Building Loading - Seismic
(Revised:
02/16/2024)
The Seismic tab defines
related design information regarding the building(s) Seismic Load.
The Seismic data defined will apply to the design
of the building(s) resistance to stress produced by earthquake loads.
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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.
Seismic
Load Information:
2020 NBCC
Site Designation
Vs30 (m/s) – average
shear wave velocity in the top 30m of soil or rock
Seismic
Category
-
SC1 through SC4
are defined in Table 4.1.8.5.-B
-
SC1 and SC2 are
Low seismic
-
SC3 and SC4 are
High seismic
2015 NBCC and IBC Codes
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 a 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 a 2 percent probability of
exceedance within a 50-year period.
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Note:
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.
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Note:
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)
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.
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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. This value may be
overridden with IBC based codes if needed, but use with caution.
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
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Note:
Consult the project site Building Code Map for
these values.
If the Seismic Zone is greater than 1, the building
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:
1991 SBC
1999, 1997,
& 1994 SBC
1993 BOCA,
1996 BOCA, 1999 BOCA
1993 ASCE
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.
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Note:
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. The Soil Profile (Site
Class) used may have a significant impact on the loading, design, and cost of
the project. Therefore, larger projects
in high seismic areas should be subject to proper soil testing and accurate
determination of the soil type.
Soil Profile
Conversion – IBC 2015 (& older IBC) to IBC 2021:
Projects with older IBC codes converted to IBC 2018
will generate corresponding Site Class defaults as shown. NOTE: the default “Unmeasured” or
“Undetermined” selection options will produce more conservative Seismic loads
on the structure than their Measured/Determined counterparts.
Soil Profile
Descriptions for Some Typical Building Codes:
2021 IBC, 2018
IBC (ASCE 7-16)
A: Hard Rock (A)
B: Rock (B) – Measured
B: Rock (B) – Unmeasured
C: Very dense soil and soft
rock
D: Stiff soil (D) – Determined
D: Stiff soil (D) – Default
<== this is the BlueScope default unless the user overrides the selection.
E: Soft soil (E)
F: Soils requiring
site-specific evaluation (F) <== a response analysis reporting
1995-2010 ASCE 7,
2000-2015 IBC, MAST (7th), MAST (8th)
A: Hard Rock (A)
B: Rock (B)
C: Very dense soil and soft
rock (C)
D: Stiff soil (D) <== this
is the BlueScope default unless the user overrides the selection.
E: Soft soil (E)
F: Soils requiring site-specific
evaluation (F) – a response analysis reporting
2005-2020 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.
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 (
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 2003-2021 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.
Flexible (or Rigid)
Diaphragms
This 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”, and the Seismic Overstrength Factor Omega is decreased by 0.5. 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 Cases and factors used for eave strut
and Portal Brace design to reflect use of the Omega reduction factor
(0.5). Seek Engineering Assistance for this function.
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.
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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.
Building has no PLAN or
VERTICAL irregularities
This check box is active beginning with the
introduction of 2018 IBC code in the software and is CHECKED by default. This box has meaning when Ss ≥ 150% and
might lower seismic loading on the building depending on other system evaluated
parameters. When the box is un-checked, the system will not apply
the option for lowering of Seismic loads.
Seek Engineering Assistance for this functionality.
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Caution: The “Irregularities...” box may be
set “by building shape”.
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 heavier 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:
See also:
§
Building Loading - Building Codes
§
Building Loading - Live Load
§
Building Loading - Wind Load
§
Building Loading - Snow Load
§
Building Loading - Tornado Load
§
Building Loading - Deflection Conditions