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Previously, when wall parameters were left as unknown, Shearwalls designed separate walls for wind design, seismic design, rigid distribution, flexible distribution, and both force directions – a possibility of 8 designed walls for each physical wall in the structure. In practice, at most 4 walls would be designed, because forces in opposing directions are similar, and often only two or three walls would result. It was left to the designer to compare these walls manually and choose the one that was strong enough for all load cases. If you wanted to see design results for the selected wall, it was necessary to “accept” the design for that case and to run the design again.
Now, the program automatically determines the worst case of wind and seismic, and for opposing force directions, and designs one wall that is evaluated for all these load cases. Optionally, you can also have the program determine the worst case of rigid and flexible diaphragms.
Worst-case Wind vs. Seismic Load Case (Feature 12)
Shearwall Design
Previously, the program determined the wall parameters needed to resist the forces from the applied wind loads, and then did so for seismic loads separately. As a result, the program could create separate wall groups for the same physical wall, one for wind design and one for seismic design.
The program now compares the walls designed for wind and seismic and selects the wall that has the highest capacity. That wall is then used to redistribute forces on the line if deflection is the force distribution criterion, and to redistribute forces to the shearlines for the rigid diaphragm procedure.
Output – Shear Design Table
The wall groups are indicated by numbers in the Shear Design table, which are defined in the Sheathing and Framing Materials by Wall Group tables. For a particular wall, the same number now appears for seismic and wind design; previously they could be different.
In the Critical Response column of the table for wind design, the program outputs an “S” beside the response ratio if the critical case was seismic and the wall had unknown parameters. Similarly, a W is printed beside the column in the seismic table if the critical case was wind. This alerts you to the reason that a wall might be designed with materials that are much higher than needed to resist the loads from the design case shown.
The legend has been modified to explain the meaning of these letters.
Worst-case Rigid Diaphragm vs. Flexible Distribution Method (Feature 69)
Many designers prefer to consider diaphragms to be semi-rigid, and in the absence of a complex numerical model of the structure, wish to design for the worst case of rigid and flexible diaphragm distribution, to cover the whole envelope of possible diaphragm rigidities. Shearwalls now allows for that approach.
Design Setting
A checkbox has been added to the Design Settings called
Worst-case rigid vs. flexible diaphragms (envelope design).
The default for new program installations that this setting is on, but this can be changed. The setting is disabled if you have not chosen to design for both rigid and flexible diaphragms (the choice is in the Structure input).
Shearwall Design
If the worst-case rigid vs. flexible setting is not selected, program determines the wall parameters needed to resist flexible diaphragm distribution forces, and then does so for rigid forces separately. As a result, the program can create separate wall groups for the same physical wall, one for rigid diaphragm design and one for flexible design.
If the setting is selected, the program first designs a wall for flexible diaphragm forces. When designing for rigid forces, if they are lower than flexible force, the program simply uses the wall designed with the flexible force. If they are higher than the flexible force, it replaces the wall designed for flexible forces with the one designed for rigid forces. For deflection-based intra-shearline distribution, the wall is then processed again for flexible forces on the next iteration of the design procedure, as the distribution of forces within the shearline may change slightly due to the new wall stiffness.
Output – Shear Design Table
The wall groups are indicated by numbers in the Shear Design table, which are defined in the Sheathing and Framing Materials by Wall Group tables. If you have selected the Worst-case rigid vs. flexible diaphragms design setting, then for a particular wall, the same number appears for rigid and flexible design. If that setting is not selected, they can be different.
Please note that if the worst-case rigid vs. flexible setting is set, a the wall materials appearing in table for rigid diaphragm design may have been designed for a higher force for flexible diaphragm design, and vice-versa. If the program designs walls that appear to be much stronger than needed, this is the most likely reason.
Worst Case of Opposing Force Directions
It is possible for the force in one direction to be slightly different than the force in the opposing direction. For wind design, this can occur for a mono-slope roof or eccentric ridge line. For both wind and seismic design, it can occur when forces are distributed due to deflection and there are asymmetries in the hold-down devices or hold-down forces. An example of this is when openings are do not line up and vertical compression forces from the floor above are added to tension forces from the floor below.
In rare circumstances, such difference could cause the program to design a different wall for the east->west direction than the west<-east direction, and similarly for north-south walls.
Shearwall Design
The program now determines the largest force on any segment in the shearline, in either direction, and designs the wall materials for that force.
Output – Shear Design Table
When different forces existed, two lines of results instead of one were output in the Shear Design table for each wall in the shearline. If different wall materials were selected by the program for these forces, a different wall design group number could be shown for the two directions.
The program still outputs separate design results for the opposing force directions if they are different, but this is to show the performance of the wall with respect to the different forces. The design group number shown for the opposing directions is now always the same.
Worst Case of Wind Shear vs. Wind C&C Design (Bug 2848)
In determining the worst-case wall on the structure, the program considers wind shear, nail withdrawal, and C&C sheathing design.
The procedure has one slight imperfection in that thicker sheathing, which is optimal for out of plane sheathing strength and for shear design, makes for weaker nail withdrawal strength due to reduced penetration. So when determining the strongest wall, one wall may be stronger for suction and for shear but another may be stronger for nail withdrawal. In such a case, the program uses the wall thicker sheathing. It is extremely rare for the wall to fail for nail withdrawal as a result.
Design Failures
There exists a small possibility that when distributing loads using the rigid diaphragm method to a stronger wall than was designed using that method, the rigid distribution routine could load the shearline to the extent that the wall fails despite being stronger than the one that previously passed.. This can happen when a wall designed for seismic is used to resist wind loads (or vice versa), or when a wall designed for flexible distribution is used for rigid diaphragm forces.
Although unlikely, it has the highest chance of happening when using deflection-based design and the effect of increased stiffness is greater than the increase in wall capacity.
If this occurs, the program alerts you to the situation via a note under the Shear Results table. The wording of the existing note that appears if this occurs for other reasons has been modified to take into account this possibility.
The same thing could conceivably happen for the flexible diaphragm method and distribution within a shearline using deflection based rigidity, but it is highly unlikely because all segments on the wall have the same shearwall materials.
SHEARLINE, WALL and OPENING DIMENSIONS Table
Because this table no longer shows a list of design groups for each wall, showing at most two for rigid and flexible design, the heading to this column is “Wall group rather than Wall Group(s). A note below the table has been added when worst-case rigid and flexible is not selected, explaining why two group numbers may appear.