NDS 3.4.3.3 Column Cap Plate 5

[StructENG82]

I am designing a column cap plate and have encountered some confusion on NDS 3.4.3.3 in regards to uplift capacity. My situation is that I have a glulam beam supporting roof trusses and the base of the beam is attached to a glulam column with a column cap plate. In checking the shear stress in the end of a beam per NDS 3.4.3.3 I am getting results in which the bolts would have to be twice (or more) higher from the bottom of the beam in comparison to using a Simpson Column Cap to get the same uplift capacity.

Am I making a wrong assumption of assuming the unloaded edge is the top of the beam? Are the Simpson Column Cap values base purely on testing? Or is there something else that I am missing.

 

[TehMightyEngineer]

I'm pretty sure most if not all of the Simpson hardware is based on tested data and thus it can "ignore" some of the NDS requirements.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[mike20793]

Simpson is moving to make some of their products' values more in line with IBC calculations from their respective references. That being said, I wouldn't hesitate to use a Simpson hanger/cap for the loads they list.

 

[Triangled]

that does seem odd.... after triple checking the math, you might call simspon engineering. I've found them readily helpful in the past.

 

[BSVBD]

I have often questioned / wondered how Simpson does it when i compare their components with my calcs in correspondence with NDS Tables 11A-R.

Now, i have never experienced such an unreasonable difference that would alarm me. Therefore, I have reasoned, in my own mind, that Simpson's testing must be legitimate enough, to a similar degree, to that of state-recognized, Material Approval numbers, like Pre-Engineered Metal Buildings, for example. In other words, after reasonable self-approval, i honor Simpson's testing and published allowables in a manner similar to that, apparent, time-established honor that certain (USA) states have granted to proven material providers.

 

[Triangled]

in your condition, simple span, uplift etc, I think I'd understand the unloaded edge as you do.

Thumbing through my old simpson catalogue 2009-2010, the footnotes under CC reference limitations to uplift based upon NDS 2005 beam shear capacity. Obviously my catalogue is out of date, but it appears that Simpson has/had considered the NDS in this particular application. If your numbers are different, I'd recommend reaching out to one of their engineers. I've found them often quite helpful, as ultimately, they want us to specify their products.

I hope you post your findings as this is a curious case.

 

[StructENG82]

Everyone,

Thanks for the responses. I reached out to Simpson as recommended and had some interesting conversations. In the old Simpson manuals they stated in the footnotes that capacities given were for an 11” beam and other depths needed to be calculated by the engineer. They excluded this footnote in the newer manuals and after conversations with the engineer it sounds like this is something they regret doing (he has put in a recommendation to get it added into the next manual edition).

Simpson provided calculations that verified my findings that for a deeper beam you will have a lower beam shear capacity in reference to uplift forces on a bolted connection near the base (higher tendency for cracking parallel to the grain).

For an example the shear capacity for a 39" deep end beam condition using the Simpson cap plate has only 500 lbs uplift capacity compared to the table value of 4670 lbs.

To summarize, if you have high uplift and deep beams make sure you are doing custom fab cap plates and not counting on the uplift values provided by Simpson for their cap plates.

 

[KootK]

Wow! Star for you for chasing this down and sharing the results with us.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

I don't see any stars, so I will add one.