Posted by Sherman in Idaho [24.32.202.166] on Thursday, July 11, 2013 at 19:19:14 :
In Reply to: Re: Question for all the engineers out there posted by Jerry in Idaho [24.223.94.244] on Thursday, July 11, 2013 at 17:46:41 :
With beams and posts, it's worth doing the calculations. They are simple and the results are often not what "seems" right. Especially around here with the potentially high snow loads, it really is good to know that you've spent the money on wood in the places where it will do the most good. I got involved in the checking of the design for a shed where the contractor had already decided to use 2x12 rafters resting on 8x10 purlins. Turns out when I ran the numbers, the purlins were over-stressed and the rafters were over-sized at 50 psf snow load. He could have used 2x10 rafters but should have used 10x10 purlins. The purlins probably won't break, but money was wasted on the oversized rafters. I usually use 1000 psi max fiber stress for structural lumber, which is on the low end, and around here at least 60 psf snow load, even though the building inspector only requires 40 psf. At the mine in Mullan, the engineers design for 100 psf snow load, even though the building inspector doesn't require it, because they think they could easily get that much and it's cheaper to build the roof extra strong than have to send guys up to muck it off if we get a heavy snow. I used 100 psf for the last shed I built on my land, because I've been there when there was 7' of snow in April, so dense that you could walk on it without snow-shoes. Built it all out of natural log poles, but made sure to do the calculations so I had good reason to believe it would carry the load.
Engineering isn't just about making it strong enough. It's also about making it as cheap as possible, and still strong enough.
The weak spot in that bracket is probably at the point that's subject to the most bending force. Around the hole, it's all compression, so not as likely to break there. Of course nowadays, even with a complex shape like that, the computer would tell the engineer exactly where the stress would be the most, and he would move metal around as needed in order to have everything stressed just exactly to the max. Back then, cast iron was cheap and of unpredictable quality much of the time, so it was better just to make everything a lot heavier than need be.