structure, and portion thereof, including nonstructural components that are permanently attached to structures and their supports and attachments, shall be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7, excluding Chapter 14 and Appendix 11A. The seismic design category for a structure is permitted to be determined in accordance with Section 1613 or ASCE 7”.
But we have some exceptions that can be summarized as follow:
1-Detached one- and two-family dwellings, assigned to Seismic Design Category A, B
or C, or located where the mapped short-period spectral response acceleration, SS, is less
than 0.4 g.
2-The seismic force-resisting system of wood-frame buildings that conform to the
provisions of Section 2308 are not required to be analyzed as specified in this section.
3-Agricultural storage structures intended only for incidental human occupancy.
4-Structures that require special consideration of their response characteristics and
environment that are not addressed by this code or ASCE 7 and for which other
regulations provide seismic criteria, such as vehicular bridges, electrical transmission
towers, hydraulic structures, buried utility lines,etc.
1-Determination of maximum considered earthquake and design spectral response accelerations:
- Determine the mapped maximum considered earthquake MCE spectral response accelerations, Ss for short period (0.2 sec.) and S1 for long period (1.0 sec.) using the spectral acceleration maps in IBC Figures 1613.3.1(1) through 1613.3.1(6).
Where S1 is less than or equal to 0.04 and Ss is less than or equal to 0.15, the structure is permitted to be assigned to Seismic Design Category A.
- Determine the site class based on the soil properties. The site shall be classified as Site Class A, B, C, D, E or F in accordance with Chapter 20 of ASCE 7. Where the soil properties are not known in sufficient detail to determine the site class, Site Class D shall be used unless the building official or geotechnical data determines Site Class E or F soils are present at the site.
- Determine the maximum considered earthquake spectral response accelerations adjusted for site class effects, SMS at short period and SM1 at long period in accordance with IBC 1613.3.3.
SMS = Fa * Ss
SM1 = Fv *S1
Fa = Site coefficient defined in IBC Table 1613.3.3(1).
Fv = Site coefficient defined in IBC Table 1613.3.3(2).
- Determine the 5% damped design spectral response accelerations SDS at short period and SD1 at long period in accordance with IBC 1613.3.4.
SDS = (2/3)SMS
SD1 = (2/3)SM1
SMS = The maximum considered earthquake spectral response accelerations for short period as determined in section 1613.3.3.
SM1 = The maximum considered earthquake spectral response accelerations for long period as determined in section 1613.3.3.
2. Determination of seismic design category and Importance factor:
Risk categories of buildings and other structures are shown in IBC Table 1604.5.
Importance factors, Ie , are shown in ASCE 7-10 Table 1.5-2. Structures classified as Risk Category I, II or III that are located where the mapped spectral response acceleration parameter at 1-second period, S1 , is greater than or equal to 0.75 shall be assigned to Seismic Design Category E.
Structures classified as Risk Category IV that are located where the mapped spectral response acceleration parameter at 1-second period, S1 , is greater than or equal to 0.75 shall be assigned to Seismic Design Category F.
All other structures shall be assigned to a seismic design category based on their risk category and the design spectral response acceleration parameters, SDS and SD1 , determined in accordance with Section 1613.3.4 or the site-specific procedures of ASCE 7.
Each building and structure shall be assigned to the more severe seismic design category in accordance with Table 1613.3.5(1) or 1613.5.5(2), irrespective of the fundamental period of vibration of the structure.
3. Determination of the Seismic Base Shear
The structural analysis shall consist of one of the types permitted in ASCE 7-10 Table 12.6-1, based on the structure’s seismic design category, structural system, dynamic properties, and regularity, or with the approval of the authority having jurisdiction, an alternative generally accepted procedure is permitted to be used. The analysis procedure selected shall be completed in accordance with the requirements of the corresponding section referenced in Table 12.6-1.
3.1 Equivalent Lateral Force Analysis
Section 12.8 of ASCE 7-10 shall be used.
- The seismic base shear V in a given direction is determined in accordance with the following equation:
V = Cs*W , where W=effective seismic weight
- The effective seismic weight, W, of a structure shall include the dead load above the base and other loads above the base as listed below:
1. In areas used for storage, a minimum of 25 percent of the floor live load shall be included.
a. Where the inclusion of storage loads adds no more than 5% to the effective seismic weight at that level, it need not be included in the effective seismic weight.
b. Floor live load in public garages and open parking structures need not be included.
2. Where provision for partitions is required in the floor load design, the actual partition weight or a minimum weight of 0.48 kN/m2 of floor area, whichever is greater.
3. Total operating weight of permanent equipment.
- Cs = SDS/(R/Ie)
Cs = Seismic response coefficient
R = response modification factor, given in ASCE 7-10 Table 12.2-1
e I = importance factor
- The value of Cs shall not exceed the following:
Cs = SD1 / [T*(R/Ie)] for T < TL
Cs = (SD1*TL) / [T*T*(R/Ie)] for T>TL
- The Value of Cs shall not be less than:
Cs = 0.044*SDS*Ie >= 0.01
- For structures located where S1 is equal to or greater than 0.6g, Cs shall not be less than:
Cs = (0.5*S1)/(R/Ie)
T = fundamental period of the structure
TL = long-period transition period, (given in ASCE 7-10 Figure 22), which is the transition period between the velocity and displacement-controlled portions of the design spectrum (about 5 seconds for Gaza Strip).
- An approximate value of Ta may be obtained from:
Ta = Ct * hn^x
hn = height of the building above the base in meters.
Ct = building period coefficient given in Table 12.8-2.
x = constant given in Table 12.8-2.
The calculated fundamental period, T, cannot exceed the product of the coefficient, Cu , in Table 12.8-1 times the approximate fundamental period, Ta .
In cases where moment resisting frames do not exceed twelve stories in height and having a minimum story height of 3 m, an approximate period a T in seconds in the following form can be used:
Ta = 0.1*N
where, N = number of stories above the base.
I will continue the rest of the procedure into another post, where I’ll try to explain the vertical distribution of seismic forces, horizontal distribution of forces and torsion, story drift, seismic load effects and load combinations, Redundancy and maybe elaborate a practical example.