Tributary Area 4

[drewtheengineer]

I just started working in the design of residential housing. I can't seem to get a straight answer here, so perhaps some of you can explain.

Let's say I'm designing to BOCA and I have a roof which bears on three points. To find the reaction on the middle support, could I just take half of the first span and half of the other span to determine the reaction or do I need to perform a continuous beam calculation? Same for a floor with three or more bearing points. Is that a standard practice backed by any code?

 

[Trussdoc]

You should always allow for continuity....when the joist or rafter is continuous. Because the lengths of solid-sawn lumber are generally limited to 20 feet or less, these are usually spliced over the internal bearing point, and function as separate beams. Trusses and Engineered Lumber are available in longer sections, thus are usually continuous.
The difference is usually not great, i.e. 1/8 of the total load in a symmetrical continuous member, but can make a difference in a critically sized support beam or foundation.

 

[Ron]

drewtheengineer...
If I understand your question, you are asking if there are three supports for a roof section (beams, trusses, joists, ..whatever), how do you appropriately apply the load to determine critical load cases (i.e, what is the tributary area for critical conditions?).

I agree with Trussdoc with one additional comment....
The initial loading on the roof SURFACE, transferred to your support, is exactly as you have described it...The center support takes 1/2 the roof area, and each outside support takes 1/4 of the roof area each.

The continuous beam calculation comes into play when the REACTIONS of your initial roof support are then transferred to whatever they might be bearing upon, such as a continuous beam.

Keep in mind the sequence...LOAD-REACTION,LOAD-REACTION...all proceeding downward to the foundation.

 

[100islands]

drewtheengineer,

Your inquiry is: To find the reaction on the middle support, could I just take half of the first span and half of the other span to determine the reaction or do I need to perform a continuous beam calculation?

- No, you don't have to perform a continuous beam calculation if you only need the reactions (especially for beams of equal spans).

 

[drewtheengineer]

The roof is made up of trusses that I do not need to design with three bearing points. They will span the entire house continuously. If I were to apply a uniform load on the entire roof area, the reaction at the middle support would be greater if analyzed as a continuous member as opposed to applying 1/2 the roof area to the center support (note: spans are not equal length).

Trussdoc is saying that I would need to analyze this as a continuous member, producing a higher load at the center support.

100islands says I do not need to analyze this as a continuous member. Is there any documentation on this from say BOCA or other codes?

 

[breaks]

If the beam you are looking at is continuous, then you need to analyze it as such. You should always strive to be as precise as possible when modeling structural elements. A possible exception to this is when a load case is so minimal that minimal design requirements would control even with a greater load case.

I doubt there is any documentation in BOCA on how to model structural members. You should model the members as they will be built, making sure to be conservative in the process.

In other words, use a continuous analysis for your beam.

 

[JAE]

One more opinion to really make your day!

The continuous truss will indeed provide a LITTLE more load to the interior support due to the continuity of the roof truss. But the magnitude is not that great. And, if the truss is wood, over the long term, it will actually begin to shed the sustained dead load back to the exterior supports, thus acting more like a pair of simple spans for dead load, but continuous for live load.

Again....the difference isn't that great so what you can do is design the interior supports for the continuous action (which is conservative) and then design the outer supports for the simple span condition (which is also a bit conservative).

This way, you have properly taken into account the continuity, that you do have, and also provided a proper consideration for the potential for long term deflection effects on the wood (if it is wood).

 

[drewtheengineer]

JAE,

That is exactly what I did/do. Thanks

Drew