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| Re: G Train: Present and Future (Opinion Piece) (1150031) | |||
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Re: G Train: Present and Future (Opinion Piece) |
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Posted by J trainloco on Tue Apr 10 13:48:40 2012, in response to Re: G Train: Present and Future (Opinion Piece), posted by Jackson Park B Train on Mon Apr 9 23:17:29 2012. the header like a beam in post and beam buildings eliminates the need for closely spaced vertical supports.Yes, and your sentence hints at the difference. Headers transfer horizontal loads from those closely spaced (or in the case of a bearing wall, completely continuous) structural members while beams and girders do not. Even when one uses a transfer beam, it generally has one or two point load(s) on it. the comment about horizontal increase was an imprecise way of suggesting that if for instance one needed to have a 4x12 for the span in question but no more than 8" vertical space one might well use a couple 8x8 bolted together to form a sufficient beam for the span. Not imprecise at all, you were pretty clear. The principle you mention starts to become ineffective when you transition to greater spans. You HAVE to make the members substantially deeper with larger spans, simply sistering more members won't do the trick. This is one reason why steel columns in buildings are generally heavier but beams are deeper. In all of these cases,the horizontal mem not only supports whatever is above but also ties together the vertical walls or post on which it rests. A header is typically not about tying vertical structure together. It's about transferring a load over an opening, except in the instance of top plates. In a brick wall, the brick wall is already tied together quite nicely by its mortar joints, even when an opening is made in it. I did work on a remodel where we used trus-joists "R" in order to support a new upper floor solely from the reinforced side walls. They were so new (1979) that the company sent out an engineer to educate/placate the building inspector. I was somewhat dubious on first look,but AFAIK no damage was reported after the Loma Prieta quake a decade later. Trusses seem to intuitively be fragile, but in structures, shape is often more important than size. The biggest problem with trusses is that since there's so little material, it heats up quick, so they perform poorly in fires. |
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