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A ceiling is an overhead interior surface that covers the upper limit of a room. It is generally not a structural element, but a finished surface concealing the underside of the floor or roof structure above.

  • A cathedral ceiling is any tall ceiling area similar to those in a church. Not everyone calls a cathedral ceiling by that term. It is also sometimes called a tray ceiling or a vaulted ceiling, and it can be enclosed with sheet rock or open to the attic rafters, depending on how it was designed.
  • A dropped ceiling is one in which the finished surface is constructed anywhere from a few inches to several feet below the structure above it. In contemporary construction this is typically done with an acoustic ceiling tile. This may be done for aesthetic purposes, such as achieving a desirable ceiling height; or practical purposes such as providing a space for HVAC or piping. An inverse of this would be a raised floor.
  • A concave or barrel shaped ceiling is curved or rounded, usually for visual or acoustical value.
  • A coffered ceiling is divided into a grid of recessed square or octagonal panels, also called a lacunar ceiling.
  • A cove ceiling uses a curved plaster transition between wall and ceiling; it is named for cove molding, a molding with a concave curve.
  • Ceilings have frequently been decorated with fresco painting, mosaic tiles and other surface treatments. While hard to execute (at least in place) a decorated ceiling has the advantage that it is largely protected from damage by fingers and dust. In the past, however, this was more than compensated for by the damage from smoke from candles or a fireplace. Many historic buildings have celebrated ceilings. Perhaps the most famous is the Sistine Chapel ceiling by Michelangelo.
  • A tray ceiling is a rectangular architectural feature that is either inverted or recessed. Tray ceilings can be plain, ornate, subtle or dramatic.
  • A shed ceiling has an angle, similar to a cathedral ceiling, to the central peak. However, the other side of the peak is a solid vertical wall to the floor.
  • A popcorn ceiling, also known as an acoustic ceiling, is a term for a spray-on or paint-on ceiling treatment used from the late 1950s into the 1980s in residential construction.

Coffer: A recessed panel in a ceiling.  Uses can include acoustic control, daylight harvesting and even taking weight out of a ceiling to lighten the structural load.

  • A coffer (or coffering) in architecture, is a sunken panel in the shape of a square, rectangle, or octagon in a ceiling, soffit or vault.  A series of these sunken panels were used as decoration for a ceiling or a vault, also called caissons (‘boxes”), or lacunaria (“spaces, openings”), so that a coffered ceiling can be called a lacunar ceiling: the strength of the structure is in the framework of the coffers.
  • A prominent example of Roman coffering, employed to lighten the weight of the dome, can be found in the ceiling of the rotunda dome in the Pantheon, Rome.
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Cantilever:  A beam or other projection that is supported at only one end and projects beyond.

  • A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress.  Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs. This is in contrast to a simply supported beam such as those found in a post and lintel system. A simply supported beam is supported at both ends with loads applied between the supports.
  • Cantilevers are widely found in construction, notably in cantilever bridges and balconies (see corbel). In cantilever bridges the cantilevers are usually built as pairs, with each cantilever used to support one side of a central section.

Beam:  A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.

  • Beams are traditionally descriptions of building or civil engineering structural elements, but smaller structures such as truck or automobile frames, machine frames, and other mechanical or structural systems contain beam structures that are designed and analyzed in a similar fashion.
  • A squared-off log or a large, oblong piece of timber, metal, or stone used especially as a horizontal support in construction.
  • Nautical: A transverse structural member of a ship’s frame, used to support a deck and to brace the sides against stress.

Moment of force (often just moment) is the tendency of a force to twist or rotate an object; see torque for details. This is an important, basic concept in engineering and physics.

  • The moment arm is the perpendicular distance from the point of rotation, to the line of action of the force.
  • The moment may be thought of as a measure of the tendency of the force to cause rotation about an imaginary axis through a point.
  • Note: In mechanical and civil engineering, “moment” and “torque” have different meanings, while in physics they are synonyms.

Torque: Torque, moment or moment of force is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object.

  • Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.
  • The magnitude of torque depends on three quantities: the force applied, the length of the lever arm connecting the axis to the point of force application, and the angle between the force vector and the lever arm. 

A shear stress, is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section. Normal stress, on the other hand, arises from the force vector component perpendicular or antiparallel to the material cross section on which it acts.

  • noun Physic.  the external force acting on an object or surface parallel to the slope or plane in which it lies; the stress tending to produce shear.
  • Force tending to cause deformation of a material by slippage along a plane or planes parallel to the imposed stress. The resultant shear is of great importance in nature, being intimately related to the downslope movement of earth materials and to earthquakes. Shear stress may occur in solids or liquids; in the latter it is related to fluid viscosity.

Post and lintel:  a structure consisting of vertical beams (posts) supporting a horizontal beam (lintel)

  • Whew! Finally an easy one!!
  • The simplest illustration of load and support in construction is the post-and-lintel system, in which two upright members (posts, columns, piers) hold up a third member (lintel, beam, girder, rafter) laid horizontally across their top surfaces. This is the basis for the evolution of all openings. But, in its pure form, the post-and-lintel is seen only in colonnades and in framed structures, since the posts of doors, windows, ceilings, and roofs are part of the wall.

Rafters:  the wooden structural support beams for a roof, sometimes visible on the exterior for certain building types and styles.

  • A rafter is one of a series of sloped structural members (beams) that extend from the ridge or hip to the wall-plate, downslope perimeter or eave, and that are designed to support the roof deck and its associated loads.
  • Not to be confused with:
    • a person who engages in the sport or pastime of rafting.
    • a flock, especially of turkeys.

Valley:  A valley is point in a roof where rafters of different angles come together.

Valley Rafter: a rafter that is attached to the ridge at the top and the valley at the bottom.

  • A part of the roof frame that extends diagonally from an inside corner plate to the ridge board at the intersection of two roof surfaces.
  • In a roof framing system, the rafter in the line of the Valley; connects the ridge to the wall plate along the meeting line of two inclined sides of a roof which are perpendicular to each other.

Truncated: cut off or cut short, usually in reference to a roof.

  • To shorten by cutting off a part; cut short.
  • A gable roof or hipped roof whose top has been cut off, forming a flat horizontal surface.

Truss:  A structural framework of wood or metal, esp one arranged in triangles, used to support a roof, bridge, etc.

  • Any of various structural frames constructed on principles other than the geometric rigidity of the triangle or deriving stability from other factors, as the rigidity of joints, the abutment of masonry, or the stiffness of beams.
  • In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in forces in the members which are either tensile or compressive forces. Moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes.

Truss Plan: A truss plan will show every truss that will make up a roof system. Each truss will be numbered and will be shown on shop drawing with dimensions and specifications.

Revolute Joint (also called pin joint or hinge joint):  is a one degree of freedom kinematic pair used in mechanisms.  Revolute joints provide single-axis rotation function used in many places such as door hinges, folding mechanisms, and other uni-axial rotation devices.

In Parts One and Two of this blog, I described the basic situation of a photosensor mounted high in a space not working and the Inverse Square Law being invoked as the obvious reason why.  Now let’s follow this to a logical conclusion.

The oft cited explanation is that the photosensor is seeing a light source such as a desk in its field of view.  That desk is a light source by virtue of its reflecting daylight from its top surface.  The desk isn’t truly a point source, but we can overlook that for now.  If we are twice as far from that desk, our photosensor will receive approximately one-fourth the light from it.  And the less light the photosensor receives, the harder it is to calibrate accurately.  That makes sense, doesn’t it?

Not really.  Something that does not change no matter how far the photosensor is above the floor is the sensor’s field of view.  Imagine I have a photosensor mounted high on a ceiling pointed downward.  If it has a field of view of 30 degrees and is 16 feet high, it “sees” a floor area of 57.74 square feet.   If I were to move that photosensor down to 8 feet, it would “see” only 14.43 s.f of floor.  That photosensor at 8 feet above the floor would probably only see one student desk.   But when the same photosensor moves to 16 feet above the floor there would be four desks in its field of view.  The math is pretty obvious.  When the photosensor is twice as far away, each desk contributes one-fourth the reflected light.  But the four desks in the field of view perfectly balance that loss, and the photosensor receives the same amount of light!  It should work just as well at 16 ft. as it does at 8 ft.!

If you want to learn more about what is really going wrong with photosensor performance, check out Part Four.