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RE: 1997 NDS Equation Error?

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Gerard Madden wrote:

>FYI: Here are my values for the 2x DF Mudsill with 7/8" dia. bolts

>k1=0.7158  (Using Section 8.2.3)
>Zii=1462# (Governs)

>Thanks again to all,

First a moot point: Using your numbers above, you should have gotten 2100
lbs for Ziiim, not 3156 lbs.

Second, I have always used the version in the 1994 UBC, where the term in
question (in the formula for k1) is shown *outside* the radical.  Using the
formulas that way, my numbers always agree with the tables in the 1994 UBC.

Third, you should use Fem=Fes=5600 psi, not Fem=6000 psi, for
wood-to-concrete connections.  At least that was the way in the 1991 NDS and
the 1994 UBC.  I realize that AF&PA's Technical Report 12 shows that the NDS
tables are based on Fe=6000 psi for concrete, but I don't know if the new
NDS tables have been updated for this.  Let's stick to the old way for now
(for purposes of this message, that is).

Using Fem=5600 psi, I get the following:

Fem=Fes=5600 (DF-L, G=0.50)
Zii=1390  (Governs)

The value 1390 lbs agrees with Table 23-III-K, page 2-1004, in the 1994 UBC.

Scott Haan wrote:

>Fes=Fem per NDS 8.2.3.

>Next look at the absurdity of mode II controlling in a wood to concrete
>Mode II is rotation of the fasteners in the members.  It is not going to
happen in a
>wood to concrete connection, the wood is going to be the failure mechanism,
>concrete bolted connection is much stronger. 

>The 1994 UBC 2211.2 dealt with this by saying that a wood to concrete
>should be designed with 1/2 the value of a double shear connection with the
>concrete being considered twice the thickness of the side members. This
>the failure to a mode IIIs because mode II doesn't apply to a double shear
>connection.  UBC Table 23-III-N 3180/2=1590 half of mode 3s in single


>Scott M Haan P.E.
>Plan Review Engineer
>Building Safety Division 
>Development Services Department
>Municipality of Anchorage

I agree that mode II should not be considered for wood-to-concrete
connections.  Also, the 1994 UBC 2311.2 does *not* say such a connection
"should" be designed with 1/2 the value of a double shear, but rather that
they "are permitted to be determined as . . . . " such.  This leaves open
the possibility that one should use the mode II failure value because it is
the more conservative value *according to the code*

To take Scott's post a little further, I personally believe that modes Im,
II, and IIIm should not apply to wood-to-concrete connections.  Looking at
the diagrams in the NDS (also shown in 1994 UBC p 2-1114) which show and
describe the failure modes, it is clear that none of these three modes apply
in a connection where the dowel is *embedded* in the main member.  Only
modes Is, IIIs, and IV should apply to wood-to-concrete connections.  The
NDS and the 1994 UBC allow any of the six modes for wood-to-concrete
connections, notwithstanding Scott's note above that 1994 UBC 2311.2
*implies* that mode II does not apply.

For support of my idea, look at the *lag screw* equations and values for the
same situation.  Take the case of a lag screw 7/8" in diameter, 1.5" side
plate of DF-L, with the minimum embedment in the main member that is
required for full design value.

Note that only modes Is, IIIs, and IV apply to lag screws.

Note that the equation for mode Is is identical to that for a single-shear
bolt, mode Is.

Note also that even though the equations for modes IIIs and IV are a bit
different (the k factor and the constants) than for bolts, the final values
are nearly identical.  Here is a table comparing bolted connection (single
shear) to lag screw:

D=0.875 in; Fem=Fes=5600 psi (G=0.50); Fyb=45,000 psi; tm=3 in (for bolt);
ts=1.5 in; 

          BOLT     LAG SCREW
Zis:     1838       1838
Ziiis:    1590       1615
Ziv:      2193       2188

The NDS could solve this confusion by clarifying that only the values for
Zis, Ziiis, and Ziv apply to wood-to-concrete connections.

Mark T. Swingle, PE, SE

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