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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

RE: WGC Trusses

(OP)
Same truss but a raised heel variant with the slightly steeper roof pitch.



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

" I am curious if anyone has experience using smaller nails but in larger quantities. "

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 have designed and performed repairs using 0.131"x3-1/2" nails in double shear and cinched. I am concerned about splitting, especially with repairs to dry Southern Pine lumber.

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

(OP)
I prefer the nails in double shear as well since in theory you are putting less fasteners into the member and less chance for splitting.

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

(OP)
Thank-you for posting that link. I've never seen that drawing before. Interesting how they are using 4d nails on those trusses.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: WGC Trusses

(OP)
This old post still has a lot of interesting tidbits. Two years later and I am still torn between clinching and smaller fasteners. Clinching is more work but the fact that the fasteners won't back out over time due to cyclic loading (wind) allows me to sleep at night.

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

(OP)
I may have mentioned this previously but I just want to re-mention it because I want to find out if anyone else has run across this similar issue with the NDS.

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

as i understand 12.1.6.5, your interpretation is correct, 6D needed for side member receiving point


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

(OP)
The nails are intended to pass clear through both side members and then clinched. So there is no nail point to speak of in this configuration. That is why I think I may be interpreting this incorrectly.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: WGC Trusses

agreed,
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

(OP)
Yes, but the exception only applies if the nail diameter is less than .148" so my common 16d at .162" is too large.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: WGC Trusses

I agree that the prescriptive exception limits you to 10d....

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

RE: WGC Trusses

I typically do not specify nails larger than 0.131" diameter. At least in my area (Southeast PA region) larger nails are not commonly stocked or used in nail guns. Also, with the SYP used in many trusses I am concerned about splitting. In this case I think the tradeoff of less nails the 8d in single shear, but more that 0.148" or 0.162" works best.

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

(OP)
I would stay away from SYP with nailed gusset plates, like you suggest there is a splitting problem with this wood type/species. I would lean towards DF No. 2 and better, a much tougher wood.

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

(OP)
The analysis of a WGC truss is also slightly different. With metal plates I usually assume that the heel and peak plates are rigid joints but with a WGC truss all of the joints should be considered rigid or semi-rigid in the matrix analysis.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: WGC Trusses

(OP)
Let's assume I go with an 8d common nail 0.131 x 2.5, even though I am in single shear I'll also project at least a 1/4" into the gusset on the opposite side of the truss. Lets assume a 4:12 pitch truss as shown below:



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

(OP)
What you end up with is something like this:



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

(OP)
Does anyone know of an online calculator that will check for nails in double shear? The AWC connection calculator will only do single shear. I'm working up a spreadsheet for double shear and it would be nice to have something to check my work against.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: WGC Trusses

(OP)
Nailing from both sides is not reasonable in my opinion, there is no way to line up the nails easily from one side to the other, better to just nail from one side which bring me back to nails in double shear.

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

(OP)
If you compare the capacity of a single shear and double shear connection in most cases you get double the capacity, see results for a 10D nail into DF and Structural 1 Plywood below:



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

(OP)
If we consider the peak joint as rigid (web joints are pin jointed), one can see that the moment at the peak is fairly substantial (145 ft-lbs):



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(PP2 + (PN x Cr)2 (Sec. 8.3.3.3)

where:

Pc' = Resultant compressive force used for determination of minimum required metal plate contact area (lbs)
CR = Reduction factor for cmopression force component across the joint interface for metal connector plate design: 0≤CR≤1.0 (lbs)
PiN = Compression force component of the wood member under investigation normal to the wood member interface (lbs)
PiP = 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

I understand the CR 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

(OP)
To allow for these same moment effects at the heel of a truss the TPI stipulates a heel reduction factor HR = 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) Teq = fb x 2dt

Where: fb = M/s and S = 2(td2/6)

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.

Ttot = Taxial + Teq

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/2td2 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

(OP)
With the nail group calculation knowing the number of required nails, row spacing, nail spacing in a row one can come up with a rectangular approximation of the area filled with this group of nails.

The force applied to the worst case nail would:

Fnail = MjointSxSyrmax/J

Where the Polar Moment of the Nail Group is given by J = bh3/12 + b3h/12: b = rx + sx, h = ry + sy

and

sx = nail spacing in x-dir
sy = nail spacing in y-dir
rmax = 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

(OP)
I next want to turn my attention to mid-panel splices:



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

(OP)
The member calcs for a WGC truss will be very similar to an mpc truss except that the matrix analysis model will have rigid joints instead of pin joints. This will introduce some minor bending into the internal webs but nothing significant since these member are not being loaded along their spans. The deflection of the truss should probably improve slightly with its higher rigidity.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: WGC Trusses

(OP)
I've been thinking about this equation this morning:

Ttot = Taxial + Teq

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

(OP)
I was checking my nail calculator spreadsheet against the AWC connection calculator and noticed a minor discrepancy. After double checking all my calcs I determined that the AWC calculator is not taking into account the tapered tip assumptions. I then emailed their technical support and the promptly responded that their calculator is based on the 2005 NDS and does not take the tapered tip assumption into account.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: WGC Trusses

When I am dealing with gusset plates it is for repairs to existing trusses. Many are SYP. With OSB gussets you can use a S.G. of 0.5. With the plywood available around here the S.G. is 0.42 (from memory). So you get better nail strengths with OSB. OSB can tolerate some water, so unless there is an ongoing leak it should not be a problem. It can dry fast in a ventilated attic, unlike when used as wall sheathing behind stucco.

RE: WGC Trusses

Years ago I worked as a draftsman for a structural engineer who made a living from truss design (it was his PhD thesis subject). I recall witnessing some tests using both plywood gussets and steel pref plates. I don't recall the specific nail sizes, but we nailed from one side and then turned it over and nailed from the other also. The joint strengths went up with more nails, not linear but still increasing. In the end we had plywood gussets with the nail heads overlapping each other and it was stronger than when they were just touching.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: WGC Trusses

Having specified gusset repairs for 30 years,
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 have not had much luck with templates or getting contractors to follow any drawing details or notes.
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

(OP)
@CTruax

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

(OP)
What I would like to compare is the following:



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

(OP)
If a moment is applied to the connection it is probably clear to everyone that the 2 row configuration is superior.

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

(OP)
Not yet sure how I'm going to create the test rig but I was looking at a crane scale to accurately measure the tensile force:

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

I have performed some relatively crude load testing using a hydraulic jack and a hydraulic load cell. I made a couple load cells using short through hydraulic cylinders that are used with external pumps. A put pressure gauge in the hoe port. I seemed to get reasonably good results with them. I wanted to calibrate them at a University testing lab, but have not had the chance.

RE: WGC Trusses

Unfortunately I am unable to find my best picture to share with you the splitting of a 2x4 truss web member with 3 rows of nails.
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 staggering of nails

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

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