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Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 10.0 and may not reflect current behavior
ASCE 7 Commentary C 27.4.6 says that the torsional load cases are due to non-uniform wind loading, which is applicable to flexible diaphragm buildings as well as rigid diaphragms. Discussions with ASCE confirmed that the intention was to apply Case 2 loading to flexible diaphragm design. Sizer now includes Case 2 loads and torsional moments for flexible diaphragm design.
Noting that the direct (non-torsional) component of the shearline force should be that determined by tributary area distribution for flexible diaphragm, this can be achieved by setting the rigidities K to the flexible diaphragm shearline force, seeing that
Fdi = F * Ki / Σ Ki ,
using the notation in the Log file, F being the total force. In that case, the center of mass CM = Center of rigidity CR, and we are including just the accidental eccentricity ea and not the eccentricity of the structure or loads. We also do not consider the torsional moment J in the other direction, as none of the loads are in the other direction. The torsional component on each line is then
Fti = T * Ki * di / (Jx)
where di is the distance of the shearline from the centre of load and
Jx = Σ Ki * di 2 ; T = F * ea
Shearlines already heavily loaded get higher contributions of accidental torsion, rather than those that are stiffer as in the case of rigid analysis.
For a simple case of a uniform load on a rectangular building, adding a certain percentage of torsional eccentricity will add that percentage of total force on the structure. For more complicated situations, the amount of force will vary due to the effect of the moment arms of the shearline locations.
To show that for this simple case, adding 15% eccentricity increases total force by 15%, consider a 40 ft wide building with 100 lb/ft force on the diaphragm.
F = 4000 lb; Fd1 = Fd2 = 2000 lb; K1 = K2 = 2000 lb; ea = 6 ft; di = 20 ft; T = 24,000 lb-ft; J = 1,600,000 lb-ft2
Ft1 = Ft2 = 24,000 lb-ft x 2000 lb x 20 ft / 2000 kN-m2 = 600 lb
= 15% F
In contrast to the Rigid Diaphragm section of the log file, factored shearline forces rather than unfactored loads are analysed, and they are the forces on the level being analysed only, being accumulated with upper level shearline forces later. The rigid diaphragm analysis includes loads and/or forces from upper levels.
A note has been added to the Log file output explaining this.
The title of the entire section of the log file for torsional analysis has been changed from “RIGID DIAPHRAGM ANALYSIS” to “TORSIONAL ANALYSIS” , in recognition that some of the output now pertains to flexible analysis. A section is added at the top of the results called, FLEXIBLE DIAPHRAGM WIND DESIGN. The assumptions given in section a), above, are shown first, and then the results are given as they are for rigid diaphragm analysis. The source of the accidental eccentricity is given as ASCE 7 Fig. 27.4-8, Case 2.
The note appears saying that only loads on the current level are analysed, and accumulation with forces from the level above is done later.
The over-rides of the 15% eccentricity and 75% loading that you can enter in the Site Dialog ( see CREF) apply to flexible diaphragms as well as rigid.
The absolute eccentricity used for flexible structures that require dynamic analysis applies to flexible diaphragms as well as rigid ( as opposed to the usual percentage eccentricity for rigid structures.).
Previously, if you added a direct shearline force in the Add Load dialog with Both selected, and Case 2 selected in the settings, the program created a flexible force at 100% and a rigid at 75% of the value entered. If only rigid or only flexible was selected the force would be created at 100%..
Because Case 2 loads now apply to both rigid and flexible diaphragms, the two forces at 75% and 100% are always created, regardless of whether both, rigid, or flexible forces are being made. This is done only for buildings greater then 2 stories high.
Above the Shear Results table, the note about the torsional load cases for low rise loads required by 28.4-1 Note 5 now appears for flexible structures more than 2 stories high, but in a modified form saying these forces cannot govern for flexible diaphragms.