![]() For a floor system you can find the individual weights of drywall, strapping, floor joists, subfloor, underlayment and carpet in an architectural handbook like Architectural Graphic Standards. To determine the dead load value for a given floor or roof system, the weight of all permanently installed materials in a given component are added together. Unlike stiffness, live loads and dead loads are added together to determine minimum design values for strength. Joists, and rafters must be strong enough not to break when loaded. Strength of a material is obviously important. Attic floors with limited storage L/240 & 10 psf.Bedrooms and habitable attic floors L/360 & 30 psf.Examples of code-prescribed deflection limits and live load values are: These limits are based on live loads and activities experienced in specific rooms of a house. Typical deflection limits referenced in code books are L/360, L/240 or L/180. Drywall attached to the underside of this system is not expected to crack when the floor joist system deflects 1/3″. ![]() ![]() For example: a floor joist appropriately selected to span 10 feet with an L/360 limit will deflect no more than 120″/360 = 1/3 inches under maximum design loads. They are expressed as a fraction clear span in inches (L) over a given number. Maximum deflection limits are set by building codes. Only live loads are used to calculate design values for stiffness. In other words, how much a joist or rafter bends under the maximum expected load. Stiffness of structural members is limited by maximum allowable deflection. Perhaps the joists were strong enough if they didn’t break! But lack of stiffness leads to costly problems. For example, first-floor ceiling plaster would crack as occupants walked across a second-floor bedroom that was framed with bouncy floor joists. Strength and stiffness are equally important. Beams, studs, joists and rafters act as a structural skeleton and must be strong enough and stiff enough to resist these loads. The house acts as a structural system resisting dead loads (weight of materials), live loads (weights imposed by use and occupancy), like snow loads and wind loads. This article will focus on how simple beams like joists and rafters react to loading. If, when the loads of the house are combined, the house weighs more than the soil can support – the house will sink until it reaches a point at which the soil can support the load. Remember when your science teacher said: every action has an opposite and equal reaction? Well every building load has an equal “reaction load”. The structural goal of a house is to safely transfer building loads (weights) through the foundation to the supporting soil. A complete analysis of wood’s mechanical properties is complex, but understanding a few basics of lumber strength will allow you to size joists and rafters with the use of span tables. Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base (foundation), supports the weight of its branches (column) and bends as it is loaded by the wind (cantilever beam). Downloads Access our comprehensive MiTek media library here.Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use.Vacancies Looking for a career in the roofing industry?.Dedicated Services Grow your business with our highly skilled resources.Software Support We’re dedicated to resolving your issues to ensure your productivity.Machinery Support Our Machinery Division provides the best customer support in the industry.Training Services At MiTek, we offer a range of SAQA accredited courses.Engineering Services Our engineers lead the industry in credibility, accuracy and efficiency.Machinery MiTek’s machinery leads the building component manufacturing industry.MiTek Pamir Software PAMIR is the industry leading roof truss, Posi-Joist & panel design software.Posi-Rafters Posi-Rafters combine the lightness of timber and strength of posi-strut steel webs to provide a quality roof system that can span further than alternative timber products.Learn more about the many benefits and find a licensed fabricator Posi-Joist MiTek manufactures the steel webs used in Posi-Joists.Ultra-Span Whatever your needs, MiTek’s Ultra-Span has got you covered.eCo Fasteners eCo Fasteners set the benchmark in timber and steel construction.Connector Plates MiTek is the world’s leading supplier of connector plates.Timeline Founded in 1955 and a Berkshire Hathaway company since 2001.The Company MiTek is the industry leader in every market we serve worldwide.
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