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Torsional Shear

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Until 1997, the UBC specifically prohibited decreasing the direct shear due
to the torsional effects. 1994 UBC Section 1603.3.3 Horizontal Torsional
Moments says, "Forces shall not be decreased due to torsional effects." In
the 1997 UBC that language has been eliminated. (1994 UBC Section 1603.3.3
has been combined with 1603.3.2 and renumbered as 1997 UBC Section
1605.2.1.) I believe the rationale behind this change is that if you
perform analyses with both plus and minus 5% eccentricity, you have
enveloped the building behavior, and if those torsional effects are large
the eccentricity is amplified per Section 1630.7. This has the effect of
increasing the magnitude of the torsional shear when it is additive to the
direct shear, and of decreasing the magnitude of the torsional shear when
it is of opposite sign to the direct shear - thus reducing the effect of
subtracting the torsional shear from the direct shear. If for both the plus
and minus eccentricity the torsional shear is still subtracting from the
direct shear for a given frame or element, the Code now seems to permit the
reduction. Finally, it lends itself better to computerized methods - a 3D
finite element analysis automatically distributes the shear to the frame
members, accounting for direct shear and any diaphragm rotational effects -
plus or minus, and under the previous requirements additional analysis was
then required to add the shear back in to those elements affected. Now the
analysis results can be used directly as long as they account for both plus
and minus eccentricity.
Allen Adams
RAM International


>
>To: seaint(--nospam--at)seaint.org
>From: julio.guerra(--nospam--at)theaustin.com
>
>Hello everybody,
>
>     Since I was introduced to Structural Engineering I know that the
>torsional shear  due to eccentricity, must not be deducted from the direct
>shear in any case. Only torsional shear that increases the direct shear
>should be taken into account. The Code is a litle ambiguous when it says:
>"Provisions shall be made for the INCREASED shears resulting from
>horizontal torsion where diaphragms are not flexible" . It doesn't adress
>the reduction of the direct shear as it was in previous codes.
>     I was reading  SEA "Seismic Design Manual", Volume , example 25 and I
>noticed that they reduced the direct shear (page 79) due to torsional
>shear.
>     Am I too conservative because I don't use this reduction? I checked
>with several structural engineers and NONE of them use the reduction.
>How is everybody else doing regarding this matter?
>
>     Thanks in advance for your response.
>


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