Floor joist direction decision?

[Lennyzx11]

Hi all, still steady working on mine. Covid put me at a stop for a couple of weeks but back at it.

I was wondering if there is a “better” way(direction) to run joists.
I have 4 bents king or queen post truss design that will span 24’. 
They will be 12’ apart. 
The two center ones will not have center support posts. Clear span.
The gables have center posts.

So I have the choice of running a tie beam perpendicular to the rafters in the center then running 2nd floor joists parallel to the rafters with tying joists approx 12’ long from wall tie beam to center tie beam.

Or.. 12’ joists perpendicular to the rafters from bent to bent. 

24-32” spacing 5x8 size is the hoist size called for. 

My main concern is which way would put less of a load on those two clear span tie beams 8x12s that are spanning 23’ approximately? These are running parallel to the rafters for clarity.

Or to have the joists ran parallel to the rafters to resist bowing out of the lower walls like conventional framing is expected.

I do plan on using the “tying joist” and rules of thumb that Jim Rogers has posted about in earlier threads and on Timberframe HQ.

Thank you,
Lenny

[Jim_Rogers]

The name of all timbers comes from its location in a frame. It also has something to do with the function of the timber.
A beam that is going from eave wall to eave wall "ties" those walls and is thus called a tie beam.
A beam going in the direction of gable end to gable end is not a tie beam. It is, normally, called a summer beam.

Each timber has to be sized to support the load. Short floor joists that connect to a summer beam can be smaller than a floor joist going from tie beam to tie beam. As they are shorter and carry less of a load.

However, now the summer beam has to be sized correctly to support those joists. And the tie beams that the summer beam connects to has to be sized correctly to support the summer beam, at the center connection. Each timber gets larger as the loads accumulate.

Careful design considerations have to be made for the size and the connection type in these circumstances.

Jim Rogers

[Lennyzx11]

Thank you Jim. I understand now the use of the term summer beam vs tie beam.

This case has 12’ joists going from eave wall>Summer beam>eave wall.
Or 12’ joists going from Tie Beam to Tie Beam. 

So length of joists are the same with same count. 
Some of the load is placed on the wall and Summer Beam one way while the load is placed on the gable wall and Tie Beam the other way.

I wondered if there was a difference in figuring the load on that clear span Tie Beam running from eave to eave. It seems that the weight of the Summer Beam would put more load on the Tie Beam to me but wasn’t sure.

[Den-Den]

Is the "tie beam" also the bottom chord of a truss?  If so, I think the truss will need engineering design.   I believe the distributed load from joists bearing on the truss will be slightly better than the concentrated load of a summer beam.

[Don P]

At a quick glance just some rambling. 

A point load at midspan produces twice the bending moment compared to the same load uniformly distributed along the same beam. You certainly would be backing up to place a sumpter at the middle of a 24' clearspan tie beam as opposed to uniformly distributing the load with joists from tie to tie.

I saw both kingpost and queenpost trusses mentioned. Those posts are not posts at all, they are ropes, tension members, dangling down from the roof to hold your floor beams up. Trusses are actually supposed to be loaded at their joints only, here's why, if the summer is supported by the kingposts there is no bending load on the tie. You have significantly increased the tension in the tie and the shear and compression in the heel joint of the truss. Member forces in a properly designed truss should be along their axis, tension or compression but not bending. In the real world...

[Lennyzx11]

Thank you DonP. 
That explanation helped me. It took some rereading to understand that the forces in a truss should be inline with the truss. I kept thinking about the joists loading the bottom chord causing a loading force and realized I was understanding something different than being explained.

For everyone. This isn’t for a 2nd floor per se. It’s planned to be open but I wanted some rigidity added to combat any “racking”.

I’m not sure of that proper term, Jim. Joists if it were for a floor or ceiling, purlins if for the roof, and girts on a wall. But open air tying bent to bent, or eave to eave may be defined differently. I called them joists.