2005 NDS

[ARLORD]

Can anyone give any examples of when to use the Diaphragm Factor, Cdi, of Section 11.5.3.

Is this factor incorporated in the Wind & Seismic Tables where minimum nail sizes and spaces are given.

 

[RARWOOD]

It is probably incorporated in the table. However with out a more specific reference, I am not sure which table you are using.

An example where you would use Cdi would be when you are designing a diaphragm using the Principles of Mechanics. For example when you want to use a nail gun nail which is not listed in the tables.

 

[ARLORD]

If required, it should be incorporated in all of the diaphragm tables. I am just trying to determine theoretically when this factor is required.

 

[KBVT]

If you attempt to manually develop diaphragm capacities with fastener shear as one of your failure modes, you can use the diaphragm factor to increase your individual nail capacity. If you are using the capacities listed in the IBC diaphragm tables, than you are not to use the increase as the values are all based on tested assemblies.

 

[ARLORD]

KBVT,

I am not sure what you mean, you wouldn't apply the Cdi factor to the values in either tables, IBC or NDS?

Also, I thought the two tables, IBC & NDS, had the same values, after applying the 0.5 factor to the ASD wind & seismic tables in NDS?

 

[RARWOOD]

I think what KBVT is trying to say, is that when you have a table that gives shear values for certain panel thickness, with a specific nail size and spacing you can't apply the Cdi factor to that.

The reason is as he explained is that not only are the table based on tested assemblies, but they already would include all appropriate adjustment factors.

The purpose of the tables is to tell you what diaphragm value you can use if you follow the nail spacing, nail size and panel thickness in the table. The table values will cover about 90% of all cases. I have been in the wood industry almost twenty years and I think I have used the Cdi factor twice.

The Cdi factor is for use when you need to design a diaphragm that is not found in the table. If you are using the tables you don't use the diaphragm factor.

I probably confused you before when I said it was probably already incorporated in the tables. The proper way of stating the answer would be as KVBT did. The tables are based on testing and the numbers in the table have already been adjusted by all appropriate adjustment factors, to reduce the test values to the published design values.

So you only use the Cdi factor when you are not using the tables

 

[ARLORD]

Ok thanks RARMBJ for the further explanation. To follow up on a similar subject. I read in a timber text book (based on the 1991 NDS) that the wind & seismic tables include the Load Duration factor, 1.6.

It further stated that if the load you are using require a different LD factor you have to adjust accordingly. I looked in the current 2005 NDS for indication that the 1.6 L.D factor was included but could not find it anywhere.

What are you thoughts on this given you last response regarding the tables. And does the NDS state that the tables include the L.D factor of 1.6.

 

[RARWOOD]

Give me a specific reference to the table in question. Prior to the end of April I was working for a glulam fabricator and had all the reference material I needed at my finger tips. I am now working for a structural consulting firm and have many steel and concrete references on my desk.

If you give me a specific table, I can go pull it from the library.

Do you have Donald Breyer's book "Design of Wood Structures"? IMNSHO his book is the best wood design book out there. It is the first one I always go to when I have a question.

 

[ARLORD]

RARMBJ

Yes I have the 3rd Edition of Breyer's "Design of Wood Structures". It states that for the Seismic & Wind tables in NDS, a 1.6 load duration factor is included. If lower duration loads are being considered, then adjust the values.

It doesn't matter which table, because the book refers to all of the NDS tables for shear walls and diaphragms. I'm am just trying to get an understanding of the principles/methods of wood design. I assume that if the 1.6 LD factor was used, some where in the NDS code it should tell you.

For example for any of the 2005 NDS diaphragms or shear wall tables in NDS.

Also back to my original question. If these adjustment factors are included in the NDS Tables(any and all), you would think that they would say so somewhere, maybe in the commentary.

I am new to wood design. I am just used to everything being called out, explicitly, somewhere in the manual/spec like in steel or concrete design. Maybe in wood things are more implicit.

 

[ARLORD]

Following up on my own question. The NDS Tables for W and Z values for connections, in the notes state to multiply the values in the tables by the appropiate adjustment factors.

I guess where tables do not state this, then what do you assume as far as which adjustment factors are incorporated.

Also regarding glulam members, are they fabricated using one grade of wood, or are the interior layers made of different grades of wood. The NDS Tables only give one grade in the W and Z connection tables.