Preface: After successfully completing over 700 projects during more than a quarter century without such a document as this, one might ask “is there really a need for one”?
Does the question presume that all of those previous projects could not have been improved upon? Experience knows differently. With the possible exception of the Tabernacle and the Temple later, there has not ever been a work of man constructed without flaw. And of course, the builders of those exceptions had the Architect of the Universe as their PM! But some Laborers may remember a column slightly (but noticeably) out of plumb on project X. What about the droopy and misaligned entryway on project Y? Then there’s the roof sheathing that’s “spongy” when trod upon, on that facility in town Z, which also has a peculiar bump in one wall. You can observe flaws on any project (to say nothing of the wasted effort correcting mistakes, etc.).
Even slight improvements will add to the value, serviceability, and longevity of Laborers work. Every project is also a learning experience for participants. Lessons learned are of enduring value only if passed along. It is hoped this humble offering will encourage others to share knowledge for future builders, and add to the worth of the Laborers for Christ program in service to Our Lord. - Wayne Valentine, P.E. (Ret.).
Acknowledgments: Many thanks to those who contributed ideas and helpful criticisms. Special thanks to my long-time friend and former colleague Stan Skousen for his careful review, suggestions, and comments, and also to my generous neighbor and computer genius Tim Wolf, who unselfishly provided the graphical adornments to this project.
Wednesday, April 8, 2009
INTRODUCTION
Laborers who understand a few basic principles of engineering, mensuration, and lay out will be able to do a better job framing the more important structural members of buildings. This includes:
* positioning and aligning members accurately
* recognizing critical load paths
* assuring adequate bearing
* avoiding weakening cuts
* making effective connections
* providing adequate but not excessive nailing
* spotting and correctly using defective material
* placing material so as to use its greatest strength
* erecting safe and economical temporary framing
This understanding can add to the quality and economy of LFC work performed for Our Savior and LCMS congregations.
Here is a legal disclaimer: By law, competent Engineers or Architects must review the design of permanent load-bearing structural members and their connection details. This Primer is only intended to acquaint Laborers with a few fundamental technical concepts, thereby exposing the need for Professional assistance in questionable or complex situations.
Never-the-less, framing and structural problems often arise during construction, and need to be dealt with quickly to avoid work delays. They may occur for several reasons, such as unforeseen difficulties in remodeling projects, the need for temporary scaffolding or supports, or even because of incomplete or poorly conceived architectural plans or revisions. When the latter happens, it may be possible to provide solutions that will keep the work going in anticipation of a design professional’s review. Knowledge of a few fundamental engineering principles will help reduce delays and improve the utilization of time and materials.
Laborers for Christ should also understand and correctly use building terminology to effectively communicate with architects, suppliers, contractors, and with each other. Speaking the same language helps the building process. Remember the project at Babel? They were doing quite well until the Lord confused their speech. Let’s try to use proper terminology and use it correctly. After all, He does not want our projects to fail like theirs.
* positioning and aligning members accurately
* recognizing critical load paths
* assuring adequate bearing
* avoiding weakening cuts
* making effective connections
* providing adequate but not excessive nailing
* spotting and correctly using defective material
* placing material so as to use its greatest strength
* erecting safe and economical temporary framing
This understanding can add to the quality and economy of LFC work performed for Our Savior and LCMS congregations.
Here is a legal disclaimer: By law, competent Engineers or Architects must review the design of permanent load-bearing structural members and their connection details. This Primer is only intended to acquaint Laborers with a few fundamental technical concepts, thereby exposing the need for Professional assistance in questionable or complex situations.
Never-the-less, framing and structural problems often arise during construction, and need to be dealt with quickly to avoid work delays. They may occur for several reasons, such as unforeseen difficulties in remodeling projects, the need for temporary scaffolding or supports, or even because of incomplete or poorly conceived architectural plans or revisions. When the latter happens, it may be possible to provide solutions that will keep the work going in anticipation of a design professional’s review. Knowledge of a few fundamental engineering principles will help reduce delays and improve the utilization of time and materials.
Laborers for Christ should also understand and correctly use building terminology to effectively communicate with architects, suppliers, contractors, and with each other. Speaking the same language helps the building process. Remember the project at Babel? They were doing quite well until the Lord confused their speech. Let’s try to use proper terminology and use it correctly. After all, He does not want our projects to fail like theirs.
FRAMING (FORCES AND STRESSES)
FORCES AND STRESSES
Remember your high-school physics and Newton’s laws, which are actually connected to rules God uses to govern the motion of planets. You learned that forces act on a straight line and that for any object at rest, the forces acting on it are met with equal but opposite forces. On objects at rest, or in equilibrium, “For every action there is an equal but opposite reaction”. We expect our buildings to be “at rest” and not collapsing!
Remember your high-school physics and Newton’s laws, which are actually connected to rules God uses to govern the motion of planets. You learned that forces act on a straight line and that for any object at rest, the forces acting on it are met with equal but opposite forces. On objects at rest, or in equilibrium, “For every action there is an equal but opposite reaction”. We expect our buildings to be “at rest” and not collapsing!
Forces in buildings come from Loads that cause Internal Stress within individual structural members that are resisting the Loads. The usual forces (stresses) concerning us are:
1) Moment (bending),
2) Compression (pushing)
3) Shear (sliding or cutting),
4) Tension (pulling),
(There are other stresses but they are generally not of concern in buildings.)
Force is the amount of push, bend, pull, etc. applied to and resisted by a structural member. We usually measure force in pounds per square inch (psi) for compression, tension, and shear; and inch-pounds or foot-pounds for bending.
LOADS
Loads are from building materials, people, stuff, and nature; e.g. shingles, beams, folks, pianos, pews, and wind, snow, or earthquakes. We classify loads in several ways.
By duration:
* Dead Load (DL) is permanent or long-term, such as the structure itself and what it continuously holds up, e.g. the roof, ceiling, walls, etc. (Somewhat like original sin.)
* Live Load (LL) is short-term, intermittent and re-occurring such as snow, wind, or people. (Somewhat like actual sin.)
* Impact is very short term shock loading, such as when a piano is dropped. Not usually considered in buildings except in seismic design. (Unlike steel and concrete, wood has a remarkable capacity to absorb short-term loading.)
By distribution:
* Concentrated Loads act in narrowly defined areas, e.g. a safe, or a beam bearing on a pier. Measured in pounds.
* Distributed Loads are either actually spread evenly over a surface, e.g. snow, wind, or carpet; or assumed to be spread evenly over a surface, e.g. people. Measured in pounds per square foot (psf) of surface, or pounds per linear foot (plf) along a beam supporting the surface.
By how they act on structural members:
* Axial Loads act along and parallel with the long axis (dimension) of the member, and produce compression or tension forces.
* Transverse Loads act at 90 degrees (“normal” or perpendicular) to the member’s long axis, and produce shear and also bending, which is actually comprised of tension and compression stresses.
Loads operate in a straight line (obvious but often un-recognized); and forces resisting them also operate in a straight line. All loads are supported by continuous paths of resistance from the building foundation to the point of application. Be aware of paths of load resistance. Identify and examine load paths to clarify critical members and joints, and to locate questionable situations. (A column should not bear in the middle of a floor with no underlying pier, for example.)
Any individual force can be “resolved” (i.e. broken down) into two or more components acting along different force lines through the same point. For example, a brace leaning against a wall will have vertical and horizontal forces acting against its weight, at each end of the brace.
Building codes specify the applicable design loads for floors, roofs, wind, etc. for each geographic area. Some typical live floor loads are 100 psf for hallways, and 40 psf for classrooms. Roof live loads vary geographically and are determined by weather (wind and snow) and seismic forces. Roof dead loads are typically 15 psf to 20 psf. Lumber weighs between 3 and 4 pounds per board-foot, depending on species.
RESISTANCE OF BUILDING MATERIALS
Resistance of Building Materials
Framing materials resist loads according to their strength. Wood, steel, and concrete are our primary structural materials and their strength characteristics are well known. The load carrying capacity of individual elements is a function of the size, shape, and inherent strength of the material, usually stated as allowable stress, and measured in pounds per square inch (psi), or, for some manufactured elements (e.g. nails, hangers), as pounds.
For rough estimating, here are some typical conservative values of allowable stresses:
Steel: 18,000 psi in tension, compression, and bending.
Concrete (plain): 1350 psi in compression and zero in tension and bending.
Structural Lumber: 1200 to 2000 psi in bending, tension and compression parallel with the grain, and 400 psi in compression perpendicular (normal) to the grain, values depending on the species and grade.
Nails: Allowable lateral load (shear) for a 16-penny nail completely driven into side grain of sound seasoned wood is approximately 80 to 100 pounds; its withdrawal load from side grain is about 30 pounds per inch penetration, and next to nothing in withdrawal from end grain.
Actual yield (total) strength is much higher but held in reserve to provide a factor of safety for un-anticipated over-loads, defective material or fabrication. Technical handbooks provide more detailed information.
Framing materials resist loads according to their strength. Wood, steel, and concrete are our primary structural materials and their strength characteristics are well known. The load carrying capacity of individual elements is a function of the size, shape, and inherent strength of the material, usually stated as allowable stress, and measured in pounds per square inch (psi), or, for some manufactured elements (e.g. nails, hangers), as pounds.
For rough estimating, here are some typical conservative values of allowable stresses:
Steel: 18,000 psi in tension, compression, and bending.
Concrete (plain): 1350 psi in compression and zero in tension and bending.
Structural Lumber: 1200 to 2000 psi in bending, tension and compression parallel with the grain, and 400 psi in compression perpendicular (normal) to the grain, values depending on the species and grade.
Nails: Allowable lateral load (shear) for a 16-penny nail completely driven into side grain of sound seasoned wood is approximately 80 to 100 pounds; its withdrawal load from side grain is about 30 pounds per inch penetration, and next to nothing in withdrawal from end grain.
Actual yield (total) strength is much higher but held in reserve to provide a factor of safety for un-anticipated over-loads, defective material or fabrication. Technical handbooks provide more detailed information.
BEAMS (BENDING RESISTANCE)
Beams (bending resistance)
Along with columns and trusses, beams are critical structural members, and they are found in virtually every building. Beams carry transverse loads such as roofs, floors and interior walls, and also plumbing fixtures, refrigerators, desks, and people; they have important load carrying jobs. Their ability to perform can be impaired by careless placing, reckless fastening, and foolish carving or cutting. This can lead to squeaky or un-even floors, sagging and leaking roofs, cracked walls, increased seismic or storm risk, and other problems. Laborers can help guard against these problems by understanding how beams work.
Beams have special names because they are for special jobs or are made in special ways. See the appendix for a list.
Beam parts:
Beam parts are best identified by reference to the end of an I-beam. There are three special areas, namely the two Flanges, and the area in between them, called the Web. Each of these parts has a separate job in load carrying. A horizontal beam properly placed to carry a vertical load has one flange on top and one on the bottom. The line along the beam halfway between the top and bottom flanges is the beam centerline, called the “Neutral Axis”.
Beams are supported by direct bearing on something else, e.g. a bearing wall, pier, truss, column, or shear through hardware connections. The number and place of supports are very important in determining a beam’s load carrying capacity. A beam supported only on its two ends is called “Simply Supported”. If there are more than two supports under a beam, it is called “Continuously Supported”. A beam overhanging a support and carrying a load on the overhang is “Cantilevered”. The distance between supports is the “Span”.
Along with columns and trusses, beams are critical structural members, and they are found in virtually every building. Beams carry transverse loads such as roofs, floors and interior walls, and also plumbing fixtures, refrigerators, desks, and people; they have important load carrying jobs. Their ability to perform can be impaired by careless placing, reckless fastening, and foolish carving or cutting. This can lead to squeaky or un-even floors, sagging and leaking roofs, cracked walls, increased seismic or storm risk, and other problems. Laborers can help guard against these problems by understanding how beams work.
Beams have special names because they are for special jobs or are made in special ways. See the appendix for a list.
Beam parts:
Beam parts are best identified by reference to the end of an I-beam. There are three special areas, namely the two Flanges, and the area in between them, called the Web. Each of these parts has a separate job in load carrying. A horizontal beam properly placed to carry a vertical load has one flange on top and one on the bottom. The line along the beam halfway between the top and bottom flanges is the beam centerline, called the “Neutral Axis”.
Beams are supported by direct bearing on something else, e.g. a bearing wall, pier, truss, column, or shear through hardware connections. The number and place of supports are very important in determining a beam’s load carrying capacity. A beam supported only on its two ends is called “Simply Supported”. If there are more than two supports under a beam, it is called “Continuously Supported”. A beam overhanging a support and carrying a load on the overhang is “Cantilevered”. The distance between supports is the “Span”.
HOW BEAMS WORK
How beams work:
Beam action demonstration: Take a Popsicle stick or tongue depressor and place it flat (wide face up) between two supports, e.g. a couple of thick books. You now have a simply supported beam (plank). Put your finger in the middle of the span and push down. You now have a loaded beam (plank), with a concentrated load at mid-span generating bending moment and shear within the beam. The beam will sag, or “deflect”. Push harder and the beam will break. Look at the break. The fibers at the top of the beam will not be severed, but the bottom fibers will be jagged and parted or broken. Now turn the beam over and repeat the exercise. This time the stick will break in two, with the fibers on both sides parted.
In each loading case, the fibers on the bottom were in tension, and the top fibers were in compression. Thus we see that a beam works by internal push in its top flange, and pull in its bottom flange. These are two opposing forces working together in harmony around the neutral axis. Sort of like the law and gospel around grace resisting the load of sin.
Your finger load also created a shearing force at the supports. But because the bending strength of the stick is weak when loaded as a plank, it failed by bending moment rather than shear.
Now, take another stick and repeat the exercise, but this time place the stick with the narrow edge on top. Unless you are as strong as Samson, you will not easily be able to break the stick, because loaded this way it is much stronger in bending. Thus we learn that a beam’s strength depends on the distance between (separation) of the flanges, or the depth of the web. This gives a basic rule of how beams work. “The flanges carry the moment, the web carries the shear”.
Beam action demonstration: Take a Popsicle stick or tongue depressor and place it flat (wide face up) between two supports, e.g. a couple of thick books. You now have a simply supported beam (plank). Put your finger in the middle of the span and push down. You now have a loaded beam (plank), with a concentrated load at mid-span generating bending moment and shear within the beam. The beam will sag, or “deflect”. Push harder and the beam will break. Look at the break. The fibers at the top of the beam will not be severed, but the bottom fibers will be jagged and parted or broken. Now turn the beam over and repeat the exercise. This time the stick will break in two, with the fibers on both sides parted.
In each loading case, the fibers on the bottom were in tension, and the top fibers were in compression. Thus we see that a beam works by internal push in its top flange, and pull in its bottom flange. These are two opposing forces working together in harmony around the neutral axis. Sort of like the law and gospel around grace resisting the load of sin.
Your finger load also created a shearing force at the supports. But because the bending strength of the stick is weak when loaded as a plank, it failed by bending moment rather than shear.
Now, take another stick and repeat the exercise, but this time place the stick with the narrow edge on top. Unless you are as strong as Samson, you will not easily be able to break the stick, because loaded this way it is much stronger in bending. Thus we learn that a beam’s strength depends on the distance between (separation) of the flanges, or the depth of the web. This gives a basic rule of how beams work. “The flanges carry the moment, the web carries the shear”.
BENDING STRESS (MOMENT)
Moment (bending stress)
The compression stress in the top flange converts to tension stress in the bottom flange. Think about how this happens. Compression within the beam is a maximum at the very top of the flange, but decreases within the beam, becoming zero at the neutral axis. From there, tension grows from zero to a maximum at the very bottom of the beam.
Therefore, bending stress is zero at the neutral axis, and maximum at the flanges! This is significant information for deciding where to poke holes in a beam. The “tension face” in a wooden member requires clear wood (i.e. knot-free), especially in the middle third of the span.
We have discovered how moment stress varies within the cross-section of a beam. Now let us look at how it varies along the beam. The beam broke in the middle, where the maximum moment stress occurred.
Therefore, the moment stress varies from a maximum in the middle of a simply supported beam to zero at the ends. This is more significant information, useful for strengthening a beam or avoiding weakening it.
The compression stress in the top flange converts to tension stress in the bottom flange. Think about how this happens. Compression within the beam is a maximum at the very top of the flange, but decreases within the beam, becoming zero at the neutral axis. From there, tension grows from zero to a maximum at the very bottom of the beam.
Therefore, bending stress is zero at the neutral axis, and maximum at the flanges! This is significant information for deciding where to poke holes in a beam. The “tension face” in a wooden member requires clear wood (i.e. knot-free), especially in the middle third of the span.
We have discovered how moment stress varies within the cross-section of a beam. Now let us look at how it varies along the beam. The beam broke in the middle, where the maximum moment stress occurred.
Therefore, the moment stress varies from a maximum in the middle of a simply supported beam to zero at the ends. This is more significant information, useful for strengthening a beam or avoiding weakening it.
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