![]() ![]() 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). If your build has a permit inspector, then the first thing to do is check with that office and verify that your plan change is allowable.Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. If your porch is bigger or you have more snow in the area then a bigger beam or better quality wood will be required. Based on my assumptions and further assuming all my calculations are correct, it looks like a 4x10 will be adequate for that span. Your architect designed for the specific values for your area and those should be noted within the design documents. For a porch 8' deep and a beam span of 16', that equates to a design load for the beam of 1920 lbs. Assuming 10 psf and 20 psf respectively, that is 30 lbs per square foot. Multiply that by the design load, which is the sum of the dead load and live or snow load. Multiply half the porch depth by the beam span to get the area. The house is holding up half of the porch roof, and the beam the other half. The load that the beam must carry is determined by the depth of your porch and the design load for your area. Using that value and a width of 6" and a depth of 10", the max load is 3140 lbs.įor reference, a 4x8 beam of EWP #2 spanning 90" like in the original design has a max load of 2840 lbs. A 6x10 beam consisting of three 2x10 nailed together has Fb of 727.35 psi. Using a built-up beam of three or more stringers nailed together allows a 15% increase in the bending fiberstress. Using that and a depth of 12" the maximum load for the beam is 2610 lbs. Going through the same process you will find that the maximum load for the 4x10 is 1970 lbs.įor a 4x12 beam the shape factor is 1.1, resulting in a Fb of 632.5 psi. Update that value in the calculator, and set the beam depth to 10". You will soon arrive at the max load for the beam of 1375 lbs.įor a 4x10 beam of #2 EWP apply the size factor of 1.2 to arrive at a Fb of 690 psi. Add 100 lbs to the load and click Show Result. Now input a load of 1000 lbs in the top field and click Show Result. Input these values and keep them constant: Span 186", width 4", depth 8", fiberstress in bending 747.5 psi, modulus of elasticity 1,1 million psi, and allowable shear 135 psi. Now go to the beam calculator and plug in the relevant numbers. ![]() For a 4x8 beam (assuming rough cut so using full 4" x 8") the F b size factor is 1.3. Table 4A in the NDS Supplement has adjustment factors that need to be considered. These are base numbers that we may need to modify depending on the manner of beam chosen. ![]() For EWP #2 those values are 575 psi, 1.1 million psi, and 135 psi. The values we need to know are for Fiber stress in bending (F b), modulus of elasticity (E) and allowable horizontal shear (F v). Table 4A gives the reference design values for Eastern White Pine. The engineering properties for graded lumber species are codified by the American Wood Council. ![]()
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