|
|
|
|
|
Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 10.0 and may not reflect current program behaviour.
The method of determining external pressure coefficients CpCg for low-rise structures from NBC 2015 4.1.7.6 and Figure 4.1.7.6.-A (or NBC 2010 Commentary Figure I-7) assumes symmetric, rectangular structures. They are based on boundary layer wind tunnel studies of buildings of that shape, verified against full scale measurements (NBC Commentary I-20).
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. These buildings can have any number of roof slopes, and coefficients CpCg for both walls and roofs depend on the slope of the roof, so Shearwalls allows a choice as to what is used as the roof slope in generating low-rise wind loads for these structures. You can generate loads on each roof panel and attached wall using the slope of the roof panel to determine CpCg or you can conservatively use the coefficient for the roof panel on a block or on intersecting blocks that has the greatest effect.
There is an option in the Generate Loads box that says
Use coefficients with greatest effect on intersecting blocks or eccentric ridge lines
This is checked by default.
When two blocks intersect such that the slope of the roof on one end or side of the block is different than that on the opposing end or side, such as block framing into another to form an L-shape, the program determines the projected area of intersection of walls and roofs on opposing sides and assigns each side the CpCg value from either side that leads to the greatest combined wind load.
The area of exterior walls beneath a sloped roof are considered to "have" the slope of the roof for the purpose of Case B load generation which depends on roof angle. If worst-case coefficients are used, then areas of intersection in plan view of a wall with a wall on the opposing side of the structure are assigned the CpCg value from either wall that yields the highest wind load.
The complementary areas of roofs are determined by the region over which a roof frames into another roof. For example, a sawtooth roof would only include the area under the intersection of the two roofs. If both sides of both roofs extend to the eave, then there is no intersecting area and no worst-case coefficients.
If hip ends on opposing ends of a block have different slopes, or there is a hip on one end and a gable on the other, then over the region of projected intersection of the areas, the coefficients with the worst effect are applied. This applies to single blocks or to multiple blocks where there is no block framing into the hip or gable ends. If there is such a block then only the extreme surfaces of the assemblage are considered, and the intermediate surfaces ignored.
The program applies the coefficients with the worst effect to the surfaces on either side of an eccentric ridge line which have different slopes. This applies to single blocks and multiple blocks where there is no block framing into the eccentric side panels. If there is, only the extreme surfaces are considered, and the intermediate ones ignored. For example, a sawtooth roof with two peaks and eccentric ridges will not consider the interior slopes.
End zones are considered when determining the complementary regions over which to compare coefficients, and it is possible to compare end zone coefficients on one side of the structure to non-end-zone ones on the opposing side.
The calculation to determine the width for the height-to-width limitations for this method in NBC 4.1.7.6.(1) no considers the maximum extent of all walls on the structure rather than the extent of a single block.
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. Different messages are provided according to whether the worst-case coefficient option is selected.
In the Load Generation Details output:
A note in the header section of the Load Generation Details output indicates that the building does not conform to strictly to NBC 2015 Figure 4.1.7.6.-A or NBC 2010 Figure I-7 for the following reasons, included if applicable to the structure – multiple blocks, eccentric ridge lines, hip roof construction, or unequal hip or gable ends.
If the load generation option for worst-case coefficients is selected, the program shows a note immediately before the first load table saying that the worst case CpCg was used for the following circumstances, included if applicable to the structure - multiple blocks, eccentric ridge lines, or unequal hip 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 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.
Previously, Shearwalls analyzed low-rise wind loads NBC 2010 Commentary Figure I-7 (NBC 2015 Figure 4.1.7.6-A from the loads in E-W and N-S directions independently, and ignored the torsional forces in the orthogonal direction.
However, Note (1) to these Figures says that the building must be designed for "all wind directions", and that Load Case A and Load Case B are the separate cases for which loads "including torsions" must be generated.
Shearwalls now includes loads 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 loads in both directions.
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, for load Case B
The program now allows you to show in Plan View the low-rise loads for Case A in the longitudinal direction and Case B in the transverse direction, as these are the cases that will ordinarily govern for design.
This option is available only for single block structures, because for a particular wind direction, multiple block structures can have Case A at the end some blocks and Case B for others, and similarly for the sides.
The option Case A Side, Case B End appears after the Case A and Case B under Wind Load Case in the Show Menu.
The large hollow arrow angled to show the general wind direction has been replaced by two orthogonal arrows in the N-S and E-W directions for this choice.
Note that these loads are not to be considered to have been generated simultaneously, and the set of loads shown is not used to generate any torsional analysis case.
The following changes have been made to the Show menus when Wind design is selected.
For wind loads, Orientation has changed to MWFRS Direction.
When the Other buildings – 4.1.7.5 wind method is selected, Load Direction is changed to Wind Direction.
When the Other buildings – 4.1.7.5 wind method is selected, the wind load case this option is now disabled. Previously the selections were available but had no effect. This happened even when seismic loads were showing.
For single block structures, Case A or Case B appear in brackets beside the wind direction, according to the ridge direction of the structure.
An explanatory note has been added to the Torsional Analysis Details output suggesting manual approaches to implementing load cases B, C, and D from NBC 2010 4.1.7.3 and NBC 2015 4.1.7.9. These cases are not generated automatically by Shearwalls, and would have an effect only on rigid diaphragm torsional forces.
In addition to changes for new or modified features described elsewhere, numerous small formatting improvements and insertions of key information have been added to the Load Generation Details output. The following give the more important changes when NBC 2010 is selected; and the corresponding improvements made using NBC 2015 provision reference numbers when that option is selected.
For windward low-rise C&C loads program was using the pressure and gust co-efficient CpCg for leeward direction, and internal pressure coefficient Cpi for positive pressures when it should have been using negative pressures. The incorrect coefficients were listed in the Load Generation Details output.
This has been corrected, and different windward and leeward coefficients are created.
For out-of-plane sheathing design, the program now determines the worst case of internal positive pressure and leeward loads, and internal negative pressure and windward loads. As it is only rare cases that the windward loads govern, this had little effect on sheathing design. Nail withdrawal design is not affected.
Windward C&C loads were being generated with the same magnitude as leeward ones, i.e. with differing interior and end zone magnitudes. They are now being generated with the correct magnitudes.
Windward C&C loads affect only sheathing design (not nailing), and this change has little design effect as end zone leeward (suction) loads almost always govern design.
For low-Rise MWFRS loads with a topographic effect, the program was not including the Cg* component from as per NBC 2010 Commentary I-21, which says you are to multiply CpCg by Cg* and divide by Cg.
It was doing this correctly for C&C loads.
Since Cg* for topographic effect is diminished relative to Cg, this was a conservative bug, and has been corrected for the NDS 2010 design code option. NDS 2015 no longer includes this calculation.
For walls under hip roof ends using the low-rise procedure and a longitudinal wind direction (Case B), end zones were not being created. This has been corrected.
For walls under hip roof ends using the low-rise procedure and a parallel-to-ridge wind direction wind direction (Case B), end zones were not being created. This has been corrected.
For walls under hip roof ends using the low-rise procedure and a perpendicular-to-ridge wind direction (Case A), end zones were created with a 1-meter width rather than 6 meters, according to 4.1.7.6.A Note 6. This has been corrected.
The note indicating that hip ends are treated as side panels in terms of low-rise coefficients has been modified to indicate that the walls under the hip ends are also use the same load case as the roof, that is, the one for side panels. A reference to 2001 AWC Wood Frame Construction Manual, Table 2.5A has been added to explain this.
The Design Setting which is now called Height for Wind Classification is now saved to the project file, it hadn’t been previously.
The disabled options for hip roof load generation pertain to the USA version of the program and have been removed completely for Canada
When Below crest is selected in the Site Information dialog box, the Site Location in the Site Information of the Design Results output says Downwind of the crest. When it is below the crest, it should be Upwind, according to Figure 4.1.7.4, and when it is above the crest, it should be Downwind. This change has been made.
n the Load Generation portion of the log file for the All-heights wind load generation method, the line of "------"s was printed at the tail end of the Wind Load Generation header instead of on a new line. This has been corrected for the new implementation in the Load Generation Details file.
In the Loads and Forces settings, the check boxes labelled Southwest, Northeast, Southeast, Northwest have been changed to show Wind from Southwest, Wind from Northeast, etc.