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Refer also to the changes listed in the section with the same name under Version 11.2,
The method of determining external pressure coefficients CpCg for low-rise structures from ASCE 7 Figure 28.3-1 assumes symmetric, rectangular structures. They are based on boundary layer wind tunnel studies of buildings of that shape, verified against full scale measurements. As relatively few low-rise structures have such a regular shape, Shearwalls now extends this method to buildings with multiple blocks and/or eccentric ridge lines.
When two blocks intersect such that the slope of a portion of a roof can be formed either by either the side panel of one block or the end panel of the other, such as one roof framing into another to form an L-shape, the manner in which the building is modelled does not affect the wind loads generated when the load generation option of treating end hip panels as side panels is chosen. However, if the ASCE 7 C28.3-2 is used, then it does make a difference. It is recommended to use the side panel option with wind load generation. If the C28.3-2 method is used, it is best to join blocks such that composite roof surfaces formed by the hip end of one block and the side panel of another are avoided.
The area of exterior walls beneath a sloped roof are considered to "have" the slope of the roof for the purpose of Case A (transverse) load generation, which depends on roof angle. If there is a hip end in the Case B (longitudinal) direction, and side panel Case A coefficients are used on the wall beneath the hip end, they are the same as the rest of the building face formed by the side panel of the other block.
A hip end panel in the same plane as a side panel on another block has the co-efficient from the same load case as the side panel, so it is equivalent to the situation where the side panel from one block forms the roof on the entire face and the other block joins that block without forming a hip end.
Previously the program disallowed structures for which the hip ends on opposing ends of a block had different slopes, or there was a hip on one end and a gable on the other. Now these structures are allowed, and the coefficients used on the roof panels and the walls below the panels are based on the angle of each roof panel.
Previously the program disallowed the case of an eccentric ridge line with different slopes on either side of the ridge. Now this case is allowed, and the coefficients used on the side roof panels and the walls below the panels are based on the angle of each roof panel. This applies to each roof on a structure with multiple blocks.
End zones are created wherever the corner of a block is an exterior corner of the structure. End zones are not created where two blocks join each other forming either a straight wall and roof, or an interior corner.
The data group Apply height to-width ratio to…has been changed to Low-rise height-to-width limit based on… and provides choices for Single block and Entire structure. The definition of low-rise structures in ASCE 7 26.2 says they must have mean roof height less than least-horizontal dimension, and in the absence of guidance in the ASCE 7, this setting is used to specify whether this limit is to be applied to each block or to the structure as a whole.
Note that the Apply height to-width ratio to… setting had no effect in previous versions of the program, as they did not allow multi-block low-rise structures.
The error messages that appeared after Generate Loads was invoked saying that load generation was not possible due to unequal hips, eccentric ridge lines, or multiple blocks, have been modified to provide warnings and suggestions for these reasons, but to indicate load generation will proceed.
In the Load Generation Details output:
A note in the header section of the Wind Load Generation Details output indicates that the building does not conform to strictly to Figure 28.3-1 for the following reasons, which are only included if applicable to the structure: multiple blocks, eccentric ridge lines, or unequal hip or gable ends.
There are now load tables for each block for multiple block structures, where previously only one table was possible.
If there are multiple blocks of which any two have orthogonal ridge lines, then both Case A and Case B loads can exist in the same wind direction on different blocks. In this case, in the Torsional Analysis Details output where it otherwise says Low-rise Case A or Low-rise Case B, it says Low-rise Case: and then Wind Generally North-South or Wind Generally East-West.
For orthogonal ridge lines, where the section for Case A previously had results for only one load direction, there are now results for both directions.
Since Case A and Case B loads can exist in the same wind direction on different blocks, for multi-block structures, in the Show menu and associated Options settings under Display,
Previously, Shearwalls analyzed low-rise wind loads from ASCE 7 Figure 28.3-1 by considering pressures in the E-W and N-S directions independently and did not consider the combined effect of these pressures on torsional analysis for rigid diaphragm force distribution to shearlines.
Shearwalls now includes loads due to pressures from both directions in the torsional analysis routine simultaneously, so that torsional forces from E-W loads are considered when determining N-S forces, and vice-versa.
Note that it is only Case B that has simultaneous pressures in both directions.
This has been implemented for the Basic Load Case only and will capture torsional effects due to structural asymmetry and end zone effects. The Torsional Load Cases in Figure 28.3-1 which specify partial loading have not been implemented in Shearwalls.
In the torsional analysis routine, the program now calculates the torsion T in the equation
Fti = T Ki li / (Jx + Jy)
as Tx + Ty for both directions, when previously Tx was used for one direction and Ty for the other. Refer to the Torsional Analysis Details output for the definitions of these variables.
In the Torsional Analysis Details file (previously the Log file) , for load Case B
The following improvements were made to the equalizing of deflections of the sheathing on either side of a shear wall, as a step in the process of equalizing deflections along the shearline as required by SDPWS 4.3.3.4.1. This affects force distribution to segments within a shearline when using the deflection-based distribution Design setting. Equalizing deflections on both sides of the wall for the 4-term equation C4.3.2-1 entails equalizing the sum of shear and nail slip terms in the equation, as the bending and hold-down terms affect both sides equally. For the linear 3-term equation from SDPWS 4.3-1, only the shear term is equalized.
For the non-linear 4-term the program now implements the Newton-Raphson numerical procedure rather than a more time consuming "brute force" procedure. The Newton-Raphson method always converges to a solution when there is one, in less than 10 iterations as opposed to 100 or more for the brute force method and converges more reliably.
The Newton-Raphson method is used to solve the non-linear equation
δi(vi) = δe(ve),
where δ is the sum of deflections from the linear shear term and non-linear nail slip term. Subscripts i and e refer to the interior and exterior sides of the segment.
This can be reformulated as a one-dimensional problem as follows,
D(ve) = δe(ve)- δi(v - ve) = 0,
where v is the known force on the segment.
The Newton method follows the slope (derivative) of D(ve) down to the ve axis.
The derivative is
dD/d(ve) = d(δe(ve)-δi(v - ve))/ dve.
It is convenient to calculate the derivatives with respect to both vi and ve , and noting vi = v - ve
dD/d(ve) = dD/d(vi) = dδe(ve)/d(ve) + dδ(vi)/d(vi),
where
dδ(ve)/d(ve) = h/gvtv + 0.75hβ(s/12γ)β veβ-1, and similarly for vi,
where s is nail spacing in inches and γ and β are parameters from SDPWS Table C4.2.2D.
When using the Design setting that linearizes the non-linear 4-term equation into a linear 3-term equation, the program was unnecessarily using the same slow iterative approach as was used for the non-linear 4-term equation. It now solves this equation algebraically. Defining α = h / 1000 Ga as the shear term factor of v in the linear 3-term equation and α i and α e represent α value on the interior and exterior,
ve = v αi/(αi + αe)
vi = v - ve
Occasionally, all forces were placed on one side of the wall when there should have been some force on both sides. This was a problem with the "brute force" iterative procedure and has been corrected in the course of implementing numerical methods.
In the Torsional Analysis Details file, distances such as center of load, center of mass, shearline location and eccentricity have been based on a fixed co-ordinate system with its origin in the southwest corner of the structure. S->N distances are positive, and N->S are negative.
The forces F, however, are based on the direction of applied force; for N->S applied force, all values in the N->S direction are positive; for S->N applied force, they are negative.
Some users find this confusing and wish to see all values relative to the same fixed co-ordinate system. Other users, however, wish to see positive shearline forces when they are in the direction of the applied force, and this corresponds to the way they are presented in the Plan view drawing.
Accordingly, we now offer a choice in the Options settings of showing forces based on a fixed co-ordinate system, or relative to the applied force.
The program now displays the loads to be used for diaphragm design derived from Fpx defined in ASCE 7 Equations 12.10-1,2, and 3, and the shearline forces derived from these loads that are used for drag strut design as per 12.10.2.1.
Note that as per 12.10.1.1, diaphragm design forces are minimum forces, and if the effects of the loads derived from structural analysis in Section 12.8 are greater, these should be applied.
Items have been added to the Show Menu under Loads saying Diaphragm Design Loads and under Forces saying For Drag Strut Design, which are enabled when viewing the Seismic load case in the Loads and Forces view only, i.e. not when in Generate Loads view.
The loads shown are those derived from Fpx, including transfer forces from the floor above, distributed in proportion to the building masses on that level, i.e. they have the same load profile as the loads for shear wall design derived from Vx from Eqn. 12.8-13, but a different magnitude.
Transfer forces from discontinuous shearlines on the level are shown in Shearwalls as point loads on the level below. When diaphragm loads are shown, this load is factored by the overstrength factor Ω0.
as per 12.10.1.1.
These loads are always shown when Diaphragm Design Loads is selected, even though they may not be the critical ones for design of every diaphragm element. In some cases, the Fpx-based loads are of a lesser magnitude than the Vx-based loads, but the increased transfer force from a discontinuous shearline makes these loads critical for some part of the diaphragm. For this reason, it is left to the judgment of the designer as to which set of loads to use to design an element.
The shearline forces shown are those used for drag strut design, that is, the largest of the forces derived from Fpx-based loads and Vx -based loads.
For Seismic Design Category A and B, only the Vx-based forces are shown, as ASCE 12.10.2.1 applies only to Categories C-F.
For the following items, definitions of have been added to the Legend to the Seismic Load Generation Details and equations added to the Equations section.
Forces from discontinuous shearlines from the floor above are shown in Plan View on the level below as a point load. However, there is also a point load in the same place from the building mass from the lower half of the wall on the level above. These two forces obscured each other. Now, the transfer force from the level above has been repositioned such that both forces are visible.
Regarding the condition "b" of ASCE 7 12.3.4.2 under which structures in Seismic Design Category are permitted to have a redundancy factor ρ = 1.0:
In determining whether the determined whether the required seismic force resisting "bays" existed, the program was not restricting the calculation to those stories that had more than 35% of the seismic base shear. Note that according to ASCE 7 12.3.4.2, virtually all stories of 5-6 story structures have 35% of the base shear, so this problem was unlikely to arise.
A literal reading of ASCE 7 12.3.4 would suggest that the factor ρ = 1.3 be applied in both directions if conditions (N-S and E-W), a and b were not satisfied in just one of them, and that is the way Shearwalls was operating. However, we applied were not being calculated separately in each direction (N-S and E-W), and if it was not satisfied in either it would result in ρ = 1.3 in both; now it is evaluated for each direction separately.
When seismic loads are selected, the Wind Load Case options are now disabled. Previously the selections were available but had no effect.
The Force Direction item has been renamed SFRS Direction, for consistency with the change to MWFRS for wind. SFRS stands for Seismic Force Resistance System
The Load Direction input has been renamed Force Direction, as the shearline and hold-down forces are the only things that could potentially change when an opposing direction is selected.
For Seismic Design Category A, in the Distribution of Base Shear table of the Seismic Load Details file, nonsensical values were shown for the vertical distribution factor Cvx from ASCE 7 Eqn. 12.8 -12. This is because Cvx is not applicable to SDC A as per ASCE 7 11.7 and 1.4.
The column for the Cvx factor now shows a "-" in this case, as does the hx * wx column, which is not relevant to SDC A either.