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CONNECTION AT CONCENTRATED LOAD

CONNECTION AT CONCENTRATED LOAD

CONNECTION AT CONCENTRATED LOAD

(OP)
Given a multiply wood truss girder with a concentrated load on the bottom chord, our truss software on the output says to "apply cluster of nails where shown", but never shows the clusters on the drawing.  Does anyone else see this on their wood truss design software?  My question regarding this is who is responsible for specifying this (i.e. the location and number of nails required).  Is it the manufacturer's responsibility, or is it the truss engineer's?  

RE: CONNECTION AT CONCENTRATED LOAD

From a previous thread (do a google search above for the thread).  This is from the state of Nevada I believe.

Basis of Design

1.    Premanufactured roof trusses are an engineered system - that is, each truss has a specified location in the overall structural system.

2.    The premanufactured roof truss system is both a vertical and lateral load system.

3.    The lateral load forces must be developed by the EOR and given to the truss engineer to design (drag truss load, collectors, etc.).

4.    All truss-to-truss connections are the responsibility of the truss engineer.

5.    All drag load (horizontal truss transfers) connections between the truss and the structure are the responsibility of the EOR.

6.    All truss-to-structure (walls or beams) connections are the responsibility of the EOR.

7.    It is the responsibility of the truss engineer to develop the loading required to implement the requirements of snow load Appendix A.

8.    It is the responsibility of the truss engineer to review all of the EOR’s design specifications, roof truss support, and drag details and to incorporate these requirements into the engineering design of the roof truss system.


Required Information from Truss Engineer:

The truss engineer should develop a truss layout plan for the truss system that clearly indicates the truss vertical support conditions, truss-to-truss connections, drag trusses and collectors, and any other field-installed reinforcement, including field-installed top chord reinforcement at eaves necessary to execute the truss system design.  The truss roof framing plan should be sealed by the truss engineer and be included with the individual truss cut sheets.  The truss engineer should also provide proper supervision of any truss company technicians.

RE: CONNECTION AT CONCENTRATED LOAD

It is the responsibility of the truss engineer.  Per ANSI-TPI 1-2002, Chapter 2, Section 2.1.2.9 (which discusses the responsibilty of the truss designer)

(Truss Designer responsible for...)"Connection requirements for: a)truss to girder truss; b)truss ply to ply; and c)field assembly of trusses."

Note:  field assembly is NOT erection, temporary or permanent bracing, but rather field spliced trusses.

woodengineer

RE: CONNECTION AT CONCENTRATED LOAD

(OP)
Is this typical of other truss design software?  That is to state that a cluster of nails are required at a concentrated load, but not specify the quantiy/type?  We use Robbins and this is how their software works.  Also, what checks/calcs do you perform to design the connection at a concentrated load?  It is very common for us to have 3 ply girders with 5000 lb. +/- concentrated loads framing into them.      

RE: CONNECTION AT CONCENTRATED LOAD

It is built into Mitek.  The only ply to ply connection Mitek does not address are floor trusses.  The designer has to determine the SDS, WS, FTC clip connection.

woodengineer

RE: CONNECTION AT CONCENTRATED LOAD

Truswal has a cluster nailing feature. We use it for loads over #2000. (This is a standard we have adopted in our company.)

2-ply floor trusses are awful and we avoid them at all costs. Try driving a 7" screw into a 1.5" wide 2x4 12" on center. Not easy. The FTC is even worse and still haunts my dreams even though it has been 18 years since I assembled them myself.

RE: CONNECTION AT CONCENTRATED LOAD

I got into a discussion about this with a engineer about a year ago that has been expert witness many times. I used to make the truss engineer seal all his connection drawings and write notes my drawings and the shop drawings that the connection design was the total responsibility of the truss engineer and thought that I was covered.

However, the guy I talked to said my notes and requiring the truss engineer to seal his connection drawings meant absolutely nothing because I was the EOR and I would be sued regardless. He said this had been decided previously in court cases (he called it "case law"), the EOR still holds the responsibility. I guess that wouldn't apply for states where it is specifically written that it is the truss engineer's responsibility as JAE posted above, don't ask me a bunch of questions about this because I really don't know. The posts above are the first times I have seen it written anywhere besides my on drawings that it is the truss engineer's responsibility.

RE: CONNECTION AT CONCENTRATED LOAD

Is that ANSI document referenced in the IBC and does anyone know if there is a similar document for metal stud truss connections?

RE: CONNECTION AT CONCENTRATED LOAD

To me it makes sense for the Truss DESIGNER to specify the truss to truss connections for the simple reason of making sure that the trusses are designed to accommodate the connection. For example, a truss carrying girder may work just fine with a 2x6 bottom chord, but the connected load may require a 2x8 bottom chord.
The connector should be indicated on the truss placement diagram and/or the truss profile sheet, where the EOR can then review and comment.

RE: CONNECTION AT CONCENTRATED LOAD

(OP)
It may be easier to start a new thread for this question, but I'll go ahead and try it here for now.  
When you have a uniform load applied to the bottom chord of a multiply wood girder truss, in order to design the fasteners, do you assume all plies take equal load, then design your fasteners for that loading?  Similarly for a concentrated load, do you divide the concentrated load by the number of plies, and then design the fasteners for that load?  
I was taught that this is how you would do this.  However, when looking at the NDS design values for single shear, I am starting to second guess this.  Say I have a 2 ply girder.  And I have a 1000 lb. concentrated load applied to the bottom chord (SYP #1).  Dividing this load by 2 I would have to design for a 500 lb. connection.  Looking at the tables for single shear (2 member) connections, it seems like I should be designing for the full 1000 lb.  I guess I may just need some clarification on how to interpret the tables.  

RE: CONNECTION AT CONCENTRATED LOAD

Great question.  Connections shall be designed to transmit load from ply to ply and connections shall be adequate to carry cumulative load of the remaining plies.

Let's consider a few examples:
*1000# into a 2 ply girder:  attach the two plies together for 500#.  Remember, the first ply will carry the load directly via hanger attachment.  You are attempting to transfer 1/2 of the load to the adjacent truss such that they act as one composite unit.

**1000# into a 3 ply girder: 2/3 of the 1000# (667#) must get transferred between the 1st and 2nd of the three plies to ensure composite action.

***1000# into a 4 ply girder:  3/4 of the 1000# (750#)must get transferred between the 1st and 2nd of the four plies to ensure composite action.

Note, since the first ply of three or more plies generally carries a smaller percentage of the load than the sum of the remaining plies, the connections must be adequate to transfer the sum of the loads on the remaining plies S7.4.5.3 ANSI TPI1-02 Commentary.

Also, note that the load distribution would be equal if the loading was concentric and not side loaded.

Other requirements...
1. Bolts are required for 4 or more ply girders.
2. There is a maximum of 5 plies for a girder loaded from one side.
3. An absolute maximum of a six ply girder which must be loaded on both sides.
(I personally will not consider a girder truss with more than 4 plies)

I hope this helps.

woodengineer

RE: CONNECTION AT CONCENTRATED LOAD

(OP)
Thanks! That helps a lot.  It's great to finally find someone who understands my question.  I still have a few questions.    
Take for example the 4 ply girder in your example.  I understand that you would design the connection to transmit 75% of the load between the first and second plies.  How about the second and third plies and the third and fourth plies?  I would think that you would need less nailing between the latter since you have already transmitted 75% to the second ply.  Would you design the connection between the 2nd and 3rd plies for 2/3 of 750?  Or do you just specify the fastening for all plies for the 750 lb?  (I am considering that the fastener only penetrates 2 plies---16d nail).

As a side note... I was trying to interpret Simpson's Technical Bulletin on their Strong-Drive Screw Applications.  They have a design example for 3 and 4 ply girders that says to transmit 100% of the total load.  Have you looked at this bulletin?  If so, any comments or thoughts?  
   

RE: CONNECTION AT CONCENTRATED LOAD

CJS,

I do not change the connection between plies.  I would design the transfer of 750# and use the same nailing for the remainder of the plies.

As for the Simpsons technical bulletin, I found it in agreement with my previous email.  It does require 100% load transfer from hanger into a mlti-ply truss if you are using a specific hanger which utilizes SDS screws to transfer the load using the hanger alone.

woodengineer

RE: CONNECTION AT CONCENTRATED LOAD

woodengineer,

thank you for your explanations. They have been very helpful.

However regarding your statement:

"1000# into a 4 ply girder:  3/4 of the 1000# (750#)must get transferred between the 1st and 2nd of the four plies to ensure composite action."

Would it not be 750# between the 1st, 2nd and 3rd plys to insure composite action?

Please explain. Thank you in advance.

RE: CONNECTION AT CONCENTRATED LOAD

(OP)
woodengineer,
After reviewing the Simpson Technical Bulletin again, the example for joining multiply wood trusses makes no mention of hangers.  It calls out a fastener spacing which makes me believe that it is not for hanger attachment.  I had a call into Simpson and the tech. support person I spoke with did not know why.  He was going to get with another engineer and review it.  I will let you know what I find out if he ever gets back with me.  

RE: CONNECTION AT CONCENTRATED LOAD

(OP)
I see that and agree.  I was referring to the design example on page 6 of 8.  The example is for joining multiply trusses.  Note on that example they are just referring to connecting the plies, not transferring loads from hangers.  And they are transferring 100% of the applied load.   

RE: CONNECTION AT CONCENTRATED LOAD

You are correct.  It is conservative.  

woodengineer

RE: CONNECTION AT CONCENTRATED LOAD

CJSchwartz,
I had a talk with an engineer with Simpson.  They are now aware of the issue and will be modifying page 6 of 8 (or at least are looking into it).

woodengineer

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