Deflection

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Author Message Curt Norton, CSI, CCS Senior Member Username: curtn Post Number: 154 Registered: 06-2002 Posted on Sunday, August 16, 2009 - 05:37 pm:    I have seen requirements in structural specifications that limit deflection to 1/180. I can't figure out what this refers to. I'm familiar with L/180 where L is the number of inches of a span. Can someone explain this to me? I would have assumed it was a typo, but I've seen it in commercial specifications that way too. Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 786 Registered: 03-2003 Posted on Sunday, August 16, 2009 - 05:46 pm:    Curt, check to see if they're using a lower case "L" instead. Sometimes it can look very much like a number 1. And, maybe it originally was a lower case "L" and during the process of revision and retyping, it was inadvertently changed to a number 1. Ron Geren, AIA, CSI, CCS, CCCA, SCIP www.specsandcodes.com Curt Norton, CSI, CCS Senior Member Username: curtn Post Number: 155 Registered: 06-2002 Posted on Sunday, August 16, 2009 - 06:35 pm:    Ron - here is a note in Speclink: "1/360 is the maximum deflection allowed by the ICC Building Code, 2006. More stringent deflection limits might be necessary for particular finishes on the under side of the floor." In the section there are three or four different references to the same and all certainly look like a 1 and not an l. Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 787 Registered: 03-2003 Posted on Sunday, August 16, 2009 - 06:49 pm:    Then it appears that someone at BSD can't tell the difference between an "l" and a "1", because the IBC clearly shows a lower case, italicized "l" (Ref. Table 1604.3). Ron Geren, AIA, CSI, CCS, CCCA, SCIP www.specsandcodes.com Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 192 Registered: 10-2007 Posted on Sunday, August 16, 2009 - 10:39 pm:    I concur with Ron. Sheldon Wolfe Senior Member Username: sheldon_wolfe Post Number: 352 Registered: 01-2003 Posted on Sunday, August 16, 2009 - 11:40 pm:    Either is correct, and both means of stating deflection limits are used in many references. I have been using the form "1/180" with the digit one rather than a lowercase L for many years. You can say the deflection is L/180 (the length divided by 180), or you can say the deflection is 1/180 of the span. Using 1/180 alone, instead of "1/180 of the span" is technically incorrect, but no more so than expressing R value, perm rating, or any other characteristic without including the dimensions. For example, I suspect most specifications say simply "R-13" rather than "R value of 13 ft²·°F·h/Btu." Similarly, simply stating "L/180" is equally incorrect, as L is not defined. Incidentally, I have seen ARCOM specs with the deflection expressed in the same way as BSD, and I have seen it expressed both ways in a single section. (Unregistered Guest) Unregistered guest Posted on Sunday, August 16, 2009 - 11:32 pm:    come on guys, its called basic math, it can be either way, they both mean the same thing when referenced correctly. Length/360 is the same as 1/360 of the length. Why use the 1/360 of length if it is longer in 'words' - because it does not use an abbreviation where the lower case l may be unknown to the reader, or where the lower case l may be misunderstood. The mathematical expression of 1/360 of the length leaves no doubt. I would not use italic either as I use that to denote revised text if the section is reissued. William Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 788 Registered: 03-2003 Posted on Monday, August 17, 2009 - 01:08 am:    William, I agree, but the form BSD used, as quoted by Curt, should have used "L" or "l" rather than "1" because they never made reference to length. The way it is currently presented is incorrect. As for italics, I never use it in specifications; I was just indicating that the IBC used an italized "l" for length; which, incidently, is not defined by the IBC, but is accepted common practice. MasterSpec actually uses both forms--even within the same paragraph: L/175 and 1/175 of clear span. Ron Geren, AIA, CSI, CCS, CCCA, SCIP www.specsandcodes.com Scott Mize Senior Member Username: scott_mize_ccs_csi Post Number: 10 Registered: 02-2009 Posted on Tuesday, August 18, 2009 - 01:24 pm:    Allow me to clarify our [BSD's] position on the issue you raise: If the term "1/180" were used in isolation, it would be misleading. However, in most of the instances where this deflection criterium occurs in the text of SpecLink+, the text reads "1/180 of span" or "1/180 span" which is the same as "L/180". If fact, it is slightly clearer (to me, at least) because "L/180" includes the implicit assumption that the user understands that "L" is the span of the member. That is a reasonable expectation for an experienced user ... but it is by no means certain that all of our users understand that convention. I checked again and while the most instances in the text say "1/180 of span" or "1/180 span", there *are* a few references in the text and master notes to 1/360, 1/240, etc., without the qualifier "of span". I will correct those immediately, as well as making sure all instances say "of span" for additional clarity. Karen L. Zaterman, CDT, LEED-AP Senior Member Username: kittiz Post Number: 53 Registered: 10-2005 Posted on Thursday, August 20, 2009 - 05:22 pm:    Scott may have a point... My initial reaction was that L/### is correct as that is how I learned it in Structures class in Architecture school. But most important, is being clear on what we state to the Contractor as part of the Contract. Good grief, would the project be in trouble if it wasn't understood? Sometimes I'm amazed anything gets built properly. The AISC Manual (LRFD) used to define L as "Span length, ft" ...among several other things, in their index. However, the 13th edition no longer has that. So it may be nomenclature that is falling out of use. In any case, I always defer to whatever my project SE prefers to use... thank you for bringing up the topic. Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 194 Registered: 10-2007 Posted on Thursday, August 20, 2009 - 07:47 pm:    I doubt that many engineers will be confused. Either L/360 or 1/360 times the span would clearly address the issue. Karen L. Zaterman, CDT, LEED-AP Senior Member Username: kittiz Post Number: 56 Registered: 10-2005 Posted on Thursday, August 20, 2009 - 08:49 pm:    I'm not so worried about engineers... they are good at figuring out all sorts of obsure, unclear things ;-) Contractor is who we write to, right? Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 195 Registered: 10-2007 Posted on Thursday, August 20, 2009 - 09:37 pm:    While we write to the contractor it is understood that these provisions will be addressed by an engineer retained by the contractor. This is especially true if your standard practice is to require that the design work be performed by a registered engineer licensed in the state where the project will be built. With Contractors the biggest concern is that they will use this example of poor practice to make the design team look bad. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1095 Registered: 03-2002 Posted on Friday, August 21, 2009 - 01:30 pm:    I agree it's understood. We are typically using these terms in specifications that have long been concerned about allowable deflection--curtainwalls, masonry veneer, etc.--where the subs and suppliers know this.