I see time and time again a post with pictures of a project in process with a question like “Is this wall load bearing, can I take it out?” or any other structural or mechanical question for that matter that would not have had to been asked if they didn’t skip one little step, “The Permit Process”. You (or someone else) draws up a plan for a proposed project submits that plan to their local building department and they will either approve the plan, reject the plan or say the 3x8’s won’t do, revise to a 3x10 and we’ll approve. That’s why we have codes in the first place and that’s exactly why we have the “Permit Process”. The last thing anyone wants is for a project to fail in any way that could result a failed inspection when trying to sell, creating more problems in the end than you started with or even worse, getting hurt. Well since it’s clear that you guys will always give the best information you can on a subject to ensure that the DIY’R competes his or her’s project up to industry standards and code, that’s great, I completely agree. I leave that to the unlicensed folks who have no professional license at stake. So occasionally I will offer what I believe to be factually accurate information that could be used by an individual to help with their design or installation. I only design projects when I have a contract with the person for whom I am doing the design, and I only take on a contract when I can personally visit the site. So I avoid designing projects for individuals on this site, since I would be potentially liable for the consequences of my actions, even if it were as simple as the individual misunderstood the instructions and improperly installed the beam. Additionally, I am subject to penalties for errors and omissions that do not apply to unlicensed individuals offering their opinions on chat groups such as this. As a licensed professional, I have obligations and am subject to laws and regulations that do not apply to unlicensed individuals. I have a license as a professional engineer in multiple states. Since you ask, I will tell you why the Engineer does not design the job for someone on this site. There is more to the design than simply sizing the beam, there are connection details and an analysis of the flex of the beam that should also be done at the same time the beam is evaluated for strength. Any structural engineer worth a nickel can tell you based upon an inspection of your site and a review of the loading what size beam you need, or in your case how much you can span with a specific size beam. If none of this makes any sense, let me make it a little simpler. Deflection is related to the fourth power of the span length, while maximum fiber bending stress is related to the square of the length, so neither factor scales directly with the beam moment of inertia. Maximum span is governed by the more restrictive of the deflection of the beam at the center, or the bending strength of the beam. The allowable span for a beam is governed by the moment of inertia of the beam (depends on the size of the beam), the maximum allowable fiber bending stress (depends on the type of wood and the grade), the modulus of elasticity of the material (depends on the species of wood), and the loading (dead plus live load, often governed by code). No, you cannot multiply the length by 1.5.
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