**Q: **I was looking over our copy of the engineer's calculations
last weekend, but it seemed like they were written in a foreign language. Is there a
glossary somewhere to tell me what all these terms and/or symbols mean?

**A: **The following list may be of help to you. Remember that
one of the main activities of engineering is the discovery, attribution and distribution
of force throughout a structure. It will not be surprising then that most of the terms you
saw are shorthand expressions for some type of force.

**Terms : Kip, Moment, Reaction, Shear, Modulus of Elasticity and
Fiber Stresses.**

**Kip ::** In the English system, we use pounds to describe
weight and feet or inches to describe distance. You may remember that 2000 pounds is
called a "ton", but did you know that 1000 pounds is called a "kip"?
[It was most likely shortened from "Kilo-Pound"] So, when you see a notation
like "kpf", think "kips per foot". 0.48 kpf would be equal to 480
pounds per foot. Engineers use kips since it makes the numbers much easier to work with
(especially in the days of slide rules) and reduces errors caused by lost zeros.

**Moment :: **A way of describing a force acting at a distance. A
teeter-totter is a good example. You (the 160 lb. adult) take your child (40 lbs.) down to
the park. You put your child on the seat which is resting in the tanbark, then you walk to
the other end to get on. Let's assume that the seats are 14 feet apart and the hinge point
is fixed at the middle of the board, 7 feet away from each seat. [Think of the hinge like
an equal sign.]

Case #1How hard will you have to pull down on the far seat to lift your child off the ground? (40 lbs.)(7 ft.) = (7 ft.)(? lbs.) Of course, the answer is 40 pounds. |

Case #2 Will the teeter-totter balance if you use the standard seat at your end? (40 lbs.)(7 ft.) equals? (160 lbs.)(7 ft.) No, unless you use your legs, your duff is in the tanbark. |

Case #3Where would you have to sit on the plank if the teeter- totter is to balance? (40 lbs.)(7 ft.) = (160 lbs.)(x ft.), x = (7/4) = 1.75 feet away from the hinge point. |

In each instance, the units are expressed as "distance"
times "force"; *foot-pounds* of "Moment". When moments are
stated, they are typically accompanied by a reference point; a location from which the
distance to the acting force is measured. In the above example, the hinge is the reference
point. Your child generates (7 feet times 40 pounds) or 280 ft.-lbs. of moment about the
hinge, while *you* generate 1,120 ft.-lbs. of moment. This is the numerical
explanation of why you and your child did not balance in Case #2. The calculation of
moments is so basic to the engineering process it is required for members, walls,
foundations and even entire buildings.

**Reaction ::** This is a fancy way of referring to the amount of
weight an element contributes to a particular location. The "reaction" of a beam
on a wall is the amount of load that beam delivers to that wall. If you see a notation
like "Rmax.", this is shorthand for the "Maximum Reaction".

**Shear ::** A type of force which wants to make adjacent
materials move past each other. This includes:

- The movement of adjacent wood fibers within a member in a direction parallel with or perpendicular to the grain;
- The movement of two individual members with respect to their original positions;
- The movement of materials with respect to fasteners such as nails, screws or bolts.

When you see the phrase "Lateral Load", this refers to shear forces acting in a direction parallel with the ground surface. The causes of these lateral loads can be wind, retained soil, water, earthquake or impact by an object(s). Stability of a structure is achieved only when lateral loads are properly distributed through the structure, into the foundation and out into the surrounding soils.

**Modulus of Elasticity (MOE) ::** One of the basic properties of
all materials. The MOE is determined by experiment using very sensitive equipment. A
measured length of wood is placed in a machine which will attempt to stretch it by pulling
from each end. A record is made of the elongation of the wood for ever increasing loads.
Given a sufficient amount of data and for loads below that which would cause wood fiber
damage, a straight-line relationship between load and elongation can be demonstrated. The
slope of this line *is* the Modulus of Elasticity. Once it is calculated, we can
predict the deformation of this material under any given load within the tested (and
safe) limits. The greater the MOE, the greater the force required to stretch a given
amount of that material. The following is a list of common materials and their respective
MOE's:

- Steel ----------------- 29,000,000 p.s.i.
- Cast Iron ------------- 13,500,000 p.s.i.
- Aluminum Alloy --------- 9,900,000 p.s.i.
- Douglas Fir Lumber ----- 1,700,000 p.s.i.

[ *For those of you who might be interested, the formula for
calculating deformation is (Deformation)=(P times L)/(A times E) where "P" is
the load in pounds, "A" is the cross-sectional area of the member in square
inches, "L" is the length of the member in inches and "E" is the
Modulus of Elasticity. As an example, a steel bar which has an area of one square inch and
is ten feet long, loaded with 10,000 pounds stretches... (10000)(120)/(1)(29000000) =
0.04138 inches. *

*Another arrangement of this equation may help you understand the
MOE better. This is (E) = (P)(L)/(A)(Deformation). The ratio of load over area (P/A) is
referred to as the "stress" on a particular material. The degree of deformation,
(Deformation/Total Length), is referred to as the "strain" on a material. The
equation for the modulus of elasticity is simply the ratio of the calculated stress to the
measured strain.*]

**Fb, Fc, Ft, Fv, Fiber Stress ::** Fiber stresses refer to the
demonstrated ability of wood to resist forces applied in specific directions with respect
to the grain of the wood. Building Codes contain long lists of these allowable stresses
for various wood species. Subscripts are used to highlight the force directions under
consideration. A short list:

- "b" is for the tension component of bending forces;
- "c" is for compression forces acting on the ends of the member (like columns or posts);
- "t" is for tension forces pulling on each end of the member; and
- "v" is for resistance to shearing forces (also called "horizontal shear" but not marked Fh).

**Section Properties : Area, Section Modulus, Moment of Inertia.**

Given the forces on a member and the allowable fiber stresses of the
material being used, you can determine the minimum acceptable section properties of that
member. There are three to be considered: The "Area" of the member, the
"Section Modulus" (or simply the "Section") and the "Moment of
Inertia" (again, simply the "Inertia"). Subscripts are used when referring
to calculated minimums versus actual amounts provided - "r" indicates the
minimum required amount and "f" is the actual amount furnished. In your
engineer's calculations you may see some notation like "*Sf > Sr, ok*".
This is just a check which shows that the selected member meets the design criteria. The
furnished amount should always exceed the required amount.

**Ar, Af, Area ::** The "area" of any wood member can
be calculated by measuring the width and depth at the cut end and calculating that square
inch area. The amount of area furnished establishes an upper limit on the reaction a
member is permitted to deliver to its support. This maximum can be expressed as
(2)(Af)(Fv)/(3) = Rmax.

**Sr, Sf, Section :: **The "section" of a member limits
the strength of the member when subjected to bending moments. For a solid wood beam with a
depth less than 12", the section it furnishes can be determined by the following: Sf
= (width)(depth)(depth)/(6). Everyone finds the units " inches cubed" a little
funny at first, but that will pass. The required section for any member is calculated
using the calculated moments and the allowable fiber stresses - Sr = (Moment)/(Fiber
Stress). If the moment is expressed in inch-pounds and the fiber stress is expressed in
pounds per square inch, the pounds cancel and you are left with inches cubed. All of the
mathematics aside, the important word to remember with regards to "section" is
the word "strength".

**Ir, If, Inertia ::** The "inertia" of a member when
used with the Modulus of Elasticity of the particular material allows us to predict the
deformation (deflection) of that member when loaded. The inertia of a solid wood beam can
be determined by the following: If = (width)(depth)(depth)(depth)/(12)...in inches to the
fourth power! There are all sorts of complicated formulas for determining deflection under
various loading conditions, but these would not be enlightening. What you should remember
is that the larger the inertia furnished (If), the smaller the resulting deflection. Or,
more simply, inertia is directly proportional to stiffness.

Engineering suffers from its share of technical jargon just like every other profession. Our advantage is that we deal with very tangible elements like beams and walls; things that everyone can relate to. When we speak about forces - gravity, the wind, the occasional earthquake - these too are part of our collective experience. Given a little clarification, anyone can understand engineering methods. I hope this information contributes to your appreciation of the man-made world around you.

**Scott McVicker, S.E. **