30734_stress Increase Factor

30734_stress Increase Factor
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  Current ASCE 7 provisions have significantly altered the familiar “one-third stress increase” —but where did it come from in the first place? A n article by Keith Muellerand Charlie Carter in theOctober 2003 issue of  ModernSteel Construction magazinetitled “The One-Third StressIncrease: Where Is It Now?” hasrenewed interest in the history of thistopic. The article focused on what is andis not permissible in ASCE 7, as well asthe various current and recent majormodel-building codes in the U.S. It didnot, however, attempt to address the his-tory of the one-third stress increase orthe rationale behind it in any greatdetail. Fortunately, these aspects arewell covered in a paper published in the4th Quarter 1977 AISC Engineering Jour-nal . The paper, by Duane Ellifritt, enti-tled “The Mysterious One-Third StressIncrease,” is paraphrased in abbreviatedform in this article. The full paper isavailable at . What was it for? Ellifritt wrote about the confusionthat stirs over the one-third stressincrease and exactly for what types of conditions it is supposed to account.Some believe it represents a low proba- bility that maximum live load and max-imum wind load would ever occursimultaneously and apply it as a four-thirds factor on the strength side. Others believe it is to account for wind loadphenomena that reduce the actual loadused in design and apply it as a three-quarter load reduction. Although theseoperations appear to accomplish thesame thing, Ellifritt contends that thethree-quarter factor is not the inverse of the four-thirds factor—that is—theallowable stress increase for wind is notthe same as the probability factor whenaccounting for simultaneous applicationof maximum live load and maximumwind load. The Survey Says … Ellifritt surveyed colleagues anddetermined a great many of them usedthe one-third increase merely becauseAISC permitted it. Most other replies fellinto one of three categories: 1. The action of wind on a structure ishighly localized and of very shortduration. Therefore, it is not necessaryto have as high a safety factor whendesigning for wind loads. 2. The properties of some materialschange with the rate of loading. Steel,when loaded rapidly, will showhigher yield strength than when it isloaded slowly. The one-third stressincrease merely reflects the increase inproperties due to rapid loading anddoes not diminish the safety factor. 3. The one-third stress increase reflectsthe low probability of maximum liveand wind loads occurring simultane-ously. Therefore, when checking D +W only, it should not be used. From the Literature Many references included some cov-erage of this principle (see the srcinalpaper for the full bibliography of refer-ences below): ➜ The oldest reference, found inTheodore Cooper’s 1896 General Spec-ifications for Steel Railroad Bridges andViaducts , permitted a 1 / 4 increase butgave no reason for this measurement. ➜ A.J. DuBois’s article entitled “TheStresses in Framed Structures” (also in1896) recommended an allowablestress for beams of 10,000 psi and anallowable for lateral bracing of 15,000psi. Although no statement is maderegarding the reason for a higherallowable stress for bracing, it could be interpreted as a 50% increase because of wind forces. ➜ Milo Ketchum, in his 1903 GeneralSpecifications for Mill Buildings , said:“When combined direct and flexuralstress due to wind is considered, add25% to the above allowable tensileand compressive stresses.” LikeCooper, he does not offer any expla-nation as to why this should be per-mitted. ➜ The New York City Building Code of 1904 contained this statement: “In cal-culations for wind bracing the work-ing stresses set forth in this code may be increased by 50%.” Again, no rea-son is given. ➜ In 1923, AISC published its first speci-fication. In it were these words: “ Com-bined Stresses —For combined stressesdue to wind and other loads, the per-missible working stress may beincreased 33%, provided the sectionthus found is not less than thatrequired by the dead and live loadsalone.  Members Carrying Wind Only —For members carrying wind stressesonly, the permissible wind stressesmay be increased 33%.” This provi-sion in AISC’s specification remained By Charles J.Carter, P.E., S.E. In the Beginning The Origins of the One-Third Stress Increase October 2004  Modern SteelConstruction another look  virtually unchanged for 51 years,except for the inclusion of seismicstresses.Supplement No. 3 added a qualify-ing paragraph in 1974: “and providedthat stresses are not otherwise*required to be calculated on the basisof reduction factors applied to designloads in combinations.” The footnoteread: “*For example, see ANSI A58.1,Section 4.2.” ➜ Robins Fleming wrote a comprehen-sive text in 1930 on wind stresses in buildings. He put forth the first logicalexplanation of the mysterious increasethat Ellifritt was able to find: “ Becausewind loads are intermittent and seldomreach their maximum , greater workingstresses are permissible for them thanfor live and dead loads.” He continued, “This is recognizedgenerally by engineers and has founda place in most building codes. In theNew York City code, where at present(1929) a working stress of 16,000 psi isspecified for tension in rolled steel, anexcess of 50% of stresses prescribedelsewhere in the code is allowed forcombined wind, dead, and live loads,provided that the sections thus foundare not less than those required by thedead and live loads alone. In Chicago,where 18,000 psi is the basic unitstress for tension, an excess of 33% isallowed for combined stress, thus per-mitting in both New York andChicago a working stress of 24,000-psitension for combined loads. This sameunit stress is followed in the code rec-ommended by the National Board of Fire Underwriters. The RecommendedBuilding Code Requirements for WorkingStresses in Building Materials , 1926, of the U.S. Bureau of Standards, favor anincrease of 25% based on 18,000 psi intension.”After listing the requirements of several codes, Fleming recommendedthe use of 24,000-psi tension on the netsection for “stresses due to wind loadscombined with live and dead loads, orfor members taking wind stressalone.” In 1940, an ASCE subcommitteeproduced a report entitled Wind Brac-ing in Steel Buildings . The report cre-ated the first departure from the all-inclusiveness of the one-third stressincrease and possible support foranswer no. 3 (previous page). “For members or details subject towind stress only, except rivets and bolts, the permissible stress should bethe same as that allowed for dead loador for dead and live load. For memberssubjected to stresses arising from thecombined similar action of wind andother loads, and for rivets and boltssubject to wind stress, the wind stressup to 33% of the other stresses may beneglected, the excess wind stress beingconsidered as equivalent to an addedlive load stress, provision being madefor it at the basic working stress fordead load and live load only.” Notethat rivets and bolts were permitted astress increase for wind only. ➜ In 1947, the American Iron and SteelInstitute published the first edition of the Light Gage Cold-Formed Steel Design Manual . The pertinent provisions fol-lowed the AISC specifications in per-mitting a one-third increase for com- bined stresses and for “wind orearthquake only.” ➜ Aquartet of British authors stated thata 25% increase is allowed and made agood argument for answer no. 1 (pre-vious page): “By Clause 25, the nor-mal permissible stresses in the mem- bers may be increased by 25% in caseswhere such increases are due solely tothe stresses induced by wind. This higher working stress isallowed because of the transientnature of the load and also becausethe structure is of a sufficiently elasticnature, which allows it to absorb suchtransient loads without permanentdefects.” ➜ In a more recent publication, thisinsight was offered by Angus McDon-ald in Wind Loading on Buildings (1975):“It should be noted that a very highwind load is a comparatively rareoccurrence and that the design windspeeds specified in most codes of prac-tice may never actually occur in the lifeof a structure. For this reason, moststructural codes allow a 25% (some-times as much as 33%) increase in per-missible stress for wind loadings.” ➜ More evidence for answer no. 1 (the“transient nature” of wind) is found inthe Building Construction Handbook  ,chapter six, by H. Stetina (1975): “Forwind or earthquake forces, acting aloneor in combination with the design liveand dead loads, allowable stresses may be increased one-third. The increase isallowed because wind and seismicforces are of short duration.” ➜ This position was also supported by Jack C. McCormac’s Structural SteelDesign (1971): “The maximum windand earthquake pressures for whichdesign is made occur at large intervalsof time and then last for only rela-tively short periods of time. It, there-fore, seems reasonable to use higherallowable stresses, such as the one-third AISC increase, for lateral forcesthan for the relatively long-term grav-ity live loads.” ➜ Adissenting opinion is was offered ina 1972 excerpt from Structural Engi-neering ,Vol. 1, by R.N. White, P.Gergely and R.G. Sexsmith: “In recog-nition of the highly unlikely occur-rence of maximum wind or earth-quake loads simultaneously with thefull value of other live loads, codesgenerally allow a 33% increase inallowable stresses under these loadcombinations.” Conclusions Ellifritt wrote that the preponderanceof literature on the subject supportsanswer no. 1—the one-third stressincrease is allowed because of the “tran-sient nature” of wind; because windloads “are intermittent and seldom reachtheir maximum;” because a very highwind load is “a comparatively rare occur-rence” and “may never actually occur;”and because wind forces are of “shortduration.” There was no support foranswer no. 2 (rate of loading), and onlytwo references to support answer no. 3(simultaneous occurrence). [Author’snote: It is interesting to find that ASCE 7-02 Commentary Section C2.4.1 state-ments about the 0.75 load factor indicatethat it is essentially the same concept asthe one-third stress increase and that it isprimarily intended to address simultane-ous occurrence.]Ellifritt continued that the one-thirdstress increase is rooted deeply in ourengineering tradition, during a timewhen wind loads were not so well under-stood; that modern methods of applyingwind loads account for some of the fac-tors used to rationalize the stress increase,such as “short duration” and “rare occur-rence.” Modern wind codes are based on better meteorological information andwind tunnels, which accurately model the boundary layer. A“rare occurrence” inwind velocity is now programmed intothe design pressure selection process inthe form of a Mean Recurrence IntervalMap. The “short duration” aspect is nowaccounted for by gust factors. Modern SteelConstruction ã October 2004  Is the one-third stress increaseappropriate for design today? Ellifritt said yes, with the followingreasoning: The gust factor as defined inANSI A58.1 (the precursor to ASCE 7 andcurrent load standard at the time of Ellifritt’s writing) represents anywherefrom a 30% to a 120% increase in the basicpressure. The increase, of course,depends on the exposure type. If windloads are increased by 30% and the stressincrease disallowed, the net effect is aload that is increased by 70%. Modern wind codes may have higherloads than previously used on some partsof a structure (such as corners, eaves, andridges), but they may also have lowerloads on other parts. The resultantshould be about the same total load as wehave always used. However, it should bedistributed differently and more prop-erly suited to location. On this basis,Ellifritt wrote that there is no valid rea-son why modern wind standards withgust factors, mean recurrence intervals,and peak coefficients should not continueto permit the designer to use the one-third stress increase.The reasoning in Ellifritt’s 1977 paperseems to contend that the one-third stressincrease should remain valid today, evenwith the latest refinements to ASCE 7.However, as outlined in the  MSC article by Keith Mueller and Charlie Carter,changes in ASCE 7 itself, as well as in themodel building codes, have significantlylimitedthe traditional applications of theone-third stress increase.  Charles J. Carter, S.E., P.E., is chief struc-tural engineer with the American Institute of Steel Construction, Inc. in Chicago. October 2004 ã Modern SteelConstruction AISC’s Engineering Journal is avail-able to all by subscription and is alsoavailable to members (including allback issues) through free electronicaccess through AISC’s ePubs at .
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