Originally Posted by GCO88
Do you attach 2 2x10s on one side, or split 1 2x10 on one side and 1 2x20 on the other?
This is at least the second time this has come up since I joined. The answer is maybe. Let me explain.
First split beam designs maybe banned by code or require signoff from a SE, but more importantly it is possible to under size the beams.
For a normal deck the load is assumed to be equally distributed across the surface for sizing purposes. Using this assumption, the length of the joists supported by a beam are linearly proportional to the width of the beam (2 by, 3 by 4 by, double 2 by, etc)If your deck supports point loads or exordinary loads, such as a hot tub, then additional calculations are required.
The attached drawing shows a simple layout with split beams. The colored boxes represent the area of load for each of the "2 by" beams. If the beam was actually a "double 2 by" then the beam on the right would carry the total
load of the blue and green area, while the beam on the left would carry the total
load of the red and purple area which in this the load from the red area far exceeds half the total load.
What does this mean in the real world? For most decks people use the beam/joist tables using the maximum joist length from the design (green and red in the attachment). As you can this maximum yields a maximum supported load that is greater than or equal to any load any beam would actually see.
On the otherhand if you calculate and size beams individually, then it is possible that some beams may be undersized when split. For example, you size a double 2 by beam to handle the red and purple loads. Then you decide to split the beam. Remeber the length of a joist (distributed load)that a beam will support is lineraly proportional to the beam width, therefore when spliting a beam each 2 by has to support half the load. In this case the red load far exceeds half the combined load.
Hope that helps. I don't like to simply accept "because the code says", there usually is a logical explanation backed by engineering for what is in the code.