Deeper is Cheaper
The marketing group at RedBuilt™ has been doing some Blog soul searching because when we talk about our blogs, they always seem to have these words associated with them: “epically long” and “epically technical.” Although we do like the idea of being epic, we would rather be epic in other ways like epic customer experiences. So, from now on, we will “try” to summarize in our blog the highly informational, well written, and thoughtful articles that our associates have poured the hearts and souls into with a direct link to the full blown “epic” versions for your perusal on our website. You can find them at www.redbuilt.com under “Articles” in the “Resources” drop down. So, following our “epic summary” above (oxymoron, we know) here is our first summarized article: This week’s blog summary is on Deeper is Cheaper by Dwaine Charbonneau, P.E. at RedBuilt™. Click here for the full length article. When you think of the price of an Open-Web Truss, you most likely think that if a truss is deeper, it will be more expensive (like an I-Joist). However, if an Open-Web Truss is deeper, it is (most often) cheaper. Please take a moment to familiarize yourself with the terms for each part of an Open-Web Truss as depicted in the picture (below), and then read on… “The primary factors that differentiate the costs of a shallow and a deep truss, given the span and loads, are the chord grades, pins, and labor. Bearing clips are the same regardless of depth, but the rest of the materials and labor are subject to quite a bit of variability, depending on a number of options available to the truss designer and his or her trusty design software.” (paragraph 6) Top and bottom chords: The material required for the bottom and the top chord can affect the grade of lumber (and therefore the price) because “The bottom chord typically supports minimal direct-applied loading – maybe some ducts, acoustic ceiling, or insulation – leaving the top chord to shoulder most of the load. Nevertheless, wood is usually weaker in tension than in compression parallel to grain, so the bottom chord may indeed become the critical design control” (paragraph 8). The top chord supports the supplied loads as well as the bending of the chord between the pins and the threat of buckling instability. These factors decide what grade of chord material will be used for an Open-Web truss and this is one of the ways in which deeper can be cheaper, “Engineers call this combination of bending and compression interaction or combined stress. To deal with it, the designer must again select the appropriate chord grade, but there is one more variable to consider: panel length. As the distance between top pins is shortened, the bending stresses and the tendency to buckle both decrease, and the chord is better able to handle the compression” (paragraph 12). Steel Tubes (Webs): “There is a practical limit to how tight the webs can be assembled. The minimum angle between the webs then determines how short the panels may be. This is one of the characteristics often found in a truss that is “maxed out:” lots of webs and closely-spaced pins” (paragraph 14). Steel Pins: If you choose a deeper truss, there will be longer panels which means there will be fewer panels and therefore, less pins. Truss Manufacturer’s Labor: If there are less panels, there is less time spent on drilling, routing, pressing and installing webs and pins. Overall, “Make the truss deeper. The chord grades drop, the number of pins is minimized, and the assembly is a breeze, relatively speaking. That’s how you save money on Open-Web trusses. There are other benefits that come with greater truss depth. It is much easier to run utilities through a roomy web layout as opposed to a cramped layout. And truss stiffness is roughly proportional to the square of the depth, so any increase in depth really pays off in minimizing deflection and improving floor performance” (paragraphs 21 & 22). Piqued your curiosity? Then click here to read the whole article. If you have further questions, contact your RedBuilt Technical Representative. Thanks for reading! We hope you enjoyed it.
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