Building Loading - Seismic

(Revised:  11/15/2016)

The Seismic tab defines related design information regarding the Building's (or Shape's) Seismic Load.

The Seismic data defined will apply to the design of the Building's (or Shape's) resistance to stress produced by earthquake loads.

  Note to Builder/Customer:

The builder is responsible for contacting the local building official or project design professional to obtain and provide all code and loading information for the specific building site.  Data supplied is assumed to be accurate and is not verified.

 

Seismic Load Information:

Spectral Response Acceleration (Ss) – IBC

Ss is a coefficient used to calculate the effect of the Maximum considered earthquake ground motion for the given geographical location.  It corresponds to the expected ground acceleration at the short period (0.2 sec.) with 5% critical damping included.  The Ss coefficient is commonly shown as a percentage of the ground acceleration (e.g., 47%g).  It is based on 2 percent probability of exceedance within a 50-year period.

Spectral Response Acceleration (S1) – IBC

S1 is a coefficient used to calculate the effect of the Maximum considered earthquake ground motion for the given geographical location.  It corresponds to the expected ground acceleration at the 1-second period with 5% critical damping included.

The S1 coefficient is commonly shown as a percentage of the ground acceleration (e.g., 24%g).  It is based on 2 percent probability of exceedance within a 50-year period.

  Notes:

In some areas of higher seismic susceptibility, the magnitude of this coefficient may change significantly within a short distance.  Therefore, one should use prudence when relying on the zip code since the zip code areas may be large or even discontinuous.  For known job site addresses, the geographical coordinates can be obtained from most GPS navigation units or from many websites, including http:Itouchmap.com/latlong.html.

  Notes:

There are several possible sources for this required input, Ss and S1, shown in the order of preference:

Local building department official.

Local or state building code (several states list county-specific minimum coefficients).

USGS web site – allows precise determination of the required parameters for 48 lower states based on:

    See < USGS Help Screen > explaining the use of the Seismic Calculator found on the USGS web site.

 

Damped Spectral Response Acceleration [ Sa(T)] – 2005-15 National Building Code of Canada (NBCC)

Sa(T) = 5% damped spectral response acceleration, expressed as a ratio of gravitational acceleration for a period of T, as defined in sentence 4.1.8.4(1).  Sa(0.2), Sa(0.5), Sa(1.0) and Sa(2.0) are values that come directly from the Canadian Building Codes for a given location.  Sa(5.0) was added with implementation of 2015 NBCC.

 

Peak Ground Acceleration (PGA) - 2015 National Building Code of Canada (NBCC)

PGA = Peak Ground Acceleration expressed as a ratio to gravitational acceleration.  PGA values come directly from the Canadian Building Code for a given location.

  Note:

2015 NBCC Appendix C, Table C-3 also includes values for Sa(10.0) and PGV (Peak Ground Velocity); however, these values are not used in BlueScope’s typical analysis. 

Seismic Design Category (SDC)

A classification assigned to a structure based on its Risk Category and the severity of the design earthquake ground motion at the site.  It is calculated automatically based on all other values input.

 

Seismic Zone

From this drop list, select the Seismic Zone Code that is specified by the regional Building Code – primarily International and Legacy Codes require this input.  This option is enabled when it is required by the Building Code.

Seismic Zone Descriptions:

Typical           UBC

Zone 0           Zone 1           Lowest probability of damaging seismic conditions.

Zone 1           Zone 2a         Relatively low probability of damaging seismic conditions.

Zone 2           Zone 2b         Greater probability of damaging seismic conditions.

Zone 3           Zone 3           Much greater probability of damaging seismic conditions.

Zone 4           Zone 4           Highest probability of damaging seismic conditions.

Zone NA        Zone NA        Not Applicable.

  Notes:

Consult the project site Building Code Map for these values.

If the Seismic Zone is greater than 1, the Building (or Shape) will be designed to resist a minimum total lateral seismic force.

The Collateral Load is automatically included with the Gravity Loads in all Seismic calculations.  A percentage of the Snow Load is also included if required by the Building Code and snow conditions.

Hazard Group

From this drop list, select the Seismic Zone Hazard Group that is specified by the regional Building Code.  This option is enabled when the Building Code requires it.

Hazard Group Descriptions
Only applicable for the following Building Codes:

%!JumpId(IDC_CODES_USE_1)   1991 SBC

%!JumpId(IDC_CODES_USE_1)   1999, 1997, & 1994 SBC

%!JumpId(IDC_CODES_USE_1)   1993 BOCA, 1996 BOCA, 1999 BOCA

%!JumpId(IDC_CODES_USE_1)   1993 ASCE

%!JumpId(IDC_CODES_USE_1)   MAST (6th)

 

Acceleration (Av)

Av is a coefficient representing the effective peak velocity-related acceleration which is used to calculate the prescribed seismic forces for the given geographical location.  It is based upon a 90% chance of not being exceeded in a 50-year mean recurrence interval.

The Av coefficient is commonly shown as a rational number with minimum value of 0.0 (zero) and maximum value of 0.4.

Acceleration (Aa)

Aa is a coefficient representing the effective peak acceleration which is used to calculate the prescribed seismic forces for the given geographical location.  It is based upon a 90% chance of not being exceeded in a 50-year mean recurrence interval.

The Aa coefficient is commonly shown as a rational number with minimum value of 0.0 (zero) and maximum value of 0.4.

  Notes:

There are several possible sources for these parameters, shown in the order of preference:

Local building department official

Local or state building code (several states list county-specific minimum coefficients)

Seismic maps included in the seismic section of the relevant Building Code or seismic maps included with 1991 and 1994 edition of the NEHRP Recommendations

Velocity-Related Zone (Zv)

From this drop list, select the Velocity-Related Zone factor that is specified by the regional or local Building Code.  This option is enabled when the Building Code requires it.

Acceleration-Related Zone (Za)

From this drop list, select the Acceleration-Related Zone factor that is specified by the regional or local Building Code.  This option is enabled when the Building Code requires it.

Zonal Velocity Ratio (v)

From this drop list, select the Zonal Velocity Ratio that is specified by the regional or local Building Code.  This option is enabled when the Building Code requires it.

 

Soil Profile (Site Class)

From this drop list, select the Soil Profile that is specified by the regional or local Building Code.  This option is enabled when the Building Code requires it.

Soil Profile Descriptions for Some Typical Building Codes:

1995-2010 ASCE 7, 2000-2015 IBC, MAST (7th), MAST (8th)

A: Hard rock

B: Rock

C: Very dense soil and soft rock

D: Stiff soil (BlueScope default if Soil Profile is unknown.)

E: Soft clay soil

F: Soils requiring site-specific evaluation (a response analysis reporting)

2005-2015 NBCC

A: Hard Rock

B: Rock

C: Very Dense Soil and Soft Rock

D: Stiff Soil Profile (BlueScope default if Soil Profile is unknown.)

E: Soft Soil Profile

F: Other

1997 UBC

A: Hard Rock

B: Rock

C: Very Dense Soil and Soft Rock

D: Stiff Soil Profile (BlueScope default if Soil Profile is unknown.)

E: Soft Soil Profile

F: Soil Requiring Site-specific Evaluation.  See UBC Section 1629.3.1.

 

%!JumpId(IDC_CODES_USE_1)   MAST (6th)

Seismic Source Type

From this drop list, select the Seismic Source Type that is specified by the regional or local Building Code.  This option is enabled when the Building Code requires it.  From 1997 UBC:

Seismic Source Type A: Faults that are capable of producing large magnitude events and that have a high rate of seismic activity.

1.)    Maximum Moment Magnitude >= 7.0, Slip Rate >= 5

Seismic Source Type B: All faults other than Types A and C

1.)    Maximum Moment Magnitude >= 7.0, Slip Rate < 5

2.)    Maximum Moment Magnitude < 7.0, Slip Rate > 2

3.)    Maximum Moment Magnitude >= 6.5, Slip Rate < 2

Seismic Source Type C: Faults that are not capable of producing large magnitude earthquakes and that have a relatively low rate of seismic activity.

1.)    Maximum Moment Magnitude < 6.5, Slip Rate <= 2

Distance to the Source

In this edit box, enter the Distance to the Seismic Source in Kilometers for both English and Metric Regional Settings.  This option is enabled when the Building Code requires it.

 

 

Reliability / Redundancy Factor – Frames & Bracing (Rho, ρ)

A Frame & Bracing reliability factor (Rho, ρ) is assigned to each principal direction of a building as a measure of redundancy.  Over the years it was observed that structures that rely on multiple lines of framing have a better chance of survival in comparison to structures where seismic-force resistance was based on fewer elements or lines of resistance.  Modern building codes use this parameter to increase the design seismic forces in cases where the failure of one element may lead to more extensive damage or collapse of the whole structure.  In other words, it would penalize the structural configurations with low redundancy.

 

The software calculates this parameter in accordance with the applicable Building Code.  For rectangular buildings, two reliability/redundancy coefficients are calculated, one for transverse and another for the longitudinal direction.  This generated value is always between 1.0 and 1.5 for 2000 IBC and either 1.00 or 1.30 for 2003-2015 IBC.  The Reliability / Redundancy factors may be overridden by BlueScope personnel only, when required.  If overridden, these parameters are identified as “USR” in the Loading Reports and the box is checked on this screen.

Acceleration Ratio – Frames & Bracing (Cs)

The software calculates the Acceleration Ratio for the Frames & Bracing that are specified by the regional or local Building Code.  This option is enabled when the Building Code requires it.  The Acceleration Ratio factors may be overridden by BlueScope personnel only, when required.  If overridden, these parameters are identified as “USR” in the Loading Reports and the box is checked on this screen.  

SNAGHTMLa129b1

 

Flexible (or Rigid) Diaphragms

A check box is active beginning with the 2006 IBC code and is ON by default.  When the box is checked, the longitudinal roof bracing diaphragm is assumed to be “Flexible”.  When unchecked by the user, the diaphragm is assumed “Rigid” and a (0.5) reduction is not applied to the overstrength factor, Omega.  The "diaphragm flexibility" button has limited use at this time as the software system does not check for horizontal irregularities, torsional redistribution, etc.  It does however adjust frame load combinations and factors used for eave strut and portal brace design to reflect use of the Omega reduction factor (0.5).

Rigid diaphragms result in more severe seismic loads.  The areas affected by the selected diaphragm type are frame load cases, eave strut design and portal brace design.

  Caution:  Even though the diaphragm setting for each shape can be accessed, the setting at the highest level controls and will be applied to all shapes.

 

Percent of Snow Load Included with Seismic Load

This field defaults to 0.0000; however, the system will generate a value (%) if required by the selected Code and show this % in Reports.  The value may be user input; but the system will automatically override the input if it is lower than that required by Code. 

For high snow areas some codes require that a reduced amount of roof snow (ranging from 75% - 80%) be combined with the building weight during a seismic event.  This equates to a system generated value ranging from 20% to 25%.

Some localities (especially in mountainous regions), require more than the code specified minimum and the software will accept an override providing it exceeds the minimum required by Code.  Verify this requirement with the local building department.

 

If the % Snow Used in Seismic is user-modified, it will be designated on Reports with “USR”:

 

Estimated Frame Weight

The Estimated Frame Weight is a system generated default (2.50 psf) that represents the uniformly distributed frame weight applied along with roof Dead weights, Collateral Loads, and %Roof Snow Load for seismic load application.  This value may be overridden when required for buildings with larger frames (Consult your Service Center).  The minimum frame weight the system will allow for input is 1.00 psf. 

 

If the Frame Weight is user-modified, it will be designated on Reports with “USR”:

 

 

Standard Controls:

§ OK, Cancel, Apply, Help

 

See also:

§ Building Loading - Building Codes

§ Building Loading - Live Load

§ Building Loading - Wind Load

§ Building Loading - Snow Load

§ Building Loading - Deflection Conditions