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Please note that some of the items in this section describe changes to the program that came about because of the analysis of ASCE 7-16 as compared to ASCE 7-10 but were not directly due to changes in the design standard. These items are indicated with an asterisk (*).
These changes pertain to the definition of Site Classes A-F and their use in determining site coefficients Fa and Fv via tables 11.4-2 and 11.4-1, which also depend on the mapped response acceleration parameters Ss and S1, respectively. Fa and Fv are then multiplied by SS and S1 to arrive at spectral response acceleration parameters SMS and SM1, which are then multiplied 2/3 to get design parameters SDS and SD1 , which contribute to the determination of the Seismic Design Category (11.6) , base shear V (12.8), vertical seismic effect (12.4.2.2), min. and max. diaphragm design forces (12.10.1.1), and wall out-of-plane (12.11.1) and anchorage (12.11.2) forces.
If Site Class D is used as the default site class because soil properties are not known, as per 11.4.3, then a minimum value of 1.2 is now applied to Fa as per 11.4.4. This applies to SS = 1.0, 1.25, or 1.5.
If Site Class B is determined through rock conditions (using Chapter 20 – Site Classification Procedure), but velocity measurements were not made, Fa and Fv are now to be set to 1.0.
In Table 11.4-1 for Fa,
The last column was for S1 greater than or equal to 0.5, but now a new column has been added for S1 greater than or equal to 0.6
A checkbox has been added to the Site Information dialog called Chosen by default. It is active for Site Class D, only and is unchecked by default. If checked, the site coefficients for SS = 1.0, 1.25, or 1.5 are set to 1.2.
As it is now possible to use site-specific procedures and enter your own Fa for class D (see ), the program will not restrict Fa to 1.0 if you do so. It is possible that ground motions are determined rigorously but soil profiles are not.
A checkbox has been added to the Site Information dialog called No velocity measurements. It is active for Site Class B, only and is unchecked by default. If checked, the program will set Fa and Fv to one.
As it is now possible to use site-specific procedures and enter your own Fa and Fv for class B (see ), the program will not restrict Fa and Fv to 1.0 if you do so, as it is possible that velocity measurements are not taken but 21.2 Risk-targeted Hazard Analysis is used for Fa and Fv.
Table 11.4-1 values have been modified to those in ASCE 7-16. A new column for SS greater than or equal to 1.5 has been added and the table interpolation modified accordingly.
For site class E and SS greater than 0.75, the input of Fa in the Site dialog is active, as described in . Note that this starts at 0.75 rather than 1.0 because interpolation is not possible between 0.75 and 1.0 with no tabulated value for 1.0.
Table 11.4-2 values have been modified to those in ASCE 7-16. A new column for S1 greater than or equal to 0.6 has been added and the table interpolation modified accordingly.
For site class E and S1 greater than 0.2, the values shown in the Fv input are given in , and the operation of this input is described . .
Refer to input of values when ground motion analysis must be done for class D as per note a.
This section pertains to the conditions under which site-specific ground motion analysis must be done to determine the site coefficients Fa and Fv rather than using the tabulated values described in item . Note that for both ASCE 7-10 and ASCE 7-16, using ground motion analysis is permitted for any structure, but Shearwalls only allowed it when it was required.
For site F, the reference to site-specific ground motion procedures in 21.0 has been changed to site response analysis in 21.1 (as opposed to also including ground motion hazard analysis in 21.2).
The following cases now require ground motion hazard analysis from 21.2 rather than using the tabulated values of Fa and Fv.
Seismically isolated or damped structures with S1 greater than or equal to 0.6. This requirement was also in ASCE 7-10.
This clause says that the analysis is required for Ss => 1.0, but because there is no value for Fa for Ss = 1.0 for interpolation, it is needed for Ss > 0.75, unless Exception 1 (below) is used. Previously ground motion analysis was not required for Class E.
For site class D, the analysis is required for Fv if S1 >= 0.2 unless the Exception 2 (below) is used. Previously ground motion analysis was not required for Class D.
For site class E, the analysis is required for Fv if S1 >= 0.2 unless the Exception 3 (below) is used. Previously ground motion analysis was not required for Class E.
For Class E, Class C coefficients can be used in lieu of ground motion hazard analysis.
The following conditions apply when Site Class D is used with S1=> 0.2 to avoid site specific analysis:
If Site Class E is used with S1 => 0.2, you can avoid site specific analysis for if the period T is less than or equal to TS, which it ordinarily is in Shearwalls. The ASCE 7-16 does not include tabulated values for S1 and Class E, but ASCE provided us with coefficients to use that will appear in the next ASCE supplement. They are Fv (0.2) = 3.3, Fv (0.3) = 2.8, Fv (0.4) = 2.4, Fv (0.5) = 2.2, Fv (0.6) = 2.0.
When Site Class F is chosen:
At the bottom of the Site Information section of the Seismic Load Generation Details file, a note now appears saying that site response analysis from 21.1 is needed.
When Class F is selected in the Site Information dialog, the default value is no longer what was in the previous entry for the other site classes; instead the program places 0.0 in the input. If it is not changed, you are prompted to enter a value.
Shearwalls now allows you to override the tabulated values for Fa and for Fv for the permitted use of coefficients from ground motion analysis on any structure, or their required use on seismically isolated or damped structures.
Two checkboxes called Use site-specific ground motion procedures in the Site Information dialog allow you to override the tabulated values for Fa and for Fv
For Site Class F, they are always inactive and checked, and the Fa and Fv boxes are active, as is currently the case.
For Site Classes A-E, the checkboxes are active and unchecked by default, and Fa and Fv are inactive. If they are checked, then the Fa and Fv values become active and show values from the Tables 11.4-1 and 11.4-2, and you may modify these values.
If ground motion analysis was not required for Fa and/or Fv according to 11.4.8, but you decided to use it for either or both, a note appears under the Site Information section of the Seismic Load Generation Details indicating site-specific ground motion hazard analysis from ASCE 21.2 was used.
If ground motion analysis was required for Fa and/or Fv unless the Exceptions 1-3 are invoked, and you decided to forego the Exceptions and enter an Fa and/or Fv, the same note appears except that it says the analysis is "required" rather than "used".
No changes were made specifically to implement seismically isolated or damped structures with S1 >= 0.6. If you have such a structure, then you check the Use site-specific ground motion procedures box for Fv and enter the result of your site-specific analysis.
To implement Exception 1, the value in the Fa input for Class E for Ss > 0.75 is set to that for Class C. This can be over-ridden by selecting the checkbox for site specific analysis and entering a different value.
For site class D and S1 >= 0.2, if the site-specific ground motion checkbox is unchecked upon load generation, for each direction:
Notes appear below the table in the Seismic Load Generation Details indicating that the exception was applied in the direction or directions, and that max CS was not applied for T <= 1.5 Ts, or that 1.5 max CS was applied for T>1.5Ts
For site class E and S1 >= 0.2 the values shown in Fv are those supplied by ASCE (see ), If T > Ts, then the site-specific ground motion checkbox is always checked and disabled, and you must enter Fv as per Exception 3.
12.3.4.1 now refers to the minimum and maximum diaphragm forces from Eqns. 12.10-2 and 12.10-3 when listing all those design provisions for which redundancy factor ρ = 1. Previously it referred only the diaphragm force Fpx from Eqn. 12.10-1
The only application of Fpx in Shearwalls is for drag strut forces, and the program was already setting ρ = 1 when evaluating Eqns. 12.10-2 and 12.10-3 for them. However, the following small inaccuracies in the Design Results were noticed and corrected:
In reference to the application of the redundancy factor under the Seismic Information table now refers to the diaphragm force Fpx as well as the drag strut forces.
This is because drag strut forces can be based on the design shear force factored by ρ if it is greater than the diaphragm force, according to 12.10.2 and 12.10.1.1.
In the drag strut table legend, the phrase
Includes redundancy factor rho.
has been removed from the definition of shear line force for perforated walls.
Previously, the value of SS, the short-term mapped seismic response acceleration parameter, was permitted to be limited to 1.5 for certain design procedures and under certain conditions. Now there is a limit on SDS, the short-term design response acceleration parameter, of the greater of 1.0 and 0.7 times the calculated SDS. As these parameters are related by SDS = 2/3 Fa Ss, the old limitation was equivalent to SDS <= Fa, so this represents a substantive change.
According to the values of Fa in Table 11.4-1, SDS is greater than 1.0 for Site Class C and SS >= 1.25, and for Site Class D with SS greater than 1.5. These are common Site Classes, and SS can be as high as 2.0 in earthquake- prone regions of the USA. This limitation will come into play in many circumstances.
Previously this limitation was applied to the calculation of seismic response coefficient CS in Eqn. 12.8-2 and its lower limit in 12.8-3. CS is multiplied by the building weight to arrive at base shear V.
It is still applied in these places, but now also to the calculation of SDS for vertical earthquake load determination in Eqn. 12.4-4a, which is Ev = 0.2 SDS D, where D is dead load.
The limit on the value of SDS is still not applied to the Seismic Design Category (11.6), min. and max. diaphragm design forces (12.10.1.1), and wall out-of-plane (12.11.1) and anchorage (12.11.2) forces. In addition, it is not applied to SDS in the calculation of TS for the new Exception 2 to the use of site-specific ground motion analysis for Site Class D (11.4.8).
The following criteria to be met for use of this limit were in ASCE 7-10 and are still to be applied:
The following criteria are new to ASCE 7-16
The program now applies a limit of the greater of 1.0 and 0.7 times the calculated SDS to the value of SDS used for CS = SDS / (R/Ie), and to the lower limit CS = 0.044 SDS Ie. Previously, the SS value in determining SDS was limited to 1.5.
The limit now applies to the SDS calculation of the vertical seismic force component of hold-down forces, Ev = 0.2 SDS D
As the building can be irregular or have a redundancy factor in one direction but not the other, the limit can be applied to hold-downs in one force direction but not the other.
As it can be torsionally irregular for rigid diaphragms but not flexible, the limit can be applied for flexible diaphragms but not rigid.
In the Plan View legend explaining the Ev force, the SDS shown is now the limited one, and the program now shows SDS values for both force directions in the unusual case that they are different. This can now be different for rigid and flexible diaphragm design.
In the note under the Seismic Information table of the Design Results explaining the Ev force, the SDS shown is now the limited one, and the program now separate notes for both force directions in the unusual case that they are different. This can now be different for rigid and flexible diaphragm design.
The limit on SDS is no longer applied to buildings in Risk Category III (Hazardous) or Risk Category III (Essential).
The limit on SDS is no longer applied to buildings in Site Classes E and F.
As seismic load generation is performed before shear walls are designed and irregularities can be detected, the program relies on the new user input of irregularities in the Site Information dialog (see 25d)ii) to determine whether the structure limit cannot be applied because the structure is irregular. If you are performing both rigid diaphragm design , if any irregularity is entered, the limit on SDS is suppressed. If only flexible diaphragm design is set to be performed, it is suppressed for any irregularity except horizontal Torsional irregularities 1a and 1b.
As the program is able to detect torsional irregularities and Horizontal Irregularity 4 – Out-Of Plane Offset, if during shear wall design these irregularities are detected but hadn’t been entered in the Site Information, or vice-versa, and if the SDS value used for CS would have been different as a result, a message appears suggesting that you reset the irregularity selections and rerun the load generation and design.
If the only irregularity set in the Site information is a torsional irregularity, and a limit that would have been imposed on SDS is suppressed, a message appears suggesting that you run load generation and design on rigid and flexible diaphragms separately. This allows the limit on SDS to be applied for flexible diaphragm design while using the calculated SDS for rigid diaphragm design.
For the purpose of calculating Cs for base shear V, the limit on SDS is applied if 1.0 or Calculate is set in the Site Information dialog for the Redundancy Factor ρ. For the purpose of the vertical seismic component Ev of hold-down forces the limit is applied if ρ is set to 1.0 or the program calculates ρ = 1.0 for the force direction.
If Calculate was set and ρ = 1.3 was calculated by the program, a message appears suggesting you set ρ to 1.3 and regenerate loads. If this happens only for rigid or for flexible diaphragm design, the message suggests running load generation and design for these cases separately.
In the Seismic Load Generation Details output, in the note referring to the applicability of the two SDS values shown,
The note no longer refers to SS and includes Ev among the calculations that are limited.
The note now also includes diaphragm design force limits and out-of-plane forces those design procedures that are not limited. Previously it mentioned only the Seismic Design Category.
The requirement that an accidental torsion of 5% of the building width be added to torsional analysis for rigid diaphragms has been limited to structures of Seismic Design Category B with Type 1b horizontal structural irregularity (Extreme Torsional) and SDC C-F with either Type 1a (Torsional) or Type 1b. The irregularities are defined in Table 12.3-1.
The calculation of Type 1a or Type 1b horizontal irregularities requires evaluation of the deflection at extreme shearlines derived from forces using the 5% accidental eccentricities and amplification factor Ax = 1.0. Ax is from 12.8.4.3
The program already did a preliminary iteration of loads analysis and design to determine the value of redundancy factor ρ and the torsional amplification factor Ax . In that iteration the program now determines the torsional horizontal structural irregularities 1a and 1b using Ax = 1.0 Torsional irregularity 1a occurs for Ax > 1.0, and 1b occurs for Ax > (1.4/1.2)2, or equivalently when δmax / δavg is greater than 1.2 or 1.4, respectively, δmax being the largest and δavg the average of the story drifts at the endmost shear lines.
The 5% accidental eccentricity is not applied for SDC A, is applied only if there is an extreme torsional irregularity for SDC B, and only if there is a torsional irregularity for all other structures.
In the Torsional Analysis Details file, if there is no accidental eccentricity:
The note that gave the reference to 12.8.4.2 now gives the SDC and presence or absence of torsional irregularities 1a and 1b, and if applicable indicates that it is 0 for those reasons.
The amplification factor Ax, which was only shown if greater than 1.0, is now always shown. If it is 1.0, it still does not appear in the line showing the calculation of torsion T.
The description of forces being transferred through the diaphragm via offsets or stiffness changes in shear walls has been formalized in 11.2 Definitions as a Transfer force, rather than being more briefly described in 12.10.1.1 Diaphragm Design Forces.
There is now a requirement that the transfer forces from discontinuous shearlines be increased by the overstrength factor Ω0 from 12.4.3.1 before being added to the diaphragm force.
The requirement that the redundancy factor ρ from 12.3.4 be applied to transfer forces from discontinuous has been removed. This is in accordance with items 5 and 6 in the list of conditions in 12.4.3.1 for which ρ = 1, which refer to the design of members, collector elements, connections, etc. where overstrength factor is either required or used.
Note that this is in seeming contradiction with the Commentary C12.10.1.1 which says that the redundancy factor applies to diaphragm transfer forces, but in a communication ASCE clarified that ρ and Ω0 are not to be applied simultaneously and that the Commentary applied to typically smaller transfer forces due to changes in lateral stiffness rather than Irregularity Type 4 Offsets to which Ω0 is applied.
An exception has been added indicating that for one- and two-family dwellings, the Ω = 1.0 so that overstrength is not required on transfer forces.
The transfer forces due to vertically discontinuous shearlines that are used for calculation of shearline forces on the level below for the purpose of drag strut force calculation have been changed as follows:
If the shearline forces from above include a redundancy factor ρ = 1.3, it is removed. The overstrength (Ω0) factor of 3.0 for bearing wall systems and 2.5 for building frame systems (from Table 12.2-1) is applied. For flexible diaphragm design, both
The requirement that transfer forces from discontinuous shearlines be factored for overstrength inspired a re-evaluation and reimplementation of the calculation of shearline forces used to create drag strut forces when they are based on the diaphragm design force Fpx.
Previously, the program determined a proportionality ratio between the diaphragm force Fpx, and the sum of the unfactored design force Fx on all levels above and including the level in question. It then multiplied each ASD-factored shearline force on that level by that factor, to convert from base-shear-based forces to Fpx-based forces. This approach was an approximate one in the following ways:
Such an approach treats discontinuous transfer forces as if they were a load due to the equivalent amount of building mass at that location, and that load is based on the diaphragm design force Fpx rather than the design shearline design force Fx.
This approach incorporated the redundancy factor ρ, as was required by 12.10.1.1, only indirectly via its contribution to the design shearline forces that are then factored by the proportionality ratio, so that the redundancy factor was effectively modified by that ratio. The much larger Ω0 factor would make this an unacceptably large discrepancy.
Manually entered seismic loads and shearline forces were not considered in this calculation, and if they exist, they would be assumed to be based on Fpx rather than the base shear. As these may represent forces from adjoining structures, this may not have been the best approach.
The assumption of a linear proportionality factor relating diaphragm-force-based shearline forces and base-shear-based forces does not consider torsional effects and non-linear stiffness calculations used in rigid diaphragm analysis. That is, for rigid diaphragms, a larger diaphragm-based force might cause a shift in distribution of forces to the shearlines that is not considered.
A proportionality factor between the diaphragm force Fpx and the unfactored design force Fx on the level in question is now applied to all the seismic loads that were created from building masses on the level x. The program then gathers these loads along with transfer forces from the level above, factored by Ω0, and user-applied loads and forces, and distributes them to the shearlines via rigid and flexible diaphragm distribution routines.
This shearline force represents the contribution of the loads on the level with the drag strut. The shearline forces from levels above on the same shearline are then added in (see ), and then the maximum of this force and the force used for shear wall design is used for drag strut force calculations.
Note that the resulting drag strut design shearline forces on the level below are now included in the Design Results output () and shown in Plan View (see ).
It is now possible to view the elemental loads and forces associated with the diaphragm force Fpx and the transfer forces used with Fpx in the Loads and Forces action of Plan View. Refer to for details.
The exception for one- and two-family dwellings has not been implemented in Shearwalls.
The following pertains to the output of the diaphragm force on each level and in each direction in the Seismic Information table of the Design Results output.
The table has been modified to show two forces for each direction, one derived from Fpx from 12.10.1.1, and the other the total force including transfer forces and any seismic forces you enter directly (as opposed to generating with building masses).
Previously the diaphragm force Fpx shown was unfactored, but as the transfer forces are factored, it was decided to show the ASD-factored diaphragm forces.
The table legend explains that
ASCE 7-16 has dropped an Exception 1 in 12.10.2.1, allowing the collector forces to be limited to those derived from the maximum diaphragm force, 12.10-3. However, this does not affect Shearwalls, which uses the other exception that refers to all 12.10.1.1, which includes 12.10-3.
Because 12.10.1.1 refers only to transfer forces from discontinuous shearlines, it was thought that forces from continuous shearlines above the level with the collector were not to be included in the "seismic forces originating in other parts of the structure" referred to in 12.10.2. However, in a communication, ASCE clarified that they were to be included, so now Shearwalls adds the design shearline forces (based on base shear) from shearlines on the upper levels, to the shearline force based on diaphragm design force Fpx.
Because Fpx -based forces are given as a minimum force in 12.10.1.1., the larger of the base-shear-based shearline force and the Fpx -based shearline force is used for drag strut force calculations. Previously the larger of the design shear force and the Fpx -based force without upper levels was used, making it much less likely that Fpx -based forces would govern.
Because there is no explicit guidance on Seismic Design Category B, 12.10.2.1, which mandates use of the Fpx -based force as a minimum force for collector for SDC C-F, was applied to SDC B as well. This is no longer the case, and the base-shear based design shearline forces are used for SDC B with no minimum Fpx -based force applied.
In the Collector Forces table of the Design Results (previously called Drag Strut Forces), for each shearline, the shearline force used to create the drag strut force is now shown.
The legend entry for Drag strut Force in the Collector Forces table of the Design Results has been modified to indicate:
12.11.1 for out-of-plane wall forces and 12.11.1.1 for wall anchorages have been reorganised and rephrased to remove the ambiguity that suggested both sections referred to anchorages. 12.1.1 now refers to wall forces only.
The only substantive change within the reorganization is that there is now a minimum force of 0.2 Wp to be applied to the wall, where Wp is the weight of the wall tributary to the diaphragm.
The anchorage force is now taken as 0.2 Wp for those cases that 0.4 SDS Ie ka < 0.2, where ka is the flexible diaphragm amplification factor defined in 12.11.1.1.