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Nailing plywood full length to trusses to reinforce them?
4

Nailing plywood full length to trusses to reinforce them?

Nailing plywood full length to trusses to reinforce them?

(OP)
I have metal plate connected 2x roof and floor trusses that need to be reinforced to carry extra loads.  

In the past we have typically nailed or screwed 5/8" or 3/4" plywood to one or both sides of the trusses to strengthen them, but with no quatatative idea of how much this reinforces them.  

How do I analyze this?  How does the addition of full lenth or partial length plywood effect how the truss behaves?  

Thank you!!
-Matt

RE: Nailing plywood full length to trusses to reinforce them?

Shearflow is dandy for homogeneous materials of constant cross-section (I), but when you throw in some knots, web members changing I, and plywood butt splices in tension, I immediately look for another solution:
Analyze the truss with the higher load. Hopefully few if any members are overstressed and can be individually strengthened. Most trusses are supplied with the majority of their member components not fully stressed.
If the added load is substantial, add an additional ply, either fabricator supplied or site-built.

RE: Nailing plywood full length to trusses to reinforce them?

I have a case where the edge of a wooden roof truss is decayed due to water penetration into the building so that the portion of deteriorated roof truss needs to be cut out somehow and resupport this truss onto the existing 8" CMU wall.  My initial thoughts on repairing this was attaching 5/8" thick plywood on each side of the truss member assuming the decayed portion is cut out and temp. shoring is provided.  Then use Simpson hanger off the wall to support this.  

My question:
Can 2- 5/8" plywoods with filler blocks between them probably provide enough strength to support the loads?  I don't know how to analyze loads acting within a plane, not perpendicular to the plane.  

Please advise, and thanks in advance.

RE: Nailing plywood full length to trusses to reinforce them?

mdmiller:

I would analyze the truss members and check stresses, then look at the plates.  2x scabs will really help with bending.  Maintain the original configuration of the truss.  

Plywood gussets should be added where ever the plates are over stressed, which is likely to be most of them.  Get enough nails on each side of splices or joints to transfer the additional load (at least).  The strength will primaily be controlled by the nails, but also check the plywood.  How?  Good question.  A quick and dirty way would be to assume something like #2 SP.  Note that plywood veneers are oriented 90 deg. at each lamination.  I wouldn't count on any tension out of the veneers that are perp. to the load.  The exterior laminations usally run parallel to the 8' dimension.  I think 3/4" plywood as 3 veneers parallel to the 8', and 2 perp.

JJOO:  

Reconstruct the truss as it was after cutting out the rotten area.  Analyze and get tension and compression forces.  Design to transfer forces through the nails, then check plywood.

RE: Nailing plywood full length to trusses to reinforce them?

The APA (american plywood association) has a specification for the design of plywood.  This provides allowable stresses for bending, axial,shear, etc.  To answer your main question:  "How do I analyze this?"  I would get that spec and do a careful analysis of how much load is in the truss initially, then distribute post-strengthening stresses to the truss and plywood in proportion to their section properties, then re-check each member for the new condition.

But that's a lot of work.  Perhaps non-plywood options would be better (i.e. steel plates and shapes).  I don't like the idea of using plywood like that on trusses.

RE: Nailing plywood full length to trusses to reinforce them?

There is more than one way to skin a cat, that is for sure.  However, I would argue that a wood truss properly designed with plywood gussests attached with ring shank nails may even be better than the pressed on plates anyway.  At any rate, I worked for a wood truss company for about a year and we did hundreds (if not thousands) of repairs and never used steel once.  We only used bolts on a couple of occasions.

RE: Nailing plywood full length to trusses to reinforce them?

Appreciate your inputs, structuralaggie & JAE!  I will certainly look through APA spec. to check plywood.  

I thought about using steel plates too, but oh well my boss didn't like the idea of using steel plates because of the weight issue. I'm looking to cut out at least two feet of wood truss at the end, so even a 1/4" plate will weigh at lease 100 lbs assuming the size of 4'W x 2.25'H.  A 5/8" plywood weighs about 19 lbs.  I guess the owner wants to avoid using a crane or something if possible...

If any other viable options come up in your mind, please let me know.  Thanks very much.

JJOO

RE: Nailing plywood full length to trusses to reinforce them?

2
I agree with Structuralaggie, you can certainly avoid steel in the repair.  Try to keep your connection ductile by using dowel type connections less than or equal to 1/4" to avoid group action factor issues.  You can get great performance out of nails or even SDS or WS high strength screws.  I also suggest using 23/32" OSB rather than plywood.  It has a higher specific gravity than plywood and its orientation is not as critical.  Take for instance an 8d common nail (.131" X 2.5"), 23/32" OSB side member, 1.5" SP#2 main member:  Nail capacity = 91# (Z Mode IIIs governs)*1.15 duration factor = 105#/Nail Versus 87#/nail (adjusted) for Plywood with appropriate strength orientation.  Be careful when specifying SDS and WS screws. Their strength capacity is high but can be spaced no closer than 4" o.c.

woodengineer

RE: Nailing plywood full length to trusses to reinforce them?

Some truss design software (Truswal) can actually replace the steel plates with plywood gussets and give you a design drawing. It will size the gussets and specify the required number of nails. Any good truss manufacturer should be able to help, or contact Truswal directly.

RE: Nailing plywood full length to trusses to reinforce them?

Thank you to both of you for your replies.

I have two questions!  

Let's say I would use 3/4" OSB gusset plates on each side of the existing 2x roof truss, and I'd like to use nails that provides 105 lbs for example (8d common nail).  Then, this 8d common nail (2.5" long) will go through the main member (1.5" thick) and penetrate into the other side member.  In this case (lateral loading connection), do I consider single shear or double shear?

2nd question: this is a challenging question.
These 3/4" OSB gusset plates will have to hold the existing wood members together when the decayed portion of each member is cut out.  Even if I model these gussets as pin-connected, I would still find the moment that would be developed at the end-bearing joint due to the weird orientation of the existing truss members.

Allow me to briefly describe the actual dimensions.
This truss spanning 65 ft consists of a non-sloping top chord (10.83 ft long) at the mid-span and a sloping top chord at each side (sloping down to the end).  Bottom chord is 5.792 ft below the non-sloping top chord, and there is a vertical member (1.625 ft long), placed within the existing 8" CMU load-bearing wall, that is between the sloping top chord and the bottom chord.  The first interior web member is from the panel point where the top chord and the vertical member are connected, sloping down to the bottom chord.  The distance from the end bearing to the first interior panel point at the bottom chord where the first interior web member is attached, is 9.08 ft.  And the length of the first interior web member is approx. 111 in.

I say this is a weird orientation because the end connection is not concentric, and under the design loading (DL 10 psf on T & B chords, LLsnow 30 psf on top chord), the bottom chord at the end would experience a great deal of compression with the first interior web member of huge~~ tension.

How would you fasten these gusset plates to this existing roof truss?  It is a big puzzle to me.  I must have a way to compute the moment due to this eccentric connection at the end, and this tension force in the web member is so huge that the number of nails required seems ridiculous to me.  Any wood experts, please help!!!

RE: Nailing plywood full length to trusses to reinforce them?

You should use nails that won't go through the truss.  The gussets would be installed one at time, so you inevitably would work one side loose by installing the other.  Double shear looks good on paper but not practical due to constructability.


I don't quite follow what you are doing exactly, but counting on the gussets to transfer significant moment is a bad idea.  Plan A should be to reconstruct the truss with the 2x's with pin connections (unless continuous through joint) and use the plywood gussets to transfer the axial forces.  If you are getting too high of loads and not enough wood to put enough nails in, then you need to go to plan B and figure out another way, which may mean get a new truss or use an interior bearing point.

Also, I know they say OSB is supposed to be interchangeable with plywood, and in this case theoretically would be better than plywood, but I just don't buy it.  That is my own opinion, I think the APA might side with woodengineer.
 

RE: Nailing plywood full length to trusses to reinforce them?

I agree with Structuralaggie,

There are specific requirements for you to use the nail in double shear.  There are clinching requirements which the contractor will never adhere to.  Rely on single shear only.

I know a lot of people who don't buy the OSB better than plywood bit.  I am repeating the testing data on OSB, but understand your point.

Sincerely,

woodengineer

RE: Nailing plywood full length to trusses to reinforce them?

I am not sure if this helps, but the Truswal engineers use single shear, and have 1 square inch of plywood for each nail required. They specify 1/2" OSB on most field applications. The Simpson SDS Screws also come in handle if your plywood gets too big. A 48x48 inch gusset is not uncommon.

RE: Nailing plywood full length to trusses to reinforce them?

The biggest problem with SDS or WS high strength screws is that they no longer have double shear values (they used to.) So Assuming you get about 240#/screw and they can be spaced at 4" o.c Staggered between sides (you can only get 1 staggered row in.)

Example:
23/32" OSB on each side of 2x4 SYP#2 web:

You could resolve 1000# using 5 screws at 4" o.c. ; say 20" (per side of break or splice)not including end distances.  Now looking at nailing (8d common) 105#/nail, say 10 nails total.  OSB scabs are each attached in one staggered row at 2" o.c. with 2" end distances you are required 12" of scab length (per side of break or splice)

The nailed connection is far more economical and yields a smaller overall gusset.  On of the other advantages is that with a smaller gusset, you are not affecting the stiffness matrix of the original truss as much.

woodengineer

RE: Nailing plywood full length to trusses to reinforce them?

To structuralaggie,
"counting on the gussets to transfer significant moment is a bad idea."  Your comment makes me hesitant to use big gussets (4' x 2.25') attached to each side of the truss because these gussets would hold all three members together (top & bottom chords and web) and the existing truss has an eccentric connection at the bearing.  To use the plywood/OSB gussets to transfer the axial forces only, the only thing I can think of is to have three separate gussets for all three members.  

To Trussme68 & woodengineer,
Considered using screws or nails, but in my case it just requires too many of them.  For instance, 6.4 kips of compression force needs to be transferred thru nails/screws in top chord.  I would need 61 nails with 105 lbs capacity, and it doesn't get much better if I use screws.  I am thinking, I gotta use thru bolts.  If I use 3/8" dia. thru bolts & 1" OSB/Plywood Str 1 Gr. gussets, AWC Connection Calculator gives me Z=702 lbs (double shear).  I would need 10 bolts.  What do you guys think?

Using steel straps instead of plywood/OSB gussets is an alternative.  1/4" steel gussets and 3/8" thru bolts (double shear) gives me 995 lbs, hence I would need 8 bolts for the top chord connection.  This 1/4" x 5" x 4'-6" LG strap would weigh about 20 lbs, so the weight doesn't seem to be an issue either.

Want to hear your thoughts.  Always appreciate your inputs, guys.  Thanks very much.

Best,
JJOO

RE: Nailing plywood full length to trusses to reinforce them?

JJOO - you should have enough nails to transfer the axial load on each side of each joint and they theoretically should align along centerlines.  There is no reason to cut the plywood into smaller pieces provided that you have enough nails in the general location of the connections.  Group them so that the centerlines of the member align with the center line of the nail group.  The continuity you have in the un-cut gusset will only help, it certainly won’t hurt.

61 nails is a lot, but that is only 32 per side.  If your gussest is big enough it is feasiable.  Is there anyway you can reduce the forces, like interior bearing points?  If you are thinking about a steel connection, I am thinking there might be a better option.

RE: Nailing plywood full length to trusses to reinforce them?

structuralaggie - This building is a 3 story condominum.  The 3rd floor ceiling is directly attached to the bottom chord, so I don't see any way to have a interior bearing point.  Maybe I can introduce new panel points by removing the first interior web and installing new web members because the existing web is quite long, 111".

Very curious about "a better option" if I use a steel connection.  Do you have anything on your mind other than having steel straps and thru bolts?

RE: Nailing plywood full length to trusses to reinforce them?

The better option I have in mind is to figure out how to reduce the load to make the nailing pattern more appealing.  When you start drilling holes in a 2x truss, you start taking out part of the section.  Nails don't do this, provided they are spaced far enough apart.  Are we talking about 2x6 chords?  I think drilling a 2x4 would be out of the question.

I don't know what your floor plan is, but if you have interior partition walls, these could be used to carry some load.  Of course, you then have to track were the load goes all the way to the foundation.  Also, these buildings typically have walls parallel to the truss span, which often times are shear walls.  Shear walls will have some foundation under them, so if you could span a beam between these walls you could use the beam as a bearing point.  You might have to reconstruct the trusses entirely with this option.

I guess I have taken one little piece of information and ran with it.  The point I am trying to make is that reinforcing the trusses may not be the best or most economical solution.  It may be that you need to take a stick framing approach to take the additional load, which would involve added beams, rafters, and ceiling joists.  There are a lot of options.


RE: Nailing plywood full length to trusses to reinforce them?

The existing truss consists of 2x6 SYP top chords, 2x4 SYP bottom chords and web members.  These roof trusses span 65 feet bearing on the exterior walls, and the interior fire walls run parallel to the roof trusses.  I thought about having a steel beam between these fire walls too, but.. these walls are +/- 60 feet apart.

I was actually considering drilling the 2x4 members to have 3/8" dia. thru bolt connection.  I have 5000 lbs tension in 2x4 SYP SS web member and 3800 lbs compression in 2x4 SYP No.3 bottom chord member.  TC is 2x6 SYP No.2 with 6400 lbs comp.  Just throwing out the design conditions.

Funny thing about this project is that the shop drawings for construction indicate contradictory information!  The original design calls different truss orientation and it has more panel points.  Based on my analysis, I see over-stress in some members under design loading. ^^;  All kinds of problems for this building already.  This project is in litigation as the trial starts next week I heard.  Anyways, this is why I'm trying to avoid using bolts because I'll be losing the critical section area and some member is already over-stressed.  But the nailing got an issue of #s required.  whew..

RE: Nailing plywood full length to trusses to reinforce them?

Well, just remember that you’re the engineer of record, not me.  I don't want to get sued too.  For all you know, I could be some lunatic.

If your members are already over stressed, definitely don't drill holes in them.  A SS web and a no. 3 bc?  Maybe thats backwards?

Surly you have some interior partition walls then.  You don't have a 65 foot open room, do you?  The best plan would be to redistribute the load somehow to take some load off those trusses, and aim it at a wall that has foundation under it on the first floor.


RE: Nailing plywood full length to trusses to reinforce them?

Good that I'm not the engineer of record, either!!  All designs were done by others.  The company I'm working for is rather providing a forensic engineering service--you know, investigate and provide a report with findings, conclusions and recommendations.

Based on my analysis, the bottom chord is overstressed.  The original design calls 2x6 SYP SS, but when I went in the attic I saw 2x4 SYP No.3.  BIG DIFFERENCE in terms of capacity.  Hard to believe but it happened already.  

The first interior web and the first interior bottom chord member experience big axial stresses according to Staad.Pro.  All other web and bottom chord members got relatively low stresses.  I am guessing this is due to this stupid orientation of the truss web members, which is different from the original design.  What's happening in the existing condition is that the first interior web is holding the bottom chord from falling down and so the first interior bottom chord member has big compression force rather than tension.  Web members are all 2x4 SYP No.3 except that the first interior web is 2x4 SYP SS.

I think I do have dry walls between the fire walls.  I'll look into the possibility to have an interior bearing point.  Thanks a lot for your advice, structuralaggie.

RE: Nailing plywood full length to trusses to reinforce them?

If you are clear spanning 65 feet, the biggest axial load in the bottom chord will be at mid span.  Are you saying your highest bottom chord force is near the support? Generally, the chords have the least amout of load at the supports, and the most at mid span.  The webs are the opposite.

If the bottom chord was supposed to be a SS 2x6 and was instead installed as a #3 2x4, it probably has failed in tension.  Check the whole bottom chord, starting at mid span.  It probably failed and now the truss is sitting on a partition wall.  

RE: Nailing plywood full length to trusses to reinforce them?

JJOO,

The situation may be even worse.  When you modeled this in Staad, did you iterate through all of the load combinations including unbalanced snow?

woodengineer

RE: Nailing plywood full length to trusses to reinforce them?

woodengineer:
Yes, I think I did.  I have a copy of roof truss design sheets that must've been used in shop fabrication, and it shows 30 psf top chord live load, 10 psf top chord dead load, and 10 psf bottom chord dead load.  I've considered those loadings and snow drift against parapet additionally.

structuralaggie:
The exactly same thing lingered in my mind when I read the staad output.  Why tension in the bottom chord? and such a big force near the support?  

Let me try to draw the existing truss orientation here:
         
                                   *
                           /     /
                   /           /
            /                /
    *                      /  
    /         /          /
    /                  /  
    *     /     /     *     /     /

  It is ugly, but don't know how to paste a copy here.
  Showing the left end of the truss, (* indicates joints)
  the first interior web is sloping down to the panel point
  at the bottom chord.  The shop drawing indicates
  otherwise; it is sloping up to the panel point at TC.

My guess is that if the truss were built based on the shop drawing, the bottom chord will deflect as nice and simple pin support parabola and therefore max. tension at midspan.  But the actual condition allows the first interior web to prevent the bottom chord from falling down at the panel point at the bottom chord, hence this bottom chord is rather hanging by the web member.  This way, the portion of bottom chord from the support to the panel point would bend upward.  Such a huge tension in the web causes big compression in the bottom chord near the support so that its magnitude is actually bigger than the tension at the midspan.  That is how I interpret the Staad output.

Please correct me if I'm wrong.  You guys are the wood experts!  This is my first time dealing with wood truss, so I'd like to learn.

RE: Nailing plywood full length to trusses to reinforce them?

I should've said "why compression in the bottom chord?"

my mistake

JJOO

RE: Nailing plywood full length to trusses to reinforce them?

If your boundary conditions have a support that is not at a panel point, then you would be relying on the bottom chord to take bending stresses.  With a 2x4 or 2x6 with the spans you are talking about, this situation will never work.  Where is your support?

If your suppport is at the end panel point, I am still suspicious of the results you are getting, but the computer doesn't lie, provided that you have modeled it correctly.  The configuration you have is different than what I had pictured, but then I still don’t know exactly what you are dealing with.  At this point I would have to model it myself before I could give you any better information.

RE: Nailing plywood full length to trusses to reinforce them?

Here let me provide the STAAD input:

UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 9.083 0 0; 3 18.083 0 0; 4 27.083 0 0; 5 37.917 0 0; 6 46.917 0 0;
7 55.917 0 0; 8 65 0 0; 9 0 1.625 0; 10 10.833 3.292 0; 11 19.833 4.676 0;
12 27.083 5.792 0; 13 32.5 5.792 0; 14 37.917 5.792 0; 15 45.167 4.676 0;
16 54.167 3.292 0; 17 65 1.625 0;
MEMBER INCIDENCES
1 1 2; 2 2 3; 3 3 4; 4 4 5; 5 5 6; 6 6 7; 7 7 8; 8 1 9; 9 8 17; 10 9 10;
11 10 11; 12 11 12; 13 12 13; 14 13 14; 15 14 15; 16 15 16; 17 16 17; 18 9 2;
19 2 10; 20 10 3; 21 3 11; 22 11 4; 23 4 12; 24 4 13; 25 13 5; 26 5 14;
27 5 15; 28 15 6; 29 6 16; 30 16 7; 31 7 17;

RE: Nailing plywood full length to trusses to reinforce them?

Hopefully, it isn't inappropriate to anyone  for me to throw the STAAD input here.  Here is the rest:

*TOP CHORD: 2X6 SYP NO.2
UNIT INCHES KIP
DEFINE MATERIAL START
ISOTROPIC 26SYPNO2
E 1600
DENSITY 2.1065e-005
POISSON 0.2
*BOTTOM CHORD AND WEB: 2X4 SYP NO.3
ISOTROPIC 24SYPNO3
E 1400
DENSITY 2.1065e-005
POISSON 0.2
*FIRST INTERIOR WEB: 2X4 SYP SELECT STRUCTURAL
ISOTROPIC 24SYPSS
E 1800
DENSITY 2.1065e-005
POISSON 0.2
END DEFINE MATERIAL
MEMBER TRUSS
18 TO 31
MEMBER PROPERTY AMERICAN
10 TO 17 PRIS AX 8.25 IY 1.55 IZ 20.8 YD 5.5 ZD 1.5
1 TO 9 PRIS AX 5.25 IY 1 IZ 5.4 YD 3.5 ZD 1.5
18 TO 31 PRIS AX 5.25 IY 1 IZ 5.4 YD 3.5 ZD 1.5
CONSTANTS
MATERIAL 24SYPNO3 MEMB 1 TO 9 19 TO 30
MATERIAL 26SYPNO2 MEMB 10 TO 17
MATERIAL 24SYPSS MEMB 18 31
SUPPORTS
1 8 PINNED
UNIT FEET KIP
LOAD 1
MEMBER LOAD
*TOP CHORD: DL 10 PSF LL 30 PSF
*BOTTOM CHORD: DD 10 PSF
*ROOF TRUSSES @16" O.C.
1 TO 7 UNI GY -0.013
10 TO 17 UNI GY -0.053
*ADDITIONAL SNOW LIVE LOAD ON TOP CHORD
*Pd=33.8 PSF LEFT END
*Pd=0 PSF @W=6.8'
MEMBER LOAD
10 TRAP GY -0.0451 0 0 6.9
17 TRAP GY 0 -0.0451 4.056 10.9605
SELFWEIGHT Y -1
PERFORM ANALYSIS
LOAD LIST 1

RE: Nailing plywood full length to trusses to reinforce them?

I'm not sure what STAAD even is, but it doesn't look too sophisticated.  It looks like old fortan code.  I use RISA 3d.  Everything is windows based and easy to use.

I really can't model if for you anyway.  If you are unsure of your model, you should get some one to help you.  Also, look at a deflected shape as a quick check to make sure everything is going in the right direction.  Statics by hand works too as a quick check to make sure its giving you axial loads that are close.

Good luck.

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