Questions Of The Week - 18 September - All Trussed Up And Nowhere To Go!

Inspection Education Topics Inspection Question of the Week
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This week’s questions deal with trusses. When answering think only of typical roof trusses for a single family residential structure. That helps keep the thinking and answers a little simpler.

Good luck!!

Q1 – This is a two part question.

What geometric shape is used as the basic concept for trusses?

Why is this shape used as the main concept for building a truss?

Q2 – On a typical truss what is the name of the point where the top and bottom chords meet?

Q3 – What are the four components of a typical truss?

Q4 – Which truss style is comprised of only one web member?

Q5 - When trusses are manufactured at large manufacturing operations what two methods are used to secure the truss plates to the truss members?

Q6 - When a truss is designed to span the entire roof from one side to the other but there is a desire to have room for mechanical equipment, a finished attic area, or other storage type area then what truss design is typically used.

Q7 – The following is a multi-part question.

Permanent bracing/lateral support is required between trusses. What three planes are required to be braced?

What are some of the materials acceptable for bracing materials in each of the planes?

Q8 – The following is a multi-part question.

Where are truss plates installed?

Are truss plates suppose to be installed in a specific orientation to the wood member it is securing?

Can truss plates overhang a wood member?

Is there any allowable tolerance for damage to truss plates and/or bending of truss plates?

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You like keeping things up top :upside_down_face:

I little I knew, but I learned some cool stuff here, hopefully I got these correct. :wink:

Q1 – This is a two part question.

What geometric shape is used as the basic concept for trusses?

Why is this shape used as the main concept for building a truss?

A triangle.

Because they allow forces to spread evenly accross the bride. They distrubut weight efficiently.

Q2 – On a typical truss what is the name of the point where the top and bottom chords meet?

Heel

Q3 – What are the four components of a typical truss?

Top Chord

Metal Gussets

Truss Web

Bottom Chord

Q4 – Which truss style is comprised of only one web member?

King Post/Common Cap or an Inverted

Q5 - When trusses are manufactured at large manufacturing operations what two methods are used to secure the truss plates to the truss members?

Ok so I’m not sure exactly which way you were going with this question so I answered both.

How they are applied

They rolled in or pressed into the wood with a hydrolic press.

The Mechanical method securing it

Steel plates that have teeth in them that are drivin in simultaneously

Plywood gusset plates that are glued and nailed

Q6 - When a truss is designed to span the entire roof from one side to the other but there is a desire to have room for mechanical equipment, a finished attic area, or other storage type area then what truss design is typically used.

Gambrel Attic truss

Q7 – The following is a multi-part question.

Permanent bracing/lateral support is required between trusses. What three planes are required to be braced?

Top Chord Plane

Bottom Chord Plane

Web member Plane

What are some of the materials acceptable for bracing materials in each of the planes?

Steelbrace

2x4 stress graded lumber

T, L, U, and Scab Braces’

Q8 – The following is a multi-part question.

Where are truss plates installed?

Basically everywhere two pieces of the truss intersect.

Are truss plates suppose to be installed in a specific orientation to the wood member it is securing?

Yes, perpendicular to the grain of the wood members.

Can truss plates overhang a wood member?

Yes, however it reduces their ability and should be accounted for. But best practice is no.
Such as this:

plate overhang
plate overhang1200×900 113 KB

Is there any allowable tolerance for damage to truss plates and/or bending of truss plates?

Not that I could find without repair. The repair needs to be evaluated by a licensed structural engineer. Additionally, The Truss Plate Institute (TPI) publication QST-88, Quality Standard For Metal Plate Connected Wood Trusses, says that the gap under a plate should not exceed 10 percent of the tooth length or 1/16,” whichever is greater. It also says that such a gap should not exceed 1/3 of the plate contact area on each member in the joint.

A couple other cool things I learned/didn’t know before starting this.

There is a truss plate institute…

This I thought was really cool, John Calvin Jureit invented the gang nail connector plate and is essentially credited with affordable housing and productiviy inceases because this was such a game changer in the construction industry.

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Good questions Manny, thanks for your time!

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WOW Chase you’re killing me!! :sweat_smile:

I was kind of hoping that these would last a little while but Chase dove right in and knocked them out!! This week’s winner is Chase! :partying_face: :partying_face:

The answers are as follows but Chase has hit the intention of all if not answered them. Chase also demonstrates how a little research can lead to more interesting information.

A1

Trusses are based on the triangle shape.

In building triangular shapes offer great rigidity as each of the three angles is held steadily in place by the side opposite it. If a load is placed on one side of a triangle it does not shift the triangles shape as could happen with a square or rectangular shape (racking). With triangular shapes loads placed on any one side are more evenly distributed to the supporting element below the truss.

A2 – The heel

A3

Top Chord

Bottom Chord

Web members

Truss plates to secure together the components noted above.

A4 – The Kingpost truss design.

A5 – Plates are typically attached with a hydraulic press or a roller press.

A6 - The “Attic” truss design or for Gambrel roof designs the “Gambrel Attic” design.

A7 - The three planes requiring bracing and sample materials for bracing are as follows. You should note that the designer does specify the required bracing and this is part of the original truss design and is typically marked as the truss bracing plan. Obviously those plans will not be available to you. The bracing is also typically consistent in placement. So if you see inconsistent or missing bracing chances are bracing has not been accomplished as specified by the designer.

Top Chord Plane – The roof structural grade sheathing, properly braced purlins, and diagonal “X” bracing
between adjacent truss top chords.

Bottom Chord Plane – Attachment of gypsum board directly to all truss bottom chords, dimensional lumber run perpendicular and across bottom chords, and rows of dimensional lumber run diagonally across bottom chords with these rows spaced no greater than 20’ apart.

WEB Member Plane – dimensional lumber attached perpendicular to web members and running the length of the installed trusses. Additionally sets of trusses will be braced with diagonally positioned dimensional lumber with these diagonal members spaced no more than 20’ apart down the row of trusses.

A8

Truss plates are installed at all points that chords and web members meet. Plates are installed on both sides of the joint.

Orientation of the truss plate does make a difference on the connection strength. However this is part of the design and manufacturing of the truss member and beyond any typical home inspection evaluation.

Truss plates that overhang a wood member are weaker connections at the overhang point. The strength of the truss plate connection is partially calculated based on the area of embedment of the truss plate teeth. As a result the truss designer may have additional blocks inserted at the truss member angle to reinforce the plate and the connection. Whether this is called for or not is obviously not known as the truss design diagrams are not available. What we can look for and question are inconsistent locations on similar trusses where blocks have been added and others where blocks may be missing for unknown reasons.

A damaged and/or badly bent truss plate is an issue and plates should not display any damage. These weaken the plate and the strength of the connection. Damage to a plate might also be a sign of significant movement of the members.

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Congrats Chase Funke!

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Way to go, Chase!

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Wahoo! Thanks Emmanuel. :smile:They were a lot of fun and I did learn a lot. It’s some of that the more you know the more you find you don’t know.
These questions are all really cool and have been a great way to learn more information. I really appreciate y’all taking time to do this.
I’ll step back a bit moving forward.:v:
Thanks again!

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Congrats again, Chase! Smokin!

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Congratulations Chase!
There’s also a good truss publication produced jointly by the Structural Building Components Association (SBCS) and the Truss Plate Institute (TPI) called the Building Component Safety Information that covers handling, installing, restraining, and bracing wood trusses

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Congratulations Chase!

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