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# Seismic acceleration (aH) limits

## Seismic acceleration (aH) limits

(OP)
Hi,

I am trying to understand an example for calculating seismic acceleration.

The equation identifies seismic acceleration limits of 1.6 and 0.3. I have checked ASCE 7 but cant seem to find any information on this matter.

Any explanation of what these numbers represent and where they come from would be appreiciated.

Daniel
Replies continue below

### RE: Seismic acceleration (aH) limits

is this referring to mechanical systems? can you provide a reference? i have my hunch

### RE: Seismic acceleration (aH) limits

(OP)
Hi,
Sorry I dont have a reference to any codes for my problem, all I can supply is the actual work through. It is a worked example from Caesar stress analysis software.

The Importance Factor IE = 1

The Component Response Modification Factor RP = 12.0, from Table 13.6-1 for "Piping in accordance with ASME B31...",

The Mapped MCE Spectral Response Acceleration Ss = 1.552, Mapped MCE Spectral Response Acceleration at short periods in accordance with section 11.4.1.

The Site Coefficient (Fa) = 1.0 for Site Class D, according to Table 11.4-1.

The Maximum Considered Earthquake MCE SMS - The MCE is adjusted for site class effects as defined in Section 11.4.3,

SMS = Fa SS = 1.552

SDS - Design elastic response acceleration at short period (0.2 sec), from Section 11.4.4.

SDS= 2/3 SMS = 2/3 * 1.552 = 1.0347

The appropriate seismic acceleration is aH

= [ (0.4aPSDS) / ( RP / IP )] ( 1 + 2z/h)

= [(0.4 x 2.5 x 1.0347)/(12.0/1.0)](1 + 2 * 0.5)

= 0.17245

Check limits on aH:

aH <= 1.6 * SDS* IP = 1.6 * 1.0347 * 1.0 = 1.65552

aH >= 0.3 * SDS * IP = 0.3 * 1.0347 * 1.0 = 0.31041

aH = 0.31041

aH = 0.2 SDS= 0.2 * 1.0347 = 0.20694,

Section 13.1.7 states, "The earthquake loads determined in accordance with Section 13.3.1 shall be multiplied by a factor of 0.7."

aH = 0.7 * 0.31041 = 0.2173

av = 0.7 * 0.20694 = 0.1149

Daniel

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