The Other Binds

A look at stresses affecting log bucking

When bucking logs sawyers are normally instructed to evaluate logs and determine what binds they are in. In engineering terms binds are compression forces that tend to compress the wood fibers. There are four common binds that sawyers are instructed to estimate. These binds are top bind, bottom bind, side bind and end bind. The first three are really bending stresses or in engineering terms, bending moments and the last one, end bind is a compression stress applied to the whole section of the log. In addition these four common binds or stresses there are other stresses that are not mentioned and can have a real impact on cutting of logs and the reactions when the cut is competed.

Three stresses that are not mentioned in sawyer manuals and training guidelines are shear, torsion and tension. Tension is essentially the opposite of end bind where a force tends to pull the wood apart. In bucking logs it is really quite rare and general of little concern because it will not pinch a saw and the cut pieces tend to move away from the sawyer. Torsion is a stress caused by a twisting force on the log. If a cut is made where one section of the log rolls or twists relative to the other, then that log was likely under some torsion. Hanging wedges can be used to control the twisting movement of a log that is under torsion. Sawyers should be able to identify logs in this condition and take any actions needed to complete the job safely. Quite large torsion stresses can be caused by a lopsided root wad or logs on steep ground when part of the log is held in place and part is free to roll down slope. The third unmentioned stress, shear, is likely as common as top and bottom binds. Shear forces will likely cause log sections to shift relative to each piece when the cut is made and may tend to cause logs to slab or split when the cut in nearly complete. Another way to think of shear forces is to think about what forces would be required to hold the log in place after the cut is made. Shear forces can act in the vertical direction as well as the horizontal direction. Horizontal shear is likely to exist when a log has some side bind. Shear forces can be very large and sawyers need to be able to recognize it and be able to deal with it safely.

The bending stress or binds and shear stresses can and almost always do vary on a single log depending on the exact location in the log. It is possible to have a log with no binds or bending stresses and at the same time/place have a high shear stress and vice versa. Assuming that all sawyers are able to estimate all the stresses in each log is likely not realistic. This does present quite a problem because their safety really depends on their ability to do a structural analysis of each log cut.

For the purpose of this write up I will give an example of an engineering evaluation of a relatively simple condition. I would not expect all sawyers to go to this much trouble on each log but it may help to demonstrate the need to think about all stresses that are acting on a log.

Assume that there is a 50’ long 3’ diameter log that needs to be cut. For simplicity the log is assumed to be a uniform diameter with an estimated weight of 320 lbs. per foot. In this example there is a trail near the right support and a cut will be made about 37.5 ft from the left end of the log.

When an engineering evaluation is done on the log, loading, shear and moment diagrams would look like the following diagrams.

The free body diagram shows the weight of the log and the reactions at each of the two locations the log is supported.

The shear force diagram shows the amount of shear force along the log in the vertical direction with the shear force in lbs.

The bending moment diagram shows how the moment or top/bottom binds changes along the length of log. The positive moment creates a top bind and the negative moment creates a bottom bind.

Note that at the location of the cut (37.5’ from the left end) there is no top bind, no bottom bind, no side bind and no end bind. However at the cut location there is about 6000 lbs of shear force that the sawyer needs to be aware of. As stated above a way to think of this shear force is to think about what force would be required to hold the log in place after the cut is made. In this case it would take 6000 lbs holding up on the left side of the cut and 6000 lbs holding down on the right side of the cut to hold the log in place after the cut is made. Of course in the field the log on the left of the cut likely would fall down with a force of 6000 lbs and the section on the right would rotate clockwise on the right support. The sawyer at this “no bind” location needs to be aware of this shear force, make a cut that will allow the movement, control possible slabbing and keep their body parts out from under the log as it drops.

As covered above, when the log is cut at 37.5’ from the left end the bending moment (or top and bottom bind) is zero and the shear stress is quite high. As you move to the right of the cut location there will be some bottom bind and moving to the left will have some top bind. If the first cut was made at a point 18.75’ from the left end, the bending moment or top bind would be high (56,250 ft-lb) but the shear stress would be zero.

The animated image at the top of this page is very similar to the example. The photo shows a log with quite a bit of shear stress as well as some bottom bind and tension. The log extended probably 40' unsupported from the right hand support. It also has a greater slope then the example with the log likely pulling some tension down hill on the well anchored root wad that was up to the left.

Anyway the whole purpose of this exercise is to point out that there are more binds or stresses then the commonly listed four binds that sawyers need to be aware of. I would suggest that anyone doing in person sawyer training or creating training aids, manuals or guides be very clear that there are more then the four stresses listed in current references.

Comments and suggestions are welcome.

Jim Thode

April 2017