LOADS - iccsafe.org

1 GENERALAll structures must be designed to support theirown weight along with any superimposed forces,such as the dead LOADS from other materials, liveloads, wind pressures, seismic forces, snow and iceloads, and earth pressures. These vertical and lateralloads may be of short duration such as those fromearthquakes, or they may be of longer duration, suchas the dead LOADS of machinery and design must consider all possible appliedforces along with the interaction of these forces onthe structure. load COMBINATIONSB ecause various LOADS may act on a structuresimultaneously, load combinations should beevaluated to determine the most severe conditionsfor design.

2 These load combinations vary from onedocument to another, depending upon thejurisdiction. There are a set of combinations for theallowable stress design and another set thatincorporates load factors for strength below provide these load 2012 IBC has three sets of loadcombinations. Section provides the loadcombinations for strength design and Section two sets of load combinations for allowablestress design. The strength design load combinationsand the first set of allowable stress design loadcombinations, found in Section , are labeledas the basic load combinations . These basic loadcombinations align closely with the load combinationsfound in ASCE 7.

3 The second set of allowable stressdesign load combinations, found in Section ,is labeled as the alternative basic load combinations .The alternative load combinationsused to beimportant to masonry designers as using these loadcombinations allowed increases in allowablestresses for load combinations including wind orseismic LOADS . Since the 2011 MSJC Code no longerpermits allowable stresses to be increased, thealternative combinations are less selection of the load combinations to beused for design is largely a matter of personalpreference. With the recalibration of the allowablestress design provisions in the 2011 MSJC Code, theresults obtained using those provisions are morecomparable to those obtained using strength addition, the design provisions themselves arebecoming more similar over time.

4 For example, theanchor bolt and shear design provisions are now thesame between the two design methodologies, justwith different factors for allowable stress design andstrength , it should be noted that the earthquakeload, E, in the load cases discussed below includesboth a vertical and horizontal component. Thevertical seismic component is typically accounted forby adjusting the factor on the dead LOADS . The verticalseismic load is discussed in Section of :RMEH 8/9/2012 9:18 AM Page ALLOWABLE STRESS DESIGNLOAD COMBINATIONSThe goal of allowable stress design is toproportion the structure so that it can resist the loadsthat are expected to occur during the life of thestructure while maintaining a factor of safety againstinelastic behavior.

5 For reinforced masonry, crackingof the masonry under tension is allowed because themasonry is not relied upon to resist tension. Whileeach one of the LOADS may be expected to occurduring the life of the structure, it is less likely thatthese LOADS will occur simultaneously. The loadcombinations below have been determined toaccount for the likelihood of the various loadsoccurring "Basic load combinations" for allowablestress design (working stress design) are given in2012 IBC Section as follows:1. Dead load plus lateral fluid pressures, [D + F],2. Dead load plus hydrostatic lateral soil pluslateral fluid pressures plus live load , [D + H +F + L],3.

6 Dead load plus hydrostatic lateral soil pluslateral fluid pressures plus either roof liveload, or snow load , or rain load , [D + H + F +(Lror Sor R)],4. Dead load plus hydrostatic lateral soil pluslateral fluid pressures plus times liveload plus times either roof live load , orsnow load , or rain load [D + H + F + (L)+ (Lror Sor R)],5. Dead load plus hydrostatic lateral soil pluslateral fluid pressures plus ( times windload or times earthquake load ) [D + H +F +( )],6. Dead load plus hydrostatic lateral soil pluslateral fluid pressures plus times ( wind load or times earthquakeload) plus times live load plus (either roof live load or snow load , orrain load ) [D + H + F + ( or ) + + (Lror Sor R)],7.

7 Times dead load plus times wind loadplus hydrostatic lateral soil [ + +H],8. times (dead load plus lateral fluidpressures) plus times earthquake loadplus hydrostatic lateral soil [ (D+ F) + + H],Footnotes to the Basic load combinations:Include lateral earth pressures in the design wherethey result in a more critical IBC does not require crane hook LOADS to becombined with roof live LOADS nor with more thanthree fourths of the snow load or one-half of thewind flat roof snow LOADS exceeding 30 psf, 20percent of the snow load shall be combined withthe seismic LOADS . Flat roof snow LOADS of 30 psf orless need not be combined with seismic floor live load should not be included if itsinclusion would result in lower stresses for thestructure or member being in allowable stresses shall not be usedwith the load combinations given in this section ofthe hydrostatic lateral soil pressures by they are permanent and they resist theprimary variable load effects.

8 If the hydrostaticlateral soil pressures are not permanent, do notconsider that they provide any resistance to otherload load combination 8 (IBC load combination 16-16), the dead load factor may be increased to special reinforced masonry shear following "alternative basic load combinations",as given in IBC Section , may be used in lieuof the basic load combinations given above forSection Dead load plus live load plus either roof liveload, snow load , or rain load , [D + L +(LrorSor R)],2. Dead load plus live load plus times thecoefficient times the wind load , [D + L + W],3. Dead load plus live load plus times thecoefficient times the wind load plus one-half times the snow load , [D + L+ W+S/2],4.

9 Dead load plus live load plus snow load times one-half coefficient times thewind load , [D + L + S + W/2],5. Dead load plus live load plus snow load plus(1 ) times the earthquake load , [D + L + S+ ],6. times the dead load plus (1 ) times theearthquake load , [ + ],Footnotes to the above combinations:Include lateral earth pressures in the design wherethey result in a more critical MASONRY ENGINEERING :RMEH 8/9/2012 9:18 AM Page 52 The IBC does not require crane hook LOADS to becombined with roof live LOADS nor with more thanthree fourths of the snow load or one-half of thewind flat roof snow LOADS exceeding 30 psf, 20percent of the snow load shall be combined withthe seismic LOADS .

10 Flat roof snow LOADS of 30 psf orless need not be combined with seismic using these alternate basic load combinationsthat include wind or seismic LOADS , allowablestresses are permitted to be increased or loadcombinations reduced, where permitted by the IBCor by the referenced standard of IBC, provided thatwhen wind LOADS are calculated by Chapters 26through 31 of ASCE 7, the coefficient in theabove equations shall be taken as For otherwind LOADS shall be taken as allowable stresses are not increased, thecoefficient may be taken as these combinations are used for foundationsfor LOADS including seismic, the vertical seismiceffect, Ev, in Equation of ASCE 7 ispermitted to be taken as these combinations are used to evaluatesliding, overturning, and soil bearing at the soil-structure interface, the reduction of foundationoverturning from Section of ASCE 7 shallnot be load combinations that include counteractingeffects of dead and wind LOADS , only two-thirds ofthe minimum dead load that is likely to be in placeduring the designed wind event shall be STRENGTH DESIGN LOADCOMBINATIONSThe goal of strength design is to proportion thestructure so that it can resist rarely occurring loadswithout reaching a limit or failure state.