... interestingly, the Code does not REQUIRE us to account for creep in our deflection calculations, per se. But, if we do account for creep, the Code references a way (e.g. the calcs we have been doing in class). It actually puts it back on the designer ... "If long term deflections are to be limited ... "
You, the architect, will need to judge the effects of long term deflection, and then design (or have us engineers, design) accordingly.
Unless there is a strong reason NOT to include the effect of creep - I generally include it.
Doc
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3 comments:
I love your blog. I wish I understood more the calculations but will ask a simple question I hope.
If most construction is designed for a L/360 floor deflection and natural tile installations require L/720 is there a general rule of thumb that can be applied to stiffen these floors up?
ie would basic framing on 12" center achieve this or is the formulations to many?
I have not heard the term Creep before. I will look more into this.
Thanks for your posts!
JW
well, here goes (and thanks for the compliment!): L/360 and L/720 refer to deflection limits (you probably already realize that). Thus, the L/720 limit is `limiting' the deflection to half that of the L/360. Think of a floor (or a floor joist) as acting like a spring. To reduce the deflection by half we either need to: 1) reduce the load by half, or 2) double the stiffness. The easiest way to reduce the load by half is to DOUBLE the number of joists. So if a certain size joist size spaced at 16 in. o.c. could handle L/360, then that same joist at 8 in. o.c. could handle L/720. Easy fix? Yeah, at double the number of joists!!! So, let's double the stiffness. The joist stiffness is measured by `I' or `EI'. So if you like the spacing you are at with certain joist that meets the L/360 limit ... find a joist with double the EI and it will meet the L/720 limit at the same spacing. Doubling the I, or EI, usually involves adding inches to the floor depth (though not doubling it). In a lot of situations additional floor stiffness is achieved by a combination of stiffer joist and closer spacing.
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