This question may be properly addressed to Mr. Levin, but I'm sure others can answer as well.
I have been asked to review a couple of timber frames by a joinery company. In regards to a previous semi-frame related job, I purchased the "Timber Frame Joinery and Design Workbook" (Red Book), and "Timber Frame and Joinery Design Volume 2" (Green Book). In the Red Book, Mr. Levin presents an analysis of a model house frame using an FEA model and designs a brace for a 3,500 lb compressive load. His approach makes perfect sense to me and I agree with the logic.
The frame I am considering for discussion consists of three bents with 10'-0" x 14'-0" bays on a 42'-0"L x 20'-0"W 1-1/2 story house and contains a bearing Ridge. The NYS ground snow loads for this area are 85 PSF, resulting in an approximately 59 PSF projected roof snow load and approximately a 12 PSF dead load. The walls and roof are sheathed using SIP's. Combined with the second floor diaphragm, wind loads and tension in the braces are not a concern. The ridge columns are 8x10 D-F-L No. 1 and the ridge beams are 8x12 D-F-L No. 1. The knee braces are 4x6 D-F-L- No. 1 and have 1-3/4"W x 3-1/2"L tenons with single 1" diameter white oak pegs, and not housed.
My FEA model is producing compressive axial forces in the braces anywhere from 8,000 lb for single braces up to 14,000 lb for pair braces. A cursory check of load and span ratios shows the 8,000 lb load on a single brace is much in line with the result Mr. Levin got for his brace. However, the larger compressive loads appear to be the action of adjacent braces transferring gravity induced compressive forces across the column to the opposite brace. Checks of these braces as designed shows they are only adequate for approximately 5,000 lb if I provide a 1/2" housing and use one half the tenon area as suggested by Mr. Levin. At the larger loads, this brace will not work no matter the housing, and braces with larger horizontal bearing area must be employed.
What disturbs me is that the joinery company informs me that in some 400 designs, an engineer has never requested a housing on this size brace of over one inch. This raises the question of why I am seeing these loads and how is it others apparently don't or ignore them? I can take a simple beam and put two braces on it and support the ends of the beam in the Y-direction, and the brace bottoms in the X & Y- directions and the results are much the same. All supports and connections are modeled as simple.
Can anyone shed some light on what I should be expecting here? Is the common theory that the beam bearing will take the load prior to the brace failing? Any help would be greatly appreciated.
BWW
