How to Avoid a Structural
98 lb Weakling
(Accompanying videos below)
The # 1 Enemy: Inadequate Planning in Addressing Tension Loads
How will you identify possible weak points in your timber structure?
Is it worth your time?
Can such a complex issue be summarized in an easy-to-grasp format?
If you are mechanically inclined even slightly, I can confidently say: Yes, it is worth your time. Yes, it is summarized in an easy to grasp format. And Yes, you will be able to identify weak points by absorbing the following information and simply applying common sense.
Part #1 What is a tension load? What role does tension play in the collapse of a structure? Part #2 Tension loads in a truss. Part #3 Examples of typical shear systems used to resist tipping over. Part #4 Tension loads in beam failure and timber grading tips.
Part #1 What is a Tension Load?
It is the force which separates or pulls structural members apart. A tension load occurs in structural members when they are resisting forces of gravity, wind, or earthquakes.
The strength of your timber structure boils down to how well it handles tension loads. Tension loads tend to be the Achilles heel with most structures, not only in timber framing. Even a structural member that fails under a compression load blows out on the side which has the weakest tension strength.
To understand a timber joint failing under tension, visualize a piece of wood getting pulled apart. Either perpendicular to grain, like a piece of firewood being split, or parallel to grain, like slamming the end grain with a sledge hammer, causing a portion of the timber to slide backwards.
In a home fire, for example, a 2x4 truss will collapse very quickly because it doesn't take long for the fire to weaken the truss members under tension load. A freestanding pavilion or porte cochere may blow over in a heavy windstorm if resistances to tension loads are inadequate. A post may buckle or a beam may collapse if wood fibers are unable to bear tension loads.
Part #2 Tension Loads in a Truss
The explanation easiest to grasp comes through in the accompanying video. In case you're short on time or can't view the video here is a short summary:
Picture yourself constructing a truss in place for a building 24' wide. First you set a 24' long 2x4 horizontally across the walls. Next, you install two shorter rafters at an angle on either side to form a peak at the center. You connect the 2x4 intersections at the walls (each end of the 24' 2x4.) Gravity holds the peak tightly together as long as the angled 2x4 rafters cannot pull or slide away from the 24' 2x4 ends. So now the peak will support a load but the mid-span of all three 2x4s will not.
What to do? Now you hang a 2x4 vertically from your solid peak and connect it to your 24' horizontal 2x4... You now have a solid place to post from to support the mid span of both angled 2x4s. After you have installed your angled supports (webs) the roof load travels down through your webs, (compression) up through your king post (tension) and down through your angled rafters (compression), which is restrained by your 24' long 2x4 (tension).
I know, I know... that was a lot of words for an easy explanation... which is why I recommend watching the video.
The critical factor again is: how well will your tension loaded joints perform? Timber framers of old were aware of this, which is why you can find many examples of joinery which lock together more tightly as the tension increases.
The video also goes through the compression and tension loading of a typical knee brace triangle. Two favorable factors regarding wind and earthquake loading is that they are not under constant load, as are trusses. Secondly, the load can be dispersed among many smaller knee braces and joints. Myself, I like big triangles with angles offset from a 45 degree to take the load more directly, provided that the resulting thrust/tension will be absorbed safely.
Part #3 Lateral Bracing Systems
In timber framing, the key component used to stiffen walls laterally is the age-old knee brace. The simple unassuming triangle. Even a rectangular sheet of plywood under a lateral load has tension from one corner to another, similar to a knee brace.
The following bracing systems are covered in the video and much easier to follow than in a written explanation. You may, however, benefit by reading the unique points listed in the short summary of each system.
Cantilevered System: This is one of the earliest forms of building. Think "pole barn". Posts buried deeply enough into the ground basically cantilever upwards. Of course, early buildings did not have pressure treated posts, so this method was discarded early on.
Knee Bracing System: Could this be called a reverse cantilevered system? The posts are locked in at the top via tension and compression, as we've covered already, and the posts need only to resist breaking under a lateral load. You'll also want to make sure your post base connection will resist the potential lateral and uplift loading it could face.
Load to Footprint System: Picture 12x12 posts, 4' long, standing on end in your garage. How much lateral (sideways) force will the 12x12 footprint take before tumbling over? This system basically uses the force and leverage compared to the footprint ratio to calculate how much connection hardware is required to keep the structure from toppling over.
Thru-Tenon System: Not unlike the Load to Footprint System except it works in a horizontal fashion. The "footprint" is the end of the horizontal tie beam where it smacks up against the blankety-blank post! Got it? It also has a built-in safety factor, in that the tenon itself would have to actually break if the tension peg or pin holding the footprint tight were to fail.
Cruck System: The Cruck System is nothing more than a glorified teepee. Early builders would use crooked trees to gain more head room. Then they got away from the circle but the basic premise remains: lean two sticks together and fasten them at the peak. You can find modern day "Cruck" systems in A-framed homes, metal pole buildings, and even churches built with specially made curved glue-lam beams.
Shear Diaphragm System: Say for example your house is 200' long and 30' wide. Any lateral load applied to the face of the 200' long wall would be transferred to each of the 30' long end walls. Force pushes lengthwise at the top of the 30' end wall in a horizontal direction. In other words, the end walls keep it from falling over.
What keeps the middle from falling over? You and I know that a 2x4 or 2x6 means nothing for a 200' span. All the itty bitty little nails used to fasten the roof plywood all work together in compression and tension to create a rigid shear diaphragm. (In this exaggerated example, a 30' wide shear diaphragm would be woefully inadequate. Periodic interior bracing/shear walls definitely would be required.)
The shear diaphragm is commonly integrated with hybrid timber framing, and even complete timber frames by using floors, walls, and roofs to beef up the structure even more. (To achieve a shear diaphragm before plywood was invented, boards were installed at an angle on the floors, walls, or roofs.)
Part #4 Tension Loading in a Timber
The good thing about beam failure is that it happens slowly and it's very obvious that you've got a problem. The wood fibers on the bottom half of the beam are failing under tension load when you have a sagging beam; chances are, it just looks very ugly and is not an imminent danger because it really takes a lot to snap a beam. But they do snap, so beef up that sagging beam or at least shovel the snow off of your roof before you have a real problem!
To your goals and momentum!