## WGC Trusses

## WGC Trusses

(OP)

Seems like I always come back to these things.

I've run a few numbers on using larger nails that go through the truss width and both gusset plates (nails in double shear). However, I've had some potential clients ask for trusses with wood gusset plates using 8d and 10d nails. I am curious if anyone has experience using smaller nails but in larger quantities. My concern is with double the nails into the truss member it will be more prone to split. Also nails in double shear are more bang for your buck in my opinion but it seems that some people think its easier to fabricate the truss by nailing off one side and then the other rather than driving a larger spike clean through and then clinching it.

Either method you can use a nail gun to shoot the nails but the clinching is definitely a manual task so I can see the downside to that.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

I've run a few numbers on using larger nails that go through the truss width and both gusset plates (nails in double shear). However, I've had some potential clients ask for trusses with wood gusset plates using 8d and 10d nails. I am curious if anyone has experience using smaller nails but in larger quantities. My concern is with double the nails into the truss member it will be more prone to split. Also nails in double shear are more bang for your buck in my opinion but it seems that some people think its easier to fabricate the truss by nailing off one side and then the other rather than driving a larger spike clean through and then clinching it.

Either method you can use a nail gun to shoot the nails but the clinching is definitely a manual task so I can see the downside to that.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

Rather than nails I was also thinking about structural screws but the labor and material cost on those might be prohibitive.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

http://www.structuremag.org/wp-content/uploads/201...

http://support.sbcindustry.com/Archive/2004/jun/Pa...

http://www.awc.org/pdf/codes-standards/publication...

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## RE: WGC Trusses

No empirical data to share, but instinctually I generally prefer more fasteners to less, and the basis is a sense of redundancy.

I have often preferred a multi-nailed connection to a few-bolted connection. Verifiability enters into my bolt vs nail thing too I admit, as in My specification of nail spacing vs hoping the carpenter doesn't overdrill the bolt holes.

## RE: WGC Trusses

I'm not sure these days why anyone would want to build trusses with plywood or OSB gussets, unless it is a DIY project.

## RE: WGC Trusses

The requests I've had for WGC trusses is usually one of the two categories:

1.) Rural location needing an ag building, where the cost of transportation gets unreasonable.

2.) The DIY'er who is a purist and has something to prove.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

https://www.ideals.illinois.edu/bitstream/handle/2...

## RE: WGC Trusses

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

http://www.eng-tips.com/viewthread.cfm?qid=392321

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

I was thinking I could use less fasteners by going with a 16d common nail (.162 Dia) in double shear however the rule in the NDS 2015 for penetration into the side members is 6D (12.1.6.5) which for a 16d nail would 0.972". This would eliminate using any gusset plates below an inch thick (ie. 7/16, 15/32, 19/32, 23/32), basically all of my standard plywood thicknesses.

What if the side member is metal? surely this side member rule of 6D does not apply. I must be reading or interpreting this wrong. Someone please clarify.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

i don't think concept applies directly to steel side plates because steel is not receiving the 'point' of the nail in the same sense as wood

## RE: WGC Trusses

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

and I think the 12.1.6.5. exception 'clinches' it

but you should be able to test your proposed system utilizing the dowel equations and come up with acceptable values

## RE: WGC Trusses

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

I think you can still develop your own values utilizing the yield limit equations if you wished

## RE: WGC Trusses

Regarding the truss with the glued gussets, they indicate that the nails are just to hold the gussets in place while the glue dries. Apparently they were just relying on the glue.

## RE: WGC Trusses

I would also not use OSB, even though some of the values are pretty good. The problems with OSB is that if it gets wet its going to lose all of its strength quite quickly, whereas plywood is not so sensitive to water. With that being said a waterproof glue is also a requirement.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

25 psf snow load with 17 psf dead load.

Using DF and Structural 1 Grade plywood 15/32" I have a capacity per nail of 85 lbs. The number of nails required between the two heel plates and the bottom chord would be: 2180.9lbs/85lbs = 25.7 nails, so round up to an even 13 nails on each side of the trus, and similarly 2298.8lbs/85lbs = 27.0 => 14 nails each side of the top chord at the heel plates.

Looking at a quick sketch that is quite a few nails, I would probably create 3 rows with the middle row staggered from the other two.

This of course ignores any contribution of the glue in transferring the axial loads, but in my mind this is offset by ignoring the moments at the joints applied to the nails. Logically this makes sense to me since the glue will be much more rigid than the nails and will prevent rotation at the joints.

The spacing between the nails in a row should actually be 4" since the nails coming from the other side will affectively double the frequency of the nails or halve the spacing. For a .131 nail the 15D spacing would be 2". From the scarf cut on the bottom chord the gusset must measure approx. 15.5" in length, depending on the pitch of the truss the total length of the plate will be somewhat longer than this.

I would probably use an end distance of 1.5" and a edge distance of .75" with 1" between the rows. Obviously a 2x6 top and bottom chord would buy a lot more real estate. Bottom line this is a lot of nails, unless I'm missing something.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

Those are 8d common nails so they aren't very big but spaced at 2" between nails in a row I would be afraid of splitting that bottom chord.

Looking at this I'm fairly convinced that going with longer nails that are in double shear and clinched give more bang for the buck, allow less nails and thereby less chance of splitting the truss members. Thoughts?

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

Using a 10d common nail (.148 x 3) and with 1/2" of protruding nail to clinch I get this layout:

with the backside looking something like this:

I probably don't need a 2.25" end distance (15D) on the plate edge toward the inside of the truss but I am using Table C11.1.6.6 from the NDS Commentary (NDS 2012).

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

So returning to our previous example we now have 2180.9lbs/176.3lbs = 12.4 => 13 fasteners in double shear, rather than 25 fasteners in single shear, a much more reasonable number of fasteners

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

Both the top chords are in compression and pushing against each other, so the member on member contact is transferring a significant amount of that compressive axial load.

To account for the component of the load that is not being transferred at this joint I can probably use the equation from the TPI 1 for MPC trusses:

Pc' = sqrt(P

_{P}^{2}+ (P_{N}x C_{r})^{2}(Sec. 8.3.3.3)where:

P

_{c}' = Resultant compressive force used for determination of minimum required metal plate contact area (lbs)C

_{R}= Reduction factor for cmopression force component across the joint interface for metal connector plate design: 0≤CR≤1.0 (lbs)P

_{iN}= Compression force component of the wood member under investigation normal to the wood member interface (lbs)P

_{iP}= Compression force component of the wood member under investigation parallel to the wood member interface (lbs)With the resultant being:

CR = 0.5

Pi = F23 = 2021.83 lbs.

PiN = Picos(Φ) = 2021.8 x cos(18.43) = 1918.1 lbs.

PiP = Pisin(Φ) = 2021.8 x sin(18.43) = 639.4 lbs.

θ = atan(PiP/PiN x CR) - Φ = atan(639.4/1918.1 x 0.5) - 18.43 = 15.3 deg. (angle between resultant compressive force and chord member)

Pc' = √(PiP)2 + (PiN x CR)2 = √(639.4)2 + (1918.1 x 0.5)2 =

1152.6 lbs.@ 15.3 deg.I kind of get the logic behind this equation but I question where does the Cr (reduction factor) come from? and why a value of 0.5?

I'm also looking at the rather large moment at this joint between the two top chords and wondering how best to factor that in.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

_{R}to address the fact that some of the compressive force may be transferred wood to wood rather than 100% through the plate.## RE: WGC Trusses

_{R}= 0.85 - 0.05(12tanθ -2.0) Sec. 8.3.2.2. I'm going to assume this is an empirical formula developed over years of testing and experience. However, for all non-heel joints this reduction factor is 1.0, which makes sense for an MPC truss but maybe not for a WGC truss, where moments at the joints are going to be larger due to the rigidity of the connection.In the TPI 1-2014 commentary there is some good discussion on combined axial and moment loading scenarios. Equation E C8.7-1 approaches this problem by converting the moment into an equivalent tension force: Teq = 6M/d.

The logic behind this is to start with the bending stress and multiply it with the cross sectional area of the plates: (1) T

_{eq}= f_{b}x 2dtWhere: f

_{b}= M/s and S = 2(td^{2}/6)Substituting into equation (1) yields the 6M/d expression.

The commentary notes that this method has proven to be conservative (which I like) in most situations however they have since supplanted it with a more complicated and accurate approach.

For analysis of the tensile capacity of the actual gusset plates (plywood or OSB), tension check, this addition of an equivalent tensile force seems to make sense to me.

T

_{tot}= T_{axial}+ T_{eq}This is essentially the same as equation 3.9-1 in the NDS and can be written in this form:

The applied stress would then be: T/2td + 6M/2td

^{2}which must be less than Ft' the allowable tensile stress of the plywood.With the analysis of the fasteners (nails) one could probably use a simplified approach like this or a more complicated nail group analysis like I've used in calculating portal frames.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

The force applied to the worst case nail would:

F

_{nail}= M_{joint}S_{x}S_{y}r_{max}/JWhere the Polar Moment of the Nail Group is given by J = bh

^{3}/12 + b^{3}h/12: b = r_{x}+ s_{x}, h = r_{y}+ s_{y}and

s

_{x}= nail spacing in x-dirs

_{y}= nail spacing in y-dirr

_{max}= distance from centroid of rectangle to corner (furthest fastener)This won't be entirely accurate since the fasteners will probably not form a perfect rectangle but it should be reasonably close.

Then one can add this worst case nail force to the average force on the nails from an axial member load (conservative) and come up with the combined load on the worst case fastener.

This becomes an iterative process:

Step 1: Compute the number of fasteners for only the tensile load.

Step 2: Given the number of fasteners and fastener group geometry from Step 1 then calculate the load on the worst case fastener in the group.

Step 3: Add the value in Step 2 to average value from Step 1 and check against the allowable for the fastener. If it exceeds the allowable then add a fastener and repeat the computation of the average fastener load due to tensile only and the load due to the moments, repeat until it passes.

It's no wonder that the truss companies have all of this stuff programmed. Technically this needs to be done for each joint at each load case, since load reversals may turn a compression loaded joint into a tension joint and vice versa, and the moments will be different for each load case.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

Nothing to exciting going on with this bottom chord splice, the splice will be loaded in tension with a moment, the same methods developed previously would apply.

In the case that the OSB or Plywood doesn't have enough strength for the combined axial + moment load then a vertical extension like what is shown could be improvised. This will not only increase the moment capacity of the gusset plate(s) but also the tensile strength. Note, that the number of fasteners into each bottom chord is not modified or decreased by the vertical extension of the gusset plate.

To be conservative I would not consider the 2x4 insert in the calculations of the updated tensile or moment strength of the gusset plate(s) even though it will contribute some unspecified amount. The filler block is primarily to secure the protruding gusset plates and to prevent them from buckling out-of-plane. To that end I don't feel that specifying the number of fasteners is overly critical. A reasonable amount of fasteners and some glue will create a cohesive unit.

In the case of top chord splice with the members being in compression the previous equation would dictate using a force that is 1/2 the compressive force, this would seem reasonably conservative but other load cases that could potentially load the splice in tension should also be investigated and compared. Again, any moments at the splice would also factor into the gusset plate sizing and fastener quantities.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

T

_{tot}= T_{axial}+ T_{eq}I most cases the worst case loaded nail will be positioned so that the moment load will be approximately perpendicular to the axial load so perhaps adding the algebraic sum may be a bit conservative, it might make more sense to add the vector sum, which will always be less than the algebraic sum unless the loads are parallel.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

## RE: WGC Trusses

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, Plymouth Tube

## RE: WGC Trusses

I recommend 10d common double shear with tip clinched.

Nails from each side increases the gusset size required to eliminate splitting of the truss member.

I use a nail density of 5.25sqin/.148"nail

1/4" diameter screws from each side work well for sheetrocked trusses to eliminate nailing vibration.

Reluctant contractors are usually convinced when plywood size savings come into calculation of material costs,

and template patterns are needed to insure that the nails/screws are staggered to prevent splitting of the wood.

## RE: WGC Trusses

I specify 0.131" nails because nobody seems to carry 0.148" nails locally, and I figure that there is a little less chance for splitting. Pre-boring requires hand nailing. I don't think that many contractors know how to use a hammer. Also, gun nailing does not affect drywall. With bottom chord repairs I have often used Simpson SDS screws to cut down on the number of fasteners.

## RE: WGC Trusses

Looking at your diagram you provided I noticed that you required a max. of 2 rows with a 2x4 member. Based on the NDS nail spacing table in the commentary (Table C11.1.6.6) I think I can get 3 rows of 10D nails into a 2x4 as I have shown above. My fastener spacing is much tighter than yours, perhaps too tight. Notice that my row spacing is only one inch between rows, but because I stagger each row from the adjacent row the actual diagonal spacing between fasteners is approx. 1.5"

Anyone have any thoughts on this?

According to the NDS commentary I should be able to make a 3 row staggered formation work on a 2x4 member but then again in practice this may be too much to expect.

Splitting of wood due to closely spaced fasteners is somewhat hard to predict in my opinion.

At this point I am inclined to do some empirical testing, nail up some test splices and test them until failure, one group with 2 rows and one group with 3 rows of 10D fasteners (double shear). My main concern is to see whether or not 3 rows leads to splitting of the main member and markedly weakens the overall connection.

One further note: With this same spacing convention (3/4" from edge, 1" btw rows) I should be able to get 5 rows of 10d fasteners into a 2x6 member.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

Same amount of nails in each test specimen. The tighter nail spacing allows for 66.6% the gusset length versus the 2 row configuration.

Any one want to venture a guess which one is stronger and by how much?

I just need to figure out a test rig and then load these two up and see what happens.

For a DF main member and OSB (7/16) I get 173.7 lbs per fastener (double shear) or 1,389.6 lbs for eight (8) 10d fasteners.

I'm wondering how much the connection will stretch at this design load (loaded in pure tension, parallel to grain of main member) and at what load will it fail, then compare the results from both configurations.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

Using larger gussets plates and increasing the nail spacing would have some benefits:

1.) Decrease the potential for splitting of the main members.

2.) Increase the moment capacity of the joint.

3.) The trusses will be stiffer and deflection will be less.

4.) More margin of safety in the gusset plates themselves since they are larger.

The downside is:

1.) Larger gusset plates means more material -> more cost.

2.) The weight of each truss is increased slightly.

Ultimately, I would like to arrive at the optimized spacing which balances size (safety) vs. cost.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

https://www.amazon.com/Hanging-Klau-Digital-Indust...

On the other side I was thinking of using a come-along or winch. Does anyone have any experience with these type of scales? I'm wondering if a sudden release of the load (rupture) would be hard on the scale. I intend to use the scale for repeated testing so I need a rig that will hold up to some abuse.

Before I get too far into this I'm wondering if any other studies have been done by others to investigate splitting due to fastener spacing, in particular nails in double shear.

A confused student is a good student.

Nathaniel P. Wilkerson, PE

www.medeek.com

## RE: WGC Trusses

## RE: WGC Trusses

There are test studies out there but if you are doing your own, other factors to consider

include: Member force, moisture content, specific gravity / Specie, and

staggeringof nailsTo split a log: Drive your wedges along the same grain...there's a reason for that.