![]() ![]() 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). Download the design software here.Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. For over 10 years the program has been under progressive and continuous development of engineering design techniques and innovations.įor novice or professional, NP Design is free, quick and easy to learn. NP Design is developed by leaders in engineering design software. NP Design assists with certified specification of engineered wood products in common applications. ![]() NelsonPine® Design (NP Design) is a powerful design program for a range of user types, from builders to structural engineers and offers a range of different building levels to accommodate various user competencies. Sustainably Managed Timber NP Design – The Design Software for NZ and Australian Standardsįast, accurate beam specification for any project, domestic or commercial. In our new ‘post-quake’ consciousness, it is the building material of choice”– Professor Andy Buchanan, University of Canterbury. “Technically, LVL is one of the most sophisticated wood products ever developed for the construction sector. NelsonPine® LVL products inspire Architects, Designers, Builders and end users to realise structural applications. Our range of LVL products are designed to span further and weigh less to optimise efficiency in design and construction. Our NelsonPine® LVL is engineered from renewable and sustainably sourced New Zealand radiata pine. LVL is manufactured to achieve consistent strength, stiffness and stability. Built up with a specific sequence of structurally graded veneer sheets, the finished product displays less variability, a higher strength to stiffness ratio and more predictable performance than sawn timber. NelsonPine® LVL provides a consistent, high-performance alternative to solid lumber and steel in structural uses. Our products are used across residential, commercial, industrial, formwork and scaffolding applications. NelsonPine® LVL is available in a variety of sizes and lengths. NelsonPine® LVL is a structural engineered wood product manufactured from rotary peeled veneers, assembled with parallel grain orientation and bonded with an exterior structural adhesive. ![]()
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