Truss bottom cords with large knots

[fisch]

Two trusses in my garage roof have a large knot equal to half of the cross sectional area in the bottom cord. Two years ago I built a garage. I used engineered wood trusses. It has 4/12 slope, 30' span, 4 ft spacing, with 2x4 bottom cord. Should I assume a 2x2 bottom cord would have done the job, or would it be a good idea to cover the area with a splice plate? This is Pennsylvania weather.

 

[andrewc]

hi fisch

i'm in the uk, so i'm not too familiar with USA practice...

having read a number of posts on various issues i'm getting the idea that, like in this country, most of your timber trusses are subcontractor designed and factory fabricated.

that said, and if yours falls into the above catagory, then it is likely that they would have selected appropriate grades of timber to use and in their consideration this timber meets with that specification.

that said... the knot does appear larger that may be ideal, based on your description..... but it may still be nothing to worry about.

i wasn't quite sure... was this the garage you built two years ago? whether it is or not... is it still covered by the manufactuers warranty? you could ask them to come and take a look. if not, then you may like to ask them or another manufactuer to comment... or even go to your local structural engineer who may be able to come take a look and comment having actually observed the knot.

hope that helps

andrew

 

[rlflower]

Truss manufacturing is a new trend, but it is based upon the tried and true methods of what the UBC building code acknowledges as "conventional construction." One aspect of conventional construction is worth mention here. Conventional construction relys upon repetition of structural members to ensure overall strength of the structure. This repetition is considered effective in the installation of joists, rafters, trusses, or any series of members spaced not more than 24 inches on center. The basic idea of repetition is to ensure that the neighboring members carry the stress of the weaker member.

If you were to take a very careful analysis of all the trusses in your garage roof, you would find several adnormalities due to manufacturing, installation, and water infiltration as well as knots, splits, chips and the like. In a general sense, the building code accounts for all these conditions. But specifically, situations do occur - such as you have mentioned - that are often the cause of concern for many people. That being said, roof truss construction is the strongest method of constructing a roof, and is very suitable for even the worst wheather conditions. Such systems are fabricated well above the required capacity.

Understand that a "cord" is typically in tension (i. e., pulled rather than compressed), which makes a solution quite easy to come by. A simple gage-metal strap, available at your local hardware store, can remedy the situation. But realise that - because of all I have mentioned above - there is no real cause for concern and I do not anticipate that anything of major consequense will occur even if you do nothing about it. Perhaps, as a homeowner, you may decide to apply the strap just for peace of mind.

 

[boo1]

Consider adding a 2x4 doubler reinforcement nailed to one side as a repair or the truss manufacture could be contact with a repair scheme.

 

[fisch]

Thanks for all the input. I'll probably put a strap over the area or maybe a doubler. Thanks again.

 

[RG88]

If you retained the truss mnfrs details, it may have the calculated axial loads in a table. From this you could ascertain the 'tension' load in the bottom chord. ANSI TPI specifications for trusses allows knots and other abnomalities, provided that the resulting area is sufficient to carry the applied load within allowable stresses. If you are ambitious you could calculate the tension yourself using the 'method of sections'.

Being a 4:12 pitch, the bottom chord is not proably designed to handle bending stresses from storage loads (typical in most building codes).

As the others mentioned, you could double it up or strap it for peace of mind.

I have seen a lot of perfectly sound trusses with knots, notches, and holes....it all depends on the axial load of the individual truss members.

 

[DTGT2002]

When lumber is graded, abnormalities such as knots affect the grade of the lumber. Additionally, design codes such as those listed above, can account for knots. The question could be whether of not the knotty lumber was allowable in the design. As an engineered system, the truss should not be a problem.

Place a strap over it and sleep better at night. For the engineer in you, contact the truss manufacturer and ask for their specs, and what size and type (direction etc) of knots are allowed inn their product.