Building Loading – Seismic Load

(Revised: 10/07/2024)

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.

„ 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. The data supplied is assumed to be accurate.

Seismic Load Information:

2024 IBC, ASCE 7-22

or

Multi-Period

Multi-period response spectrum (MPRS) is the preferred option, a technical advancement over the previous version as it provides higher accuracy data based on new 2018 USGS Seismic Hazard Maps.

Users need to access the ASCE 7 Hazard Tool, and then enter the project location and basic parameters, to get the results, and then select the Details button. Next, navigate to the “TABLE” or “CSV” tab under “Multi-Period Design Spectrum” and then type in the table values for S_a corresponding to T = 0.1 seconds thru T = 3.0 seconds into Vision in the column labeled S_a MCE-R. The Hazard Tool lists values for T = 0 seconds thru T = 10 seconds, but Vision only uses T = 0.1 thru T = 3.0.

Two-Period

Two-period response spectrum (TPRS) is the traditional three-domain spectrum based on T = 0.2 s and T = 1.0 s, which is unchanged from previous editions. Values can be found for a specific project location using the ASCE 7 Hazard Tool. The ASCE 7 Hazard Tool uses the nomenclature Sms for T = 0.2 s and Sm1 for T = 1.0 s. This is the only option for locations where MPRS parameters are not developed yet.

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

S_s 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 S_s 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.

„ 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.

„ Note:

There are several possible sources for this required input, S_s 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)

S_a(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). S_a(0.2), S_a(0.5), S_a(1.0) and S_a(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 is 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 input values. 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

„ 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:

%!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 (6^th)

Acceleration (Av)

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

Acceleration (Aa)

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

„ 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

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 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 Soil Profile 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.

Site Designation; Site Class:

2024 IBC, ASCE 7-22

A: Hard rock

B: Medium hard rock

BC: Soft rock

C: Very dense sand or hard clay

CD: Dense sand or very stiff clay

D: Medium dense sand or stiff clay

Default – Unknown

DE: Loose sand or medium stiff clay

E: Very loose sand or soft clay

F: Other (Requires site analysis)

Soil Profile for other 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 (7^th), MAST (8^th)

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.

%!JumpId(IDC_CODES_USE_1) MAST (6^th)

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 2003-2021 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 Seismic Factor (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.

Diaphragm Condition: Flexible, Rigid, or Semi-Rigid

Flexible – diaphragm is significantly more flexible than the vertical SFRS it is connected to

Rigid – these diaphragms are significantly stiffener than the vertical SFRS they are connected to

Semi-Rigid or semi-flexible – this broad category covers anything in between the two categories listed above. Many diaphragms in low-rise metal buildings would be included here.

ASCE 7-16 and older below: ASCE 7-22, IBC 2024 below:

The Flexible Diaphragm option became active with the 2006 IBC code and is ON by default. When selected, the longitudinal roof bracing diaphragm is assumed to be “Flexible,” and the Seismic Overstrength Factor Omega is decreased by 0.5. When not selected, the diaphragm is assumed “Rigid,” and a (0.5) reduction is not applied to the Overstrength Factor Omega.

Selecting Flexible Diaphragm 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 the 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.

„ 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 the 2018 IBC code in the software and is CHECKED by default. This box has meaning when Ss ≥ 150% and might lower the building's seismic loading depending on other system-evaluated parameters. When the box is unchecked, the system will not apply the option to lower Seismic loads. Seek Engineering Assistance for this functionality.

„ 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, if required by the selected code, the system will generate a value (%) and show this value in reports. The value may be user input, but the system will automatically override it if it is lower than that required by code.

For high-snow areas, some codes require that a reduced amount of roof Snow (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 provided it exceeds the minimum required by the 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:

§ OK, Cancel, Apply, Help

See also:

§ Building Loading - Building Codes

§ Building Loading - Live Load

§ Building Loading - Wind Load

§ Building Loading - Snow Load

§ Building Loading - Tornado Load

§ Building Loading - Rain Load

§ Building Loading - Deflection Conditions