Timber Frame Truss - Combining Connection Capacities

[b_cec]

Hey everyone,

I am designing a timber frame truss to resist uplift due to wind loading, and this question comes from looking at the Rafter/Ridge Beam/King Post connection. I have two ways to consider load distribution (load is applied from an end reaction on the ridge beam): either the load is distributed through the whole frame like a truss, or the king post resists all of the load through a pegged tenon & mortise connection. (see attachment for connection details/options)

The pegged connection does not have enough capacity to act on its own, so I will probably design as a truss and most of the load will be carried by the wood screws (of which I'll have to specify more than the 2 screws shown).

However, I am wondering if anyone knows of a good way to consider the capacities of both the wood screws (through truss action) AND peg connection (at king post) simultaneously. This way the pegs would be more than just decorative and I could probably get away with just the 2 screws indicated.

Thanks in advance!

Truss layout:

 

[b_cec]

Here is the truss layout, nothing too special

 

[BAretired]

I don't understand the first drawing. Is the ridge beam 8 x 12? If so, how do you install the pegs at midspan?

The second drawing is not a truss layout. It is a truss elevation. Why not carry the uplift on the truss and forget about a ridge beam?

BA

 

[b_cec]

Yes, it is a truss elevation.

And yes, ridge beam is 8" x 12". Drawings given to me show the tenon & mortise oriented the wrong direction, they would be rotated 90 deg. around y-axis in order to work.

The ridge beam spans about 15' between two of these timber frame trusses and picks up roof load from typical sawn lumber rafters, so the ridge beam cannot be removed.

 

[BAretired]

I partially understand, but if the existing roof beam spans 15' between two of these trusses, then those two trusses must also be existing, yet you say you are designing them. Could you provide a layout showing the plan view of the area in question and indicate what is existing and what is proposed?

If you don't need trusses, why not simply use sawn rafters throughout and suspend the ceiling members from the ridge beam?

BA

 

[KootK]

As you've detailed it, the screws would pull the ridge beam apart in cross grain tension under the truss scheme. So that's questionable. Perhaps you could use a strap across the top instead.

In this arrangement, I suspect that the M&T load path will be much stiffer than the truss load path. As such, I would not recommend combining capacities, I worry that your pegs would give out before the truss mechanism really kicked in.

Can you perhaps add another row of pegs above the one proposed?

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.

 

[BAretired]

The two 1" dia. Red Oak dowels are only 1.5" from the bottom of the beam and 1.5" from the top of the tenon. I doubt that these meet minimum end distance requirements and I would not want to rely on those dowels to carry 1734 pounds.

The two screws tend to pull out of the top chord members parallel to the grain and apply tension across the grain of the beam, a questionable connection. The chords should be tied together with a steel strap to prevent them from separating.






BA

 

[jtreybal]

Your best bet is to run the king post continuous to the top of the rafters. House, mortise and tenon the ridge beam into the side of the king post, and then mortise and tenon and house the rafters into the king post as needed to "hang" the king post from the rafters. This avoids tension connections through the use of wood to wood bearing.

 

[BAretired]

If I'm not mistaken, the truss is existing, so adding pegs or modifying the height of king post are not options. If so, the best bet would be to design a connection which can transfer the uplift force (less dead load) from the beam to the truss at Point N2 or Point N4.

BA