NDS Connection Lateral Design Values 3

[mpoint]

I'm trying to reporduce Table 11L from the 2001 NDS For Wood Construction. I'm using the Yield Limit Eqations found in Table 11.3.1A but can't come up with the same values listed in Table 11L. If I understand the code correctly, the minimum value obtained using the 6 formulas should be what is listed in the table. Am I reading this correctly? If so does someone have an example that shows the correct results for values found in the table? Any guidance is appreciated.

 

[UcfSE]

How far are you from the tabulated values? Are you following the assumptions made in the table footnotes?

 

[mpoint]

My values are 15-40 percent higher then the table values. I've followed the footnotes at the bottom of Table 11L and chapter 11 of NDS 2001 as best I can tell but will go through the book again to make sure and I've run through a set of calculations by hand and used a spreadsheet to check my work. The main member dowel bearing length lm is one value I'm not 100 percent sure on, I set lm = 10D. I played with changing this value to obtain the correct printed values but didn't see a consistent pattern.

 

[JAE]

mpoint - I don't have the 2001 NDS but I did create some nail/bolt spreadsheets using the 1997 NDS and got their answers in the table using the lowest value of the given Z modes of failure.

Be sure to read all the footnotes (I know you said you did above) and hand check the calcs if you are using a spreadsheet. Sometimes there could be a mixed unit issue within the equations.

 

[RARWOOD]

Are you applying the reduction terms from Table 11.3.1B? You could also go back to the 97 NDS and calculate the values comparing the two numbers.

It might also have to do with whether you have rolled threads or full body threads. Compare the 97 NDS to the 2001 NDS values for lags. The 2001 values are for reduced bogy lags.

 

[mpoint]

I've done the calculations long hand and in Excel. I get the same results. I've recreated the NDS 97 table without any problems.

The example I used is #8 wood screw, D = 0.164", Table 11.3.1B D < 0.25" therefore Rd = Kd = 2.2 for D ? 0.17"
Wood is southern pine, Gm = Gs = 0.55 therefore Fem = Fes = 5550 psi
ts = ls = 1/2", I assumed tm = p = 10D as per Table 11L note 2 therefore tm = lm = 1.64in
Fyb = 90,000 psi for D = 0.164", Table 11L note 2
Zmin = ZIIIs = 119.7 lbs... Table 11L Z = 91 lbs.

Probably something real simple I'm missing but I'm missing it.

 

[RARWOOD]

I will look at your example. If you can recreate the 97 NDS then there must have been a small change made in the 2001 NDS.

I think the difference is that the tables in the 97 NDS are based on cut thread wood screws & the tables in the 2001 NDS are based on rolled thread wood screws. See appendix L of the 2001 NDS.

If you look at the tables for lag screws you will find that the 2001 NDS is based on reduced body diameter lag screws while the 97 NDS table is based on full diameter lag screws.

 

[MCurry]

mpoint - I get the same value for Z as you (119.7#, Mode IIIs governs). But I read Z=82# from Table 11L.

Nothing obvious to me about the discrepancy.

 

[CJJS]

Did you consider the entire length of the lag screw, or omit the tapered portion at the tip?

 

[mpoint]

Appendix L NDS 2001 shows L going from the top of the flat head to the tip of the screw but L doesn't enter into the Z formulas. Table 11L, footnote 2, states "minimum screw penetration, p, into the main memeber equal to 10D". I used lm = p for my calculations.

 

[CJJS]

Try subtracting E (Length of tapered tip)from L. Use that value for lm and see what you get.

 

[RARWOOD]

Set the screw diameter = 0.131 and Lm=1.31. Then run your spread sheet and then compare the results to the 2001 NDS.

 

[CJJS]

If the lag bolt penetrates a distance of 10D or greater into the main member, then the dowel bearing length lm is taken as the total length of the lag bolt in the main member. If the lag bolt penetrates a distance less than 10D into the main member, the dowel bearing length lm is taken as the total length of the lag bolt in the main member minus the length of the tapered tip E of the lag bolt.

 

[CJJS]

Sorry mpoint, I just realized that subtracting the taper is only for lag bolts.

 

[CJJS]

RARSWC is correct. Read page 158 of NDS 2001 (Appendix I.5). Tabulated lateral design values for rolled thread wood screws are based on root diameter Dr. Sorry if I led you in the wrong direction previously.

 

[mpoint]

Thanks for the help. Dr was it.

 

[RARWOOD]

There are several programs on the American Wood Council web site that will calculate bolt,lag, and screw values.

The AWC table appears to subtract the tapered tip of wood screws. Based on Section 11.3.4 of the 2005 NDS it would appear that if the lag or screw penetration exceeds 10D that you aren't required to subtract the tapered tip.

In the past when calculating penetration for lag screws you subtracted the tapered tip, but when calculating the penetration for screws you included the tapered tip.


Thank you mpoint for this post I learned a lot as I was trying to help you with your problem