This translation is the key to any structural sizing problem. In beam-speak you say: this header must carry X-pounds per lineal foot. The first step is the same for sawn- and engineered wood materials: add up all the loads acting on a header or beam and then translate this load into terms of how much load each lineal foot of header or beam will feel. Here is a simplified approach that will help you specify the appropriate material for many applications. However, the process for sizing these structural elements can be complicated if you are not an engineer. The beam must be strong enough so it doesn’t break (Fb value) and stiff enough so that it doesn’t deflect excessively under the load (E value). The idea behind sizing headers and beams is straight-forward: Add together all live loads and dead loads that act on the member and then choose a material that will resist the load. They transfer loads from above to the foundation below through a network of structural elements. The job of headers and beams is a simple one. Part II will review sizing procedures, performance and cost of these materials for several applications (see “ Sizing Engineered Beams and Headers” for part 2). Part I will show you how to trace structural loads to headers and beams. In this 2-part series we will review how sawn lumber and these engineered materials measure up as headers and beams. Parallam, Timberstrand, Laminated Veneer Lumber and Anthony Power Beam are examples of alternative materials that provide builders with some exciting choices. ![]() Sawn lumber limits design potential and in some cases just doesn’t work. You can’t beat sawn lumber for most small window headers, but as spans and loads increase, stronger materials are a better choice. Too often builders gang together 2-inch dimension lumber to support roof and floor loads without considering other options. A neat solution, but is this an efficient and cost effective use of material? The same is true for beams like structural ridge beams and center girders. These headers work to support most residential loads and coincidentally keep the window tops to a uniform height. Most builders automatically choose double -2 x 8 or -2 x 10 headers to frame windows and doors in every house they build. ![]() ![]() Understanding how loads are transferred through a structure and act on structural members is the first step to sizing headers and beams Some information contained in it may be outdated. I agree that the flat-wise 2x4 purlins are outside the scope of the prescriptive design allowances I'm aware of and would likely need to be eliminated to prescriptively design the roof in the OP, in favor of one of the prescriptive options per Table R803.1 or Table R503.2.1.1(1).Īs to the rest of your questions, those are all things you need to know to apply the span tables found in the prescriptive allowances.Please note: This older article by our former faculty member remains available on our site for archival purposes. Use of span tables from the WFCM is allowed per R301.1.1, although I haven't checked if it has an prescriptive tables that would be helpful for 48" o.c. Which makes it clear that if you double the member spacing and halve the load, you'll get the same load per unit length and the same allowable span. And Appendix A.3 of that document tells you that the allowable span is based on, among other things, the load per unit length on the rafter, which it says is based on the psf loading and the member spacing. ![]() That document provides rafter span tables for loading up to 20 psf dead and 50 psf live, with a choice of L/180 or L240 deflection criterion. OK, but 2021 R802.4.1 says " For other grades and species and for other loading conditions, refer to the AWC STJR."
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |