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RE: 1997 NDS Equation Error?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "seaint (E-mail)" <seaint(--nospam--at)seaint.org>
- Subject: RE: 1997 NDS Equation Error?
- From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dcp.state.ma.us>
- Date: Tue, 13 Nov 2001 11:56:36 -0500
- Cc: "mts (E-mail)" <mswingle(--nospam--at)earthlink.net>
Gerard Madden wrote: >FYI: Here are my values for the 2x DF Mudsill with 7/8" dia. bolts >D=0.875 >SG=0.50 >Theta=0 >Fyb=45000 >ts=1.5 >tm=3 >Ktheta=1 >Fem=6000 >Fes=5600 >Re=1.07 >Rt=2 >k1=0.7158 (Using Section 8.2.3) >k2=1.3409 >k3=2.0154 >Zis=1838# >Zii=1462# (Governs) >Ziiim=3156# >Ziiis=1615# >Ziv=2230# >Thanks again to all, >-gerard >SJ, CA ******* 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: D=0.875 Fem=Fes=5600 (DF-L, G=0.50) Fyb=45000 tm=3 ts=1.5 theta=0 k1=0.679 k2=1.317 k3=2.077 ktheta=1 Re=1 Rt=2 Zim=3675 Zis=1835 Zii=1390 (Governs) Ziiim=2015 Ziiis=1590 Ziv=2190 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 connection. >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, the >concrete bolted connection is much stronger. >The 1994 UBC 2211.2 dealt with this by saying that a wood to concrete connection >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 limited >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 shear. snip >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 ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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