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Of the books on the market for Timber Frame construction that I have seen only one slightly impressed me. “Timber Framing for the Rest of Us” by Rob Roy. As a matter of fact all of his books have impressed me so far. Most of the books on the market are picture books that tell you about Timber Frame homes. They are mostly to get you interested in having one built for you. Rob’s book goes into some good detail about the simpler frame construction and design. He also uses homes he has built as examples. He tells you how you can mill your own beams and other interesting details. He gives you some very basic information in regards to the engineering of beam size for floors or roofs. None of the books go into how you design or calculate the strengths in a frame.
Most homes in the USA are Stick Frame, meaning 2 inch by so many inches sized lumber in construction. Timber frame means structural frame made from beams or post that are larger than 5×5 inches in dimensions. You probably think of Amish or maybe Mormon made homes when you think of timber frame. Why are most homes not timber frame? Large trees are difficult to come by in many areas, therefore beams or logs must be imported. Not a lot of builders specialize in timber frame and are therefore not extremely skilled or efficient at this type of construction. Also if you do not have some nice blueprints or design specs for a frame you want to build, then you would have to possibly hire an Architect or a Structural Engineer to design the frame. And if you want old fashioned joinery with wooden pegs an Architect or Engineer may not even be able to help because they only have calculations which deal in metal bolts, pins and plates for connections. When constructed under code restrictions beams must be graded, so that means you must either buy them as pre-made, pre-graded beams or if you mill them then you must find someone to grade them.
In the past history of this kind of construction most frame designs and construction details and joinery were handed down generation to generation. Copied construction designs and use of the same species of wood generation to generation meant being assured that the structure was sound, as it had already past the test of time. Today if you are designing the frame from scratch then you are probably a structural engineer. You would have a text book to guide you in calculations which would determine the strength of any kind of design you choose. I’ll list some sources of information relating to this.
- National Design Specifications
- International Building Code
- Timber Construction Manual
- Uniform Building Code (Western USA)
- Standard Building Code (South Eastern USA)
- National Building Code (North Eastern USA)
- Design of Wood Structures (College Text Book)
- Lumber and Forestry Associations also have information.
- Lumber Producers have information.
Now did that just scare the living poo out of you? Yes wood is not a simple material to work with at all on the scale of timber frame, post and beam, beam and stringer. Its not even simple in furniture. You certainly want to make sure your design is safe for occupancy. Even engineers make mistakes right? When human lives are at stake the design details are all the more important. This is why architects and engineers are paid the big bucks. However I must say that after reading the text book, “Design of Wood Structures”, I have begun to really get the jest of what is going on when they calculate the strengths of a given design. Though I have not worked the problems myself. Merely reading that book has greatly broadened my overall understand and comprehension of the nature of wood and working with wood.
So how do they determine the strengths of a piece of wood? In a lab they gather 100 high quality identical samples 2x2x30 inches of a given species of wood for testing. They stress each piece with equipment which show the pounds of pressure or stress per square inch that is being applied. They stress each piece until failure. 100 recordings are made. The list is sorted least stress to greatest stress. The lowest 5 are thrown out, so that the wood is determined to fail a the 95th lowest value or greater. In formulas a safety factor of 1.25 is applied depending on the method used to calculate the safety. One method uses 1.25 another uses a more situational dependent value for safety. Anyway the value which goes in a table for the strength of the wood is the 95th lowest value needed to break that wood times a safety margin of 1.25. This is called a design reference value. It is specified in pounds per square inch for compression, tensile, bending and other types of stresses. An engineer will get these design reference values mostly from the National Design Specifications. Phew, I hope I explained that clearly enough.
Stresses become a lot more complex with wood than pounds per square inch applied in vertical compression. I don’t even want to go into it in this writing. Probably the best that the common individual can hope for practically is in calculating floor loads and roof loads on floor joist and rafters. Aside from that please go find a qualified structural engineer or copy very strictly some design that has already been engineered and tested. Where you might find pre-engineered designs? I have no idea, maybe a reader can clue me into this. One way might be to examine a structure which has been standing for 30 years and simply copy its design, making sure that you are using the same species of wood. Over-engineering can work where feasible. Use the formula variable P for Plenty. Though I think without a good measure of common sense and some prior wood working experience and some study, self engineering is risky business.
I think in my case if I do my own calculations for use of timber in a structure, the structure will either use timber only for a floor or roof, not walls and not for holding up the floor or roof. Or it will be for simple symmetrical one story designs, (Post and Beam).
Personally I still can’t help but love the look of timber frame. I can’t help but try to imagine how timber frame, or at least beams might fit into my own construction designs and projects. Some advantages in using timbers are.
- I can make my own beams.
- Fewer pieces are needed in the structure.
- The structure has the look of impressive long lasting strength.
- The structure is on display and can always be inspected for its integrity.
- Varying types of infill for walls can be used (Earth, Straw, Stone etc.)
Regardless of engineering complexity, I personally will continue to study how to engineer with wood. As I learn more, I learn more about what I may be capable of and when I must say, “Nope, we would need an engineer for that.”
A last note. I have been told the best chain saws are Husqvarna and Stihl. Husqvarna parts are easier to find I hear. Stihl has been recommended as the absolute best however. Saws of around 100cc or 10 horse power are best for milling lumber. Run them at 90% throttle if possible. Sharpen the chain often. Use good oil. etc. A special chain called a rip chain may be needed and can be purchased from grandberg.com.
If you visit Lee Valley’s web site you will find a wood working catalog and near page 200 will be a thing called a Beam Machine. It cost around $50. This is a very simple guide tool. It is shaped like a 2×4 in order to fit over a 2×4 so that it can slide back and forth along the board. This uses the board as a straight edge. The board would be nailed to the log. There is a C or U shaped clamp on the side of this bracket with 2 bolts. These bolts tighten up on the chain saw bar so that the chain saw bar is held at a 90 degree angle to the flat side of the 2×4. There is also a small bubble level on the bracket. The chainsaw bar is kept at a perfect angle as it is moved along. Once you get one side sawn, simply rotate the log 90 degrees and move the 2×4 to the flat side. Then saw of the next side of the beam. Repeat this 4 times and you have a beam. I am about to give this one a try myself soon.
Also at Lee Valley or grandberg.com you may buy the Granberg Mill. This mill would be a jig or frame that completely supports the chain saw. It supports the saw on both sides of the log. The top of this frame has rollers which roll along on the log or on a board which is nailed to the top of the log. The frame holds the saw a given distance beneath the rollers.
One of the more inexpensive type of highly portable band saw can be found at lumbersmith.com for around $2200. And one more tip, you may be interested in getting what is called a Log Arch for moving logs around your lot. This can be pulled behind a four wheeler. It moves one log which is chained up and suspended from the arch and boom. This can be easily made or purchased for around $700. I talked to a trucker from West Virginia at the time of this writing who told me that the going rate for Logs (as logs) were $0.30 to $0.45 cents per board foot. He said that standing timber would be about 40% of that cost. Personally I don’t intend to do the logging. And at least one author has agreed with me that if you are not a professional logger then its probably best to stay away from this. I do intend to fell some trees myself and cut boards/beams from the logs at the felling location. I might haul one or two logs per run sometime if I feel like it. I followed a 16 foot dump truck around Atlanta Georgia one day. He had a load of good sized 24″ diameter or better logs cut just long enough to fit the dump bed. He pulled a trailer with a bobcat and a stack of plywood. I assume he used the plywood to make a ramp for rolling the logs into the dump bed. He probably used ropes or chains and the bobcat to get the logs into the dump bed from a ramp on the side. I’m sure he used the bobcat to maneuver single logs into position for rolling up the ramp one at a time. If you are in a position where you might jump and run 24/7 think about finding some excavators. Excavators are constantly pushing down trees simply to burn them in dozer piles. They could possibly give you the logs/trees. They would even let you saw the lumber at the site most likely.
Stick Frame 2×4 2×6 2×8 construction.
I only want to say that the book, “Design of Wood Structures” covers engineering of this kind of construction really well. Engineers can certainly calculated the load bearing capacity of 2×4 walls. They can certainly calculate the load bearing capacity of various truss designs. A more modern type of truss design is the plywood or particle board I Beam. This is made by using a sheet of plywood that is say 20 feet long and only say 3 feet high. 2×4’s are nailed on each side at the top and bottom of the plywood. Joints are staggered. 2×4’s are nailed on each side over plywood joints. This is a strong truss for flat or near flat roofs and floors. When plywood and sheet rock is nailed to the 2×4 wall/ceiling/floor frame it is called sheathing and acts as a “diaphragm”. Both structural board (plywood and osb and particle board) and sheet rock act as a continuous brace for all parts of the frame. With close nail, screw or staple spacing (schedule), this forms a very rigid and strong structure.
Fasteners and Plates
The book “Design of Wood Structures” also covers plates and fasteners in very good detail. Plates are a fairly simple matter. Any stress on the metal plate will simply be trying to tear the plate and therefore puts it in tension. All one needs to know is the tensile strength of the plate for the given type of metal and thickness. Fasteners though are a lot more complicated. The book covers bolts, lag screws, screws and nails. Fasteners over 1/4 inch in diameter are large dowel type fasteners and need pilot holes or pin(bolt/dowel) holes. Fasteners below 1/4 inch in diameter are small nail or screw type fasteners and need no pilot holes. When using the larger type of fastener the computations and considerations become more complex. For example the strength of the member may need to be recalculated minus the area of the pilot hole or pin hole. Also angle of load to grain is figured into the calculations. Crushing strength of the wood members must be considered. This is why nails and screws are so popular. Some kinds of nails have greater withdrawal strength than others. Though in most connections withdrawal is not a consideration or much of one. Screws of course have the greatest withdrawal strengths.
All metal dowel type fasteners have a strength property know as yield limit. This is the pounds of force needed to bend the fastener. Actual shear or failure is not calculated because the fastener will bend or withdraw long before it ever breaks. Basically there are 6 modes of failure(bending and withdrawing and crushing) for every connection type. The engineer will calculate all 6 modes and use the mode with the lowest failure strength for the final value. It could work out that anyone single mode might be the lowest.
Anyway lets say you find that a bolt would hold up 100lbs as its lowest mode of failure. If you have 2 bolts then the connection will hold 200 lbs. If four then 400lbs etc. Same for nails or screws. Also washers and plates will prevent bolts heads and nuts from pulling through the wood member because they distribute the load over a wider area.
Of course there is more to it than this. Each calculation can be modified based on many different sets of criteria. For examples wet vs dry service. Moisture content of the wood. Repeated loading. Impact loads. Terrain modifiers. Occupancy categories and so on. There are factors for wind, snow and seismic loads. To thoroughly engineer the structure two different systems must be calculated. One is for lateral loads, called the lateral force resisting system. The other is the vertical force resisting system. In the lateral system sometimes walls and floors change roles. Yes joist and rafters become post. Lateral of course are wind and seismic loads. In an earthquake you have cantilever forces in play as if the ground was turned on its side 90 degrees so that it was vertical and the structure is hanging from the foundation like a shelf on a wall. In this wall/shelf example inertia is the force acting on the shelf (building) and not gravity. And in wind, braces may be in compression one moment and tension the next. Most homes are designed to withstand 80 mph winds.
Stresses or forces at play in a wood frame.
- Compression (crushing)
- Tension (pulling)
- Torsion (twisting, I only mention this, its not usually a consideration in wood construction)
- Shear (tearing, cutting)
- Bending (Bending Modulus or Moment, as in beams, rafters, joist)
- Modulus of elasticity (pliability)
- Cantilever (overhanging)
- Axial (along an axis, such as in post and braces)
- Self Straining (from shrinking)
- Creep (deformation from repeated loading)
- Deflection (bowing due to temporary loading)
In wood you need to remember that there are different values for all of these forces across grain vs with grain, and longitudal (top to bottom). The book “Design of wood Structures” covers all the do’s and dont’s when it comes to loads to grain considerations. It cover’s all other hard earned do’s and dont’s for just about everything that can be thought of. You might call these rules of thumb.
To keep things simple, lets say that the engineer is going to calculate the vertical force resisting system (vertical loads) of a wooden frame. He will divide the structure into minor tributary areas, then combine them along lines or points. Line being a joist or rafter, point being a post or brace. The forces actually flow from the very top of the roof down to the foundation in very much a similar fashion as water flowing from streams to creeks to runs to small rivers to larger rivers to lakes or seas or oceans. Tributary area is a great metaphor for describing flow of forces. And its a great way to break down the problem. Forces go from pounds per square foot on floors and roofs to pounds per linear foot on joist and rafters to pounds per square inch on post and foundations.
What software could one use? The mighty spread sheet would be the main tool. Special software for this purpose would have to be carefully made and certified by qualified people. I wonder if this is why you do not find free engineering software. Liability might be a huge problem when human lives are at stake. If I get time I will write some software for my own purposes for specific design problems but I probably will not be sharing it. Once a spreadsheet template has been setup for a given design problem however that one template can function again and again for different projects with little or no change. A good free spreadsheet can be found in “Open Office”. Simply google for “Open Office”, download and install it. They call the spreadsheet Open Office “Calc”. There is also and Open Office “Math” but I think this is only for constructing formulas visually for documents.
Types of Joinery that I have read about or heard of by name…
- Mortise and Tenon
- Open Mortise and Tenon
- Blind Mortise and Tenon
- Stub Mortise and Tenon
- Through Mortise and Tenon
- Step Lapped Rafter Seat
- Single Shoulders
- Double Shoulders
- Joist Pocket
- Lap Joint
- Through Half Lap
- Mortise and Tenon with Diminished Haunch
- Dovetail Lap Joint
- Lapped Half Dovetail Collar Tie
- Rectangular Tenon
- Square Tenon
- Wedged Dovetail Tenon
- Birds Mouth
- Soffit Tenon
- Housed Lapped Dovetail
- Half Lap Scarf
- Stop Splayed Scarf
- Bladed Scarf
- Beveled Edges
- Dressed Shoulders
Some ideas for lifting or raising..
- Shear Poles
- Gin Pole
- Block and Tackle
- Pike Poles
- Tread Wheel
Some tools of the trade
- Felling Axe
- Broad Axe
- Race Knife
- Boring machine
- Corner Chisel
- Draw Knife
- Riving horse/bench
- Scraping horse/bench
- Moisture Content Meters
- More Moisture Content Meters
Some of the names of the parts of frames.
- Collar Tie
- Cross beam
- Ridge Pole/Beam
- Front Plate
- Top Plate
- Summer Beam
- Corner Post
- Chimney Post
- Dragon Beam
- Major Purlin
- Knee Braces
- Jettied Post
- King Post
- Queen Post
- Hammer Beams
- Gunstock Post
- Joweled Post
- Tie Beam
- Ridge Beam
- Principle Rafter
- Common Purlin
- Anchor Beam
- Chimney Girt
- End Girt
- Full Plate
- Shakes (Wooden Shingles–3 foot Riven boards)
Common Design names
- Salt Box
- Cape House
- Two Story Colonial
- Banked Colonial
- Greek Revival
- English Barn
- Dutch Barn
- Hip Roof
- Gable Roof
This next set of photo’s shows the operation of the Grandberg Alaskan Saw mill. I am using my Husquvarna 55 with 18″ bar. It took about 30 minutes to saw off that first side. I was resting me and the saw a lot. It really moves along a lot faster than that. I bought two 10 foot 2×4’s for the straight edge. I intended to cut 10′ boards from this old dried elm stump. I realized after I finished that I really needed 12′ boards instead to hang over 1 foot on each end. However 9 feet or so is all the practical length I can get from this log.Next I intend to cut of at least one side or maybe both with a tool called the Beam Machine and my chain saw. Then I will cut 2″ planks from this log.
I demonstrate the use of the Beam Machine which uses a single 2×4 as a guide to cut off the side of a log. I realized after I started that I should have been 1″ further in. It looks to me like lumber could be made using this Beam Machine. If you didn’t want to use the lumber made this way for your home then it could always be used for barns, chicken coops and dog houses.
|Design of Wood Structures|
|Timber Framing For the Rest of Us|
|Timber Frame Construction|
|Old ways of working wood|
|The craft of modular post & beam|