![]() 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). Protect yourself, get the professional services, it's $ well-spent.Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. This is typical of what they may find: contractors substituting cheaper fasteners or structural materials which are not rated for the level of weather exposure or load can cause the entire load-bearing system to fail, possibly with tragic results. In the event of a failure of a structural component, the plaintiff attorneys supoena the design documents, including the plans review, the inspections, product approvals for the components right-down to the fasteners, and then a forensic engineering team reviews them with compliance to the building code at the time of construction. I am a plans examiner and a Life Safety Code inspector, and I've seen a lot of stupid, dangerous things done by amateurs and professionals. ![]() Considering what's at stake, you don't need a catastrophic failure so using the services of a structural engineer is a matter of safety for you and anyone around the structure. Yes, you may be able to use the free design skills of your local supply store, but here in Florida, you don't get a permit for structural work like that without having a professional engineer (P.E.) submitting sealed plans for the structural calcs. I adapted a 'cherry-picker.' I'll post that up in a new thread. I even went as-far as-to buy a short-web I-beam with the idea to bolt it to the underside of the box beam at the ridge, but I discovered that between the I-beam girder, the trolley, and the rigging, I would lose precious headroom, and be unable to lift much over 3' in height onto the loft deck. If it was an I-beam girder I could use an overhead trolley on the lower surface flange of the beam, but due to the box construction, that was out. One concession I made to getting heavy things up & down to/from the loft, I thought about what would make an easy load-lifter? The roof ridge beam is also a 1/2" wall thickness box beam with plates welded to it for the ceiling joists. For access, I use an A-frame ladder, but since it's used for storage, it's not an everyday trip I make. The structure has no stairs to access the loft, not-even an attic 'pull-down' stairs. The beam is sandwiched on both sides by a 2" x10" (ripped to 8" to match the box beam dimension) wood beam to allow easier attachment of the loft floor joists, which are 2" x 8" & 18" o.c. ![]() The engineer did the calcs and gave us a front-of-the loft steel box beam, 1/2" wall thickness, 4" x 8" welded into steel flitch plates cast into the masonry side walls. We used a structural engineer, and I asked for a capacity capable of supporting the weight of multiple motorcycles. The garage is a nominal 20' x 22' & the loft is a nominal 13' x 22'. That's all we could afford to do, we kept the same roof instead of bumping it higher. I had built a loft which isn't full height above my two-car garage. ![]()
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