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Archives for December 2007

Roof Rafter Design

Rafter Design: An Integral Part of the Construction Process

The rafter design of a structure is an integral part of any new roof construction. Rafters serve as the aesthetic framework of a roof as well as structural supports for the building. Roof designs are generally sorted into two categories: vented or non-vented.

Vented roofs are used to adjust the roof temperature to prevent melting snow and ice from damaging the structure. They can also release hot air trapped in the attic of a structure to help provide a cooler living environment beneath the attic space. Ventilation openings are also useful for releasing built-up condensation in the structure.

It can also help equalize the pressure between the inside of the structure and the outdoor environment. Insulation and rafter placement also play an important role in controlling condensation and interior climate.

Proper placement of insulating layers can limit or eradicate the growth of mold, corrosion of the structure’s elements, and decay of the wooden supports. Rafter design should not only be crafted to support the structure’s roof and provide a visually appealing exterior, but it should take proper ventilation and insulation procedures into consideration as well.

The vented roof space should not be connected to the interior living space of a structure. This would encourage airflow and condensation. This is accomplished by installing an air barrier in the ceiling line of the structure and limiting the free space between ceiling and essential internal components such as sprinkler systems and air ducts to two inches. This is necessary in cold or very cold climates.

Rafter spacing must be adjusted in heavy snowfall areas to prevent undue deflection and warping of the roofing surface. Adjustments for the natural climate must be made to ensure proper stability and ensure integrity of the structure.

The pitch of the roof also aids in proper heating, cooling and moisture control for a structure. Utilizing a designing software program to calculate the proper pitch and rafter placement is recommended. Not only must the structural design meet the specifications of the client while adhering to building codes, but it must be built for practicality and longevity as well.

Rafter design involves:

  • combining the structural needs of the building
  • the building codes for the geographic area
  • environmental and climate considerations
  • and the design preferences of the consumer

These complex factors make roof and rafter design one of the major vital elements of any new roof construction.

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Flitch Beam Design & Software

Accurate Flitch Beam Design Made Easier with Software

Flitch beam design software is a useful tool for architects, engineers, designers, and builders. Flitch beams are a common type of composite construction. Composite construction materials are formed by combining two or more materials in a way that allows them to function as a single component structurally. Flitch beams are created by layering wood beams with steel plates or plywood in order to form a wider, lighter structural beam. Bolts hold the layered components together to form a single unit.

These advantages make flitch beams a desirable and attractive choice in light frame construction projects:

  • they can support heavier loads over longer distances
  • are thinner than solid wood or steel beams with similar load-bearing qualities
  • can be nailed to other components of wood structures during construction
  • are much lighter than solid beams.

Using computer software to design a flitch beam can greatly improve the cost effectiveness of a project by allowing for a more exact and efficient design.  Software packages precisely calculate the needed thickness, depth, and length of each beam much easier than any kind of hand calculation.

Utilizing flitch beam design software eliminates the possibility of using beams that are too thick or too closely spaced together.  This can drastically reduce construction costs by allowing each beam to be more fully utilized to its capacity.

One of the more difficult calculations associated with a flitch beam is that of the Deflection the beam will undergo.  Software packages will carefully calculate the deflection of flitch beams.  Properly constructed flitch beams ensure that all of the components deflect by exactly the same value. The relative stiffness value of steel and wood is vastly different.  When used correctly, structural analysis software will accurately determine the proper interaction of multiple materials.

Bolt size and Spacing in the construction of flitch beams is crucial. A separate article will briefly discuss a simple way of determining the bolt spacing for a flitch beam.

Flitch beam design software is a must-have tool for the careful architect, engineer, or designer.

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Structural Engineering

Structural Engineering: the Basis for Residential Dwelling Construction

Structural engineering is a complex process that is vital to the construction of any residential dwelling. The knowledge base of the structural engineer aids in the calculation the values and measurements of the construction materials, their placement within the structure, and the types of materials selected for the project.

Calculating Values
Calculating the necessary measurements and properties of construction materials for a residential dwelling is a complex and time-consuming process.

For example, when calculating the necessary size of I beams in residential buildings, you must consider multiple factors:

  • maximum bending moment of the beam
  • maximum deflection of the beam at the center of the span
  • width, length, and depth of the beam
  • moment of inertia
  • constant psi rating for the material the beam is comprised of

You must also consider the exterior dimensions of the home, the span if the beams and floor joists the dead and live loads for the structure, and the design style of the roof. If any interior walls will support the weight of the roof, this will affect the necessary I beam size throughout the residential dwelling.

Required calculations for determining the size of a residential steel beam include the allowable bending stress for structural steel, the moment of inertia, and the section modulus of the required beam.

A structural engineer generally performs these calculations. Certain computer software designed specifically for the calculation of

  • beam design
  • floor beam span
  • rafter design
  • header size and span
  • floor joist load
  • cantilever floor joist load
  • residential I beam spans

are useful for quickly calculating these values.

Determining Building Materials

Steel and solid sawn wood are the traditional construction materials used in creating structures. Modern technology has resulted in the emergence of new composite materials and combinations of natural materials that improve the construction process, cost, weight, strength, and stability of a structure.

Solid sawn wood, structural composites, tube steel, solid steel, glulams (glued- laminated timbers), manufactured beams, and I joists are all used in differing combinations during the construction process.

Since each of these materials behaves differently under the stress of a load, calculating the required measurements and values for construction becomes even more complex. The physics behind the effects of weight, wind, water, temperature, and snow directly affect the construction process.

Residential dwelling construction requires a broad knowledge base, prior construction experience, and an understanding of the physics related to the construction materials and the forces that affect them.

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Normal Stress, Bending Stress, & Shear Stress

Stresses in Beams
In a separate article entitled “Structural Analysis of a Beam” there was a brief discussion of stresses and their function in structural analysis.  In this article there will be a more in dept discussion of normal, bending, and shear stress.

Normal Stress
A normal stress is a stress that occurs when a member is loaded by an axial force.  The value of the normal force for any prismatic section is simply the force divided by the cross sectional area.

Normal Stress

A normal stress will occur when a member is placed in tension or compression.  Examples of members experiencing pure normal forces would include columns, collar ties, etc.

Bending Stress

Beam Design

When a member is being loaded similar to that in figure one bending stress (or flexure stress) will result.  Bending stress is a more specific type of normal stress.  When a beam experiences load like that shown in figure one the top fibers of the beam undergo a normal compressive stress.  The stress at the horizontal plane of the neutral is zero.  The bottom fibers of the beam undergo a normal tensile stress.  It can be concluded therefore that the value of the bending stress will vary linearly with distance from the neutral axis.

Bending Stress

Calculating the maximum bending stress is crucial for determining the adequacy of beams, rafters, joists, etc.

Shear Stress
Normal stress is a result of load applied perpendicular to a member.  Shear stress however results when a load is applied parallel to an area.  Looking again at figure one, it can be seen that both bending and shear stresses will develop.  Like in bending stress, shear stress will vary across the cross sectional area.

Shear Stress

Calculating the maximum shear stress is also crucial for determining the adequacy of beams, rafters, joists, etc.

Final Considerations
When doing any kind of beam design using structural design software will greatly ease the entire process of calculating stresses.  There are several different engineering design software packages available for beams, columns, or foundation design.  StruCalc, Enercalc, Risa, and BeamChek will all take in to account normal and shear stresses when doing any kind of beam design.

Josh Parker, E.I.T.
Cascade Design Group

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