On the Beam
‘Every single project I design and build,’ wrote David Tisherman near the top of his Details column in July 2005, ‘is fully, individually engineered, and I refuse to make any assumptions on my own about what might be needed in a set of plans to create a sound structure. If any builder anywhere thinks that he or she knows enough to get by without support from a structural engineer, well, that’s just asking for trouble.’
‘I know what I don’t know, frankly, and I sleep well at night knowing that my plans begin with their work and always move forward with their ongoing support.’ He continued:
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‘One of the big areas where builders who don’t work with competent technical support will often lose their way – and rightfully should be losing some sleep – comes when they do their own “engineering” with remodeling projects.’
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‘Time and again, I’ve seen builders add spas to swimming pools (one of the most common of all remodeling requests) by simply doweling a few pieces of steel into the top of the bond beam and then building a spa on top of it. Anywhere from six months to six years later, such a spa will almost invariably crack. Most often, the problem is that the builder didn’t allow for the proper overlapping of steel from the existing structure or, even worse, relied exclusively on a bit of doweling.’
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‘Adding something like a spa to an existing structure isn’t that tricky if you have the right plan and stick to it during construction. I guess it’s this simplicity that inclines builders to approach the work too casually.’
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‘Nowadays, many of the spas added to pools are simply set inside the existing shells. It’s a nice approach because it conserves space and makes the spa look more as though it was part of the original construction. It isn’t that difficult to do, either. The problem here is visual: When you pour the new wall for the part of the spa that butts up against the existing shell, the easy way out is to create a portion of the pool/spa wall that will be twice as thick as the walls of the rest of the pool. This creates a visual interruption that, to my eyes anyway, is not the cleanest possible look.’
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‘[T]here’s a [better] visual option. . . . It’s more complicated structurally, but what [you do is] cantilever the spa wall over the existing bond beam, thereby preserving the appearance of a wall with uniform thickness.’
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‘It’s more work to do it this way, of course. The first step involves demolition of the top of the bond beam and lowering of the existing wall down to the needed level. That level will vary from project to project, of course, and obviously you’ll take the wall down further with a pool-level spa than you would with a raised spa. Chipping away the top of the bond beam and leaving it rough (but clean) has the added advantage of providing a good mechanical bond with the new spa wall.’
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‘The next step in the process is forming the spa. This . . . is all about establishing a surface that will withstand the pressures exerted by shooting gunite or shotcrete up against it.’
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‘For all of the importance of proper forming, however, it doesn’t mean a thing unless the project is properly engineered. In this example, the engineering details specify the extent of the overlap for the steel between the existing structure and the new spa; the thickness of the floor; and the layout of the steel structure.’
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‘This plan,’ David concluded, ‘rolls forward through the entire construction process, shaping our thoughts about the plumbing and where the lines will run . . . to serve the needs of the new spa. It also sets the tone for the electrical system, spillway design and numerous tile details.’
‘With this information in hand and application of proper construction practices, there’s virtually no chance that the structure will fail. . . . In this case, a cantilevered wall that was created with aesthetics solely in mind is possible only because it could be reliably engineered.’
Where do you come down in the every-project-engineered discussion? Do you see it as practical overkill, or is it something without which you wouldn’t even consider tackling a project? Please share your insights by commenting in the space below!
David Tisherman is the principal in two design/construction firms: David Tisherman’s Visuals of Manhattan Beach, Calif., and Liquid Design of Cherry Hill, N.J. He can be reached at [email protected].