Plywood gusset design at Heel of Truss
Plywood gusset design at Heel of Truss
(OP)
I'm looking at the forces on a plywood gusset at the heel of a common fink truss. At first glance this analysis should be pretty simple. Look at the fasteners and look at the shear thru thickness of the plywood. Then I got thinking about it further and realized that the angle of the bottom chord where it meets the top chord creates a scarf cut so theoretically the gusset plate is in tension and in shear. Maybe I am overthinking this one but for now I am slightly baffled on how best to check the strength of the gusset itself.
For a bottom chord splice that is strictly in tension this is a simple tension check.
However, for a top chord splice in compression and bending once again things become more complicated.
Does anyone have any sample calcs for situations which deal with multi-axial loading of plywood gusset plates.
For a bottom chord splice that is strictly in tension this is a simple tension check.
However, for a top chord splice in compression and bending once again things become more complicated.
Does anyone have any sample calcs for situations which deal with multi-axial loading of plywood gusset plates.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com





RE: Plywood gusset design at Heel of Truss
Engineering the members for a plywood (wood) gusset connected (WGC) truss should not be any different from analyzing the wood members for a MPC wood truss. The only real difference I can see is with the plates/gussets.
With metal plates you typically look at three items in the analysis:
1.) Lateral Resistance: How much the plate teeth can hold before tearing loose from the member to which they are attached. This is a function of the of the angle the plate teeth make with the load and the wood grain.
2.) Tension or Shear: This is to check the strength of the steel plate itself to see if it might fail typically along the joint line between the connected members.
3.) Net Section: Checks the failure of the wood in tension where it might tear out at a chunk where the plate directly makes the connection to the member beneath it.
With plywood gusset plates item 3 above would probably go away since the gusset plates will typically be much larger than metal plates hence the net section is the entire section of the member which is already being checked when the member checks are done.
Lateral resistance checks of nails comes straight out of the NDS tables or formulas and should be straight forward enough. The one additional thing to watch for would be the nail spacing requirements to prevent splitting of the wood members, this in fact might control the gusset plate size in certain cases.
Item 2 should be fairly straightforward as well, tension and shear thru thickness values are available which should allow one to check the strength of the plywood along the joint / scarf lines. I am still somewhat wondering the best course of action for the heel joint but I can probably safely use the same methods that I have employed for MPC trusses based on the TPI 1-2007/2014.
The one item that still stands out as unresolved however is how to deal specifically with a splice plate (top chord or bottom chord) that is in tension or compression as well as a moment load. The equation developed for this particular situation is rather complex (ANSI TPI1-2007, Sec. 8.7.1) as can be seen below:
Ma=Cm/2.5[T1(Wp+y+z−d1)/2+T2(4Wp+2y+4z−3d1)/6+Cs(d1−y−z)/2+Cw(d1−y)/2]
Can a similar equation or method of analysis be utilized for a plywood gusset plate that replaces the metal plate?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
Why not analyze your trusses with pins at the splice locations? then you don't care about the moment. It's conservative and likely results in larger members but it certainly makes analysis easier and more reliable.
RE: Plywood gusset design at Heel of Truss
If you analyze the truss as pinned and disregard the moment AT THE NAIL GROUP... I think the nail group and plywood design could be UN-conservative. I'm thinking this is a separate consideration from the truss members themselves.
Seems like there is eccentricity of the axial forces acting on the nail group in the separate pieces, especially at the top chord/bottom chord joint at the truss ends. Very spall eccentricity but very large axial forces.
RE: Plywood gusset design at Heel of Truss
RE: Plywood gusset design at Heel of Truss
http://www.eng-tips.com/viewthread.cfm?qid=374730
First find the critical fastener load due to the moments given the number of nails and the nail grid. Then add this value to the average fastener load from the tension applied to the member, thereby giving the worst case shear load on the fasteners.
For the plywood gusset plate in bending and tension one could apply the NDS eqn. 3.9-1 however Ft' and F*b are both the same value for plywood -> Ft' so the equation would become:
(ft + fb)/Ft'
The applied stress would then be: T/bd + 6M/bd2 which must be less than Ft' the allowable tensile stress.
Notice that my equation is much simpler than the one given by the TPI 1 in a similar situation for metal plates. I think the primary reason is the distribution of the stress due to the bending (moment) is shared between the steel and the wood-on-wood contact of the members. I am making the assumption that the plywood gusset plates are assuming this full moment load and ignoring any contribution or load sharing by the wood-on-wood contact of the bottom chord members.
However in the case of a top chord splice could one ignore the compression load on the gusset plates and only consider the tensile stress on the bottom side since most of the compressive load would be transferred through the contact of the chord members? And wouldn't this tensile load actually be decreased by the applied compressive load?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
For argument sake lets assume a 1.5" truss ply thickness and 1/2" gusset plates each side giving a total thickness of 2.5". An 8D common nail is 2.5", however I would not consider it in double shear in this application. If I were to use a 10D thru 16D common nail in this situation I would have at least 1/2" of nail or more to clinch so in those cases I think I could safely assume clinching was possible and nails are loaded in double shear. Would less than 1/2" of nail protrusion be too small to clinch?
To open up the calculations to as many options as possible I'm considering 8d, 10d, 12d and 16d nails with all the three possible nail types: common, box, sinker.
I also considering 6d and 7d nails but I'm not sure if I will allows those yet.
The plywood or OSB thickness will be: 3/8, 7/16, 15/32, 19/32, 23/32.
Giving this even more thought it would seem that certain gusset thicknesses and nail combinations would not be optimal if the possibility for clinching and double shear is not possible. For instance if I have 23/32" gusset plates on both sides and 1.5" truss ply for a total thickness of 3". If I were to use a common 10D nail or 12D nail I probably could not clinch and therefore double shear is not possible, hence I have to nail the truss from both front and back. Would this not tend to cause the main member to have more tendency to split since there are double the nails in it.
I'm also going to assume that the osb/ plywood is Structural I, this would be my recommendation anyways in an effort to eliminate defects and require a stronger material for the gusset plates. This affects both the shear values of the gusset plates and the lateral loading capacity of the nails.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
I'm not sure these different shear and tensile planes makes sense but it is the best I've come up with so far.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
Also with respect to tension capacities at different angles relative to the strength axis of the OSB/Plywood wouldn't the proper equation to use be the Hankinson formula to make this interpolation? I've noticed the TPI 1 uses the Hankinson in a number of applications but maybe this only applies to wood in compression or bearing and not tension.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
I think that the Hankinson formula only applies to material with a continuous grain orientation throughout the thickness. Sheathing materials may be a different animal because they have layers that alternate with respect to grain orientation. In this application, I'd expect Hankinson to apply to the lumber but linear interpolation to apply to the gussets. Of course, with members considered to carry only axial loads, Hankinson becomes a bit moot.
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.
RE: Plywood gusset design at Heel of Truss
Based on this section it would appear that I cannot use 16D common nails in double shear with 23/32" gussets. I'm also going to assume that I need at least 3D protrusion of the fastener in order to properly clinch it.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
Did you hand calculate the forces you are using at the joints or did you use a program to generate the forces then are designing the gussets for those forces? If you uses a truss design program to generate the forces make sure you look into the fixities of the truss so you make sure you are accounting for if the truss is taking large moments in the heel plates and such.
In regards to double shear make sure you are put a note so that they apply clinching of the nails properly if you are going to utilize the double shear method and the 75% rule he mentions is a good one to follow.
He is saying that the effective width of the gussets should be limited to 2 times the width of the member... so 2x4s... (3.5" dimension x2 = 7) therefore the effective width of the gusset should not generally be wider than that... if that makes any sense to you? If you look at figure 3 he shows you the required gusset width... 12.4", 12", and 8.2" which are greater than the 7" he recommends by the rule of thumb.
RE: Plywood gusset design at Heel of Truss
I can assign rigid, pinned or semi-rigid at any joint. Up until now with MPC trusses I have assigned all web-to-chord joints as pinned, heel joints as rigid and the option to either pin, fix or semi-fix the peak joint. With wood gussets it would seem to make more sense if I were to consider all joints as rigid since the large gusset plates will offer more resistance to rotation than the smaller metal plates, even at the web-to-chord junctions.
I understand what he is saying about the 2 times the width of the member but I would like to know the reason why? Where is the rule of thumb coming from? What is the logic behind it?
Also with regard to clinching and my previous statement of staggering the nails so that they are nailed through both front and back. It probably makes the best sense from an engineering standpoint but from the construction standpoint, you would have to nail the front, then flip the truss over, clinch the ends, nail the back, then flip the truss again and clinch the back nails. Whereas if you just nail the front you only flip once and clinch.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
For example if one were to choose 23/32" gusset plates then the only viable option would be a 16D Box Nail (0.135" x 3.5") or a 20D Box Nail (0.148" x 4"), all other 12D and 16D options would not work because they do not extend at least 3 x Dia. Larger diameter nails do not work since they violate the 6 x Dia. rule (NDS 2012 Sec. 11.1.6.5).
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
Gusset Plate Total Thickness Nails
3/8" = 2.25": 10d Commmon, 10d Box, 10d Sinker
7/16" = 2.375": 10d Commmon, 10d Box, 10d Sinker, 12d Sinker
15/32" = 2.4375": 10d Commmon, 10d Box, 10d Sinker, 12d Sinker
19/32" = 2.6875": 12d Common, 12d Box, 12d Sinker, 16d Box, 16d Sinker
23/32" = 2.9375": 16d Box, 20d Sinker
This would allow for at least 3 x diameter for clinching, and also does not violate NDS 2012 Sec. 11.1.6.5.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
I've got the front end working now so you can look at the input options. It doesn't do anything yet (actually defaults to metal plates if you choose the wood option), but it gives an idea of the gusset options.
With wood gussets the web-chord joints are going to be much stiffer than with metal plates so I will probably set up the matrix analysis to consider all joints as rigid instead of pinned. I'm actually still thinking about that and talking to other engineers with regards to pinned, semi-rigid or rigid joints with this type of construction.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Plywood gusset design at Heel of Truss
RE: Plywood gusset design at Heel of Truss
Thanks!
EIT
www.HowToEngineer.com
RE: Plywood gusset design at Heel of Truss
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com