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RdRo design values for plastered straw bale shearwalls

RdRo is used in the National Building Code of Canada 2010 (NBCC) to define the ductility and overstrength of lateral resisting systems. The NBCC and the Ontario Building Code (2006) (OBC) do not define values of RdRo for straw bale shear walls with plaster. Bruce King's 'Design of Straw Bale Building', chapter 4.4.9, references the Uniform Building Code 1997 (UBC) R-value. One can interpret the value of R (UBC) compared to RdRo (NBCC) conservatively as follows.


For use with OBC 2006, it is reasonably conservative to treat the stated R value in 'Design of Straw Bale Buildings' as being equal to the product of Rd Ro.

King's analysis finds that plastered straw bale walls have in-plane strengths in a similar range to that of plywood sheathed stud walls. A brief comparison of the seismic forces on a squat nailed plywood shear wall building in Toronto on a class C site yielded the following results (further details below).

V/W = 0.034 (OBC 2006)

V/W = 0.040 (UBC 1997)

The discrepancy between these results is reasonable, since the comparison is sensitive to the value of Sa(0.2) used. UBC 1997 has a more broad based approach to 'seismic zones', and OBC uses more refined site specific values for Sa(0.2).

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Both OBC 2006 and UBC 1997

Site class C

I = 1.0 (importance factor)

T = 0.2 seconds

OBEC 2006

Rd = 3.0, Ro = 1.7 (RdRo = 5.1)

Sa(0.2)  = 0.26

Fa=1.0, Fv=1.0, Mv=1.0

4.1.8.11 Equivalent Static Force Procedure

V/W = S(Ta)MvIe / RdRo = 0.051

V/W < 2/3 S(0.2) Ie / RdRo = 0.034

UBC 1997

R = 5.5 (bearing wall system)

Zone 1, Ca = 0.09, Cv = 0.13

(30-4) V/W = Cv I / RT = 0.118

(30-5) V/W < 2.5 Ca I / R = 0.040

UBC 1997 differentiates between 'bearing wall systems' and 'building frame systems'. Building frame systems ignore the contribution of shear walls to gravity support, and thereby incorporate greater redundancy and have better R values.