![]() Builders can quickly choose a standard rafter size from tables that works for the building width. The tables indicate the maximum distance the rafter can safely span before additional support is needed. They can have 1 or 2 harp points (king or queen post style).Table R802.5.1(1) – Allowable rafter spans for common lumber species (ft-in)Įxcerpt of Rafter Span Table from 2018 IRC (Credit: Woodbin) Puts the ends of the cables at the top of the glulam and harp the cables to the bottom near center span. I have many times used steel plates across the ends of the beams with threaded swaged ends on the cables to tension them. Anchorage of the cable ends is usually the tricky part, it's hard to get enough bolts in the end anchorage to develop the cable. Typically I tension them to not exceed the dead load on the beam, so as to not put tension in the top or compression in the bottom that would cause lateral instability. I have post tensioned glulams before with cables. Other problems for glulams from the 60's include the use of casein glue (which softems in moist environs) and scarf joints (which can separate). ![]() Typically beams were cambered for 1.5 x dead load delta, figuring that the. RE: Permanent Glulam deflection cause Triangled (Structural)ġ" seems like too little camber to begin with. And, compare the starting and final deflection of the beams as you do the work. Apply face grain perpendicular to framing, stagger panels and nail with Bd Per CBC Table 23063.1, uno. You might also jack the existing beams up while you a tightening the turnbuckles. load tables based upon roof live load and framing spacing. should show if that’s a reasonable trade off. fibers, while still adding a small P/A stress. But, if you harp the rods from near the beam top at the two bearings, to a few inches below the beam at about the third points, maybe 12' from both bearings and 10' in the middle, maybe with another harp point at the center line you do reduce the bending normal stress on both the top and the bot. Yes it adds a small P/A compression component. I would look further at post tensioning, that was not bad first thought. Usually, some detailing oriented the beams, t&b. plys were a better grade than the interior plys, but those outer plys were likely the same grade. That came along later than the early 60's. The 4x4 purlins are attached to the beams somehow and would offer some stability, it’s been standing for 50 years and doesn’t show any instability, does it? In the early 60's I don’t recall GlueLam beams which had different Lam strengths on the top than those on the bot. isn’t a product called ‘Tectum Decking?,’ it was a wood fiber decking product which would span 4', or a like product. Who was the original engineer, any plans there or at the local bldg. Maybe he was smart enough to save a copy of the plans and specs., check and see. owner for more than 50 years, and smart enough to ask questions when the thing was being built, and now again. I'd appreciate your further thoughts on this one. I am suspecting that the contractor installed them upside down explaining the immediate ponding, then the beams deflected another inch or so just like the engineer anticipated.Įverything is painted so I cannot see "TOP" stenciled on the beams, but then again, I don't know that laminators stenciled "TOP" in those days, and also, I suppose there was a reason why they always (I hope) stencil "TOP' on laminated beams now. I am suspecting that the design team never designed for drainage properly, or the contractor didn't follow the plans, and that the engineer specified 1" camber just to level out the exposed beams after dead load deflection. ![]() He therefore made the contractor install that drain in the center of the roof, which is also visible in the center of the open ceilinged room from below. He said that upon completion of the roof, he flooded it with water and it immediately ponded. I spoke with the owner who also was present when this building was built in the early 1960s. There is a roof drain visible from the inside, in the center of the room ceiling, elbowing then extending along the underside of the ceiling to the outdoors. There is on obvious spot, perhaps 4'圆' oblong in one portion of the ceiling, somewhat towards the center. The fascia all around creates a nice lip to increase the depth of a pond by about 1". The entire roof is about 40' wide x 56' long with four roof drains located about 6' in from each corner. The bearing point at one end of each beam is level with the bearing point at the other end of each beam. The 4.25 x 16.25 glulam beams are approximately 40' long, approximately 34' from bearing to bearing with 3' cantilevers each end. The typical beam deflection is 2.5" not 4" as reported before.
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