Structural Load Determination: 2018 IBC And ASCE/SEI 7-16 PDF

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CHAPTER2Load Combinations2.1IntroductionIn accordance with IBC 1605.1, structural members of buildings and other structures must bedesigned to resist the load combinations of IBC 1605.2, 1605.3.1 or 1605.3.2. Load combina tions that are specified in Chapters 18 through 23 of the IBC, which contain provisions for soilsand foundations, concrete, aluminum, masonry, steel and wood, must also be considered. Thestructural elements identified in ASCE/SEI Chapters 12, 13 and 15 must be designed for the loadcombinations with overstrength of ASCE/SEI 2.3.6 or 2.4.5. These load combina tions and theirapplicability are examined in Section 2.5 of this publication.IBC 1605.2 contains the load combinations that are to be used when strength design or load andresistance factor design is utilized. Load combinations using allowable stress design are given inIBC 1605.3. Both sets of combinations are covered in Sections 2.3 and 2.4 of this publication,respectively. The combinations of IBC 1605.2 or 1605.3 can also be used to check overall struc tural stability, including stability against overturning, sliding and buoyancy (IBC 1605.1.1).It is important to understand the difference between permanent loads and variable loads andtheir role in load combinations. Permanent loads, such as dead loads, do not change or changevery slightly over time. Live loads, roof live loads, snow loads, rain loads, wind loads andearth quake loads are all examples of variable loads. These loads are not considered to be per manent because of their inherent degree of variability with respect to time (see the definitionof “Loads” in IBC 202).According to IBC 1605.1, load combinations must be investigated with one or more of the vari able loads set equal to zero. It is possible that the most critical load effects on a member occurwhen one or more variable loads are not present.ASCE/SEI 2.3 and 2.4 contain load combinations for strength design and allowable stress design,respectively. The load combinations are essentially the same as those in IBC 1605.2 and 1605.3with some exceptions. Differences in the IBC and ASCE/SEI 7 load combinations are covered inthe following sections. In ASCE/SEI 7-16, the load combinations with seismic load effects havebeen removed from ASCE/SEI Chapter 12 and placed in ASCE/SEI Chapter 2 in sections sepa rate from the basic load combinations.Prior to examining the various load combinations, a brief introduction on load effects is given inSection EffectsThe load effects that are included in the IBC and ASCE/SEI 7 load combinations are summa rizedin Table 2.1. More details on these load effects can be found in those documents, as well as insubsequent chapters of this publication (see the Notes column in Table 2.1, which gives spe cificlocations where more information can be found on the various load effects).502 Fanella Chapter 02 p005-030.indd 526/07/18 7:15 PM

6  Chapter 2Table 2.1 Summary of Load EffectsNotationDDiEEmFFaHLLrRSTWWiLoad EffectDead loadWeight of iceSeismic load effect defined inASCE/SEI 12.4.2Seismic load effect including over strength defined in ASCE/SEI 12.4.3Load due to fluids with well-definedpressures and maximum heightsFlood loadLoad due to lateral earth pressures,ground water pressure or pressure ofbulk materialsRoof live load greater than 20 psf andfloor live loadRoof live load of 20 psf or lessRain loadSnow loadCumulative effects of self-strainingforces and effectsLoad due to wind pressureWind-on-ice loadNotesSee IBC 1606 and Chapter 3 of this publicationSee IBC 1614, Chapter 10 of ASCE/SEI 7 andChapter 4 of this publicationSee IBC 1613, ASCE/SEI 12.4.2 and Chapter 6of this publicationSee IBC 1613, ASCE/SEI 12.4.3 and Chapter 6of this publication—See IBC 1612, Chapter 5 of ASCE/SEI 7 andChapter 7 of this publicationSee IBC 1610 and Chapter 3 of this publicationfor soil lateral loadsSee IBC 1607 and Chapter 3 of this publicationSee IBC 1607 and Chapter 3 of this publicationSee IBC 1611 and Chapter 3 of this publicationSee IBC 1608, Chapter 7 of ASCE/SEI 7 andChapter 4 of this publicationSee ASCE/SEI 2.3.4 and 2.4.4See IBC 1609, Chapters 26 to 31 of ASCE/SEI7 and Chapter 5 of this publicationSee IBC 1614, Chapter 10 of ASCE/SEI 7 andChapter 4 of this publication 2.3  Load Combinations Using Strength Design or Loadand Resistance Factor DesignThe basic load combinations where strength design or, equivalently, load and resistance factordesign is used are given in IBC 1605.2 and summarized in Table 2.2. These equations establish theminimum required strength that needs to be provided in the members of a building or struc ture.These load combinations apply only to strength limit states; serviceability limit states for deflec tion, vibration, drift, camber, expansion and contraction and durability are given in Appendix Cof ASCE/SEI 7.The load factors were developed using a first-order probabilistic analysis and a broad survey ofthe reliabilities inherent in contemporary design practice. The equations in Table 2.2 are meantto be used in the design of any structural member regardless of material in conjunction with theappropriate nominal resistance factors set forth in the individual material specifications.Refer ences 2.1 and 2.2 provide information on the development of these load factors along withaddi tional background material.Factored loads are determined by multiplying nominal loads (that is, loads specified in Chapter 16of the IBC) by a load factor, which is typically greater than or less than 1.0. Earthquake and windload effects are an exception to this: a load factor of 1.0 is used to determine the maximum effectsfrom these loads because they are considered strength-level loads.02 Fanella Chapter 02 p005-030.indd 626/07/18 7:15 PM

Load Combinations  7Table 2.2 Summary of Load Combinations Using Strength Design or Loadand Resistance Factor Design (IBC 1605.2)IBC Equation No.ƒ1 ƒ2 Load Combination16-11.4(D F)16-21.2(D F) 1.6(L H) 0.5(Lr or S or R)16-31.2(D F) 1.6(Lr or S or R) 1.6H (ƒ1L or 0.5W)16-41.2(D F) 1.0W ƒ1L 1.6H 0.5(Lr or S or R)16-51.2(D F) 1.0E ƒ1L 1.6H ƒ2S16-60.9D 1.0W 1.6H16-70.9(D F) 1.0E 1.6H1 for places of public assembly live loads in excess of 100 psf and for parking garages0.5 for other live loads0.7 for roof configurations (such as sawtooth) that do not shed snow off the structure0.2 for other roof configurationsLoad combinations are constructed by adding to the dead load one or more of the variable loadsat its maximum value, which is typically indicated by a load factor of 1.6. Also included in thecombinations are other variable loads with load factors less than 1.0; these are companion loadsthat represent arbitrary point-in-time values for those loads. Certain types of variable loads, suchas wind and earthquake loads, act in more than one direction on a building or structure, and theappropriate sign of the variable load must be considered in the load combinations.The seismic load effect, E, that is to be used in IBC Equation 16-5 (ASCE/SEI load combination 6)is equal to the following (see ASCE/SEI 12.4.2):E Eh E vwhere Eh horizontal seismic load effect defined in ASCE/SEI ρQEEv vertical seismic load effect defined in ASCE/SEI 0.2S DS Dρ redundancy factor defined in ASCE/SEI 12.3.4QE effects of horizontal seismic forces applied to the structureS DS design spectral response acceleration parameter at short periodsThus, IBC Equation 16-5 (ASCE/SEI load combination 6) can be written as follows:(1.2 0.2 S DS ) D 1.2 F ρQE f1 L 1.6 H f2 SIn IBC Equation 16-7 (ASCE/SEI load combination 7), the seismic load effect that is to be usedis E Eh Ev (see ASCE/SEI 12.4.2). Therefore, this equation can be written as follows:(0.9 0.2 S DS ) D 0.9 F ρQE 1.6 H02 Fanella Chapter 02 p005-030.indd 726/07/18 7:15 PM

8  Chapter 2According to the first exception in ASCE/SEI, the vertical seismic load effect, Ev, can bedetermined from the following equation, which is applicable to structures that have significantresponse to vertical ground motion:Ev 0.3Sav DIn this equation, Sav is the design vertical response spectral acceleration, which is equal to twothirds the value of the MCE R vertical response acceleration SaMv determined in accordance withASCE/SEI 11.9.2. This provision is invoked only by ASCE/SEI Chapter 15 for certain nonbuildingstructures.In the second exception in ASCE/SEI, Ev is permitted to be taken as zero for either ofthe following conditions: In ASCE/SEI Equations 12.4-1, 12.4-2, 12.4-5 and 12.4-6 for structures assigned to SeismicDesign Category (SDC) B In ASCE/SEI Equation 12.4-2 when determining demands on the soil-structure interface offoundationsFluid load effects, F, occur in tanks and other storage containers due to stored liquid products.The stored liquid is generally considered to have characteristics of both a dead load and a liveload. It is not a purely permanent load because the tank or storage container can go throughcycles of being emptied and refilled. The fluid load effect is included in IBC Equations 16-1through 16-5 where it adds to the effects from the other loads. It is also included in IBCEquation 16-7 where it counteracts the effects from uplift due to seismic load effects, E. Becausethe wind load effects, W, can be present when the tank is either full or empty, F is not incorpo rated in IBC Equation 16-6; that is, the maximum effects occur when F is set equal to zero.Two exceptions are given in IBC 1605.2. According to the first exception, factored load combi nationsthat are specified in other provisions of the IBC take precedence to those listed in IBC 1605.2.The second exception is applicable where the effect of H resists the primary variable load effect.In cases where H is not permanent, the load factor on H must be taken equal to zero (that is, H isnot permitted to resist the primary variable load effect if it is not permanent). The 1.6 load factoron H accounts for the higher degree of uncertainty in lateral forces from bulk materials (whichare included in H) compared to that from fluids, F, especially when considering the dynamiceffects that are introduced as the bulk material is set in motion by filling operations.The load combinations given in IBC 1605.2 are the same as those in ASCE/SEI 2.3.1 with thefollowing exceptions: The variable f1 that is present in IBC Equations 16-3, 16-4 and 16-5 is not found in ASCE/SEIcombinations 3, 4 and 6. Instead, the load factor on the live load, L, in the ASCE/SEI 7 combi nations is equal to 1.0 with the exception that the load factor on L is permitted to equal 0.5 forall occupancies where the live load is less than or equal to 100 psf, except for parking garagesor areas occupied as places of public assembly (see exception 1 in ASCE/SEI 2.3.1 and 2.3.6).This exception makes these load combinations the same in ASCE/SEI 7 and the IBC. The variable f2 that is present in IBC Equation 16-5 is not found in ASCE/SEI combination 6.Instead, a load factor of 0.2 is applied to S in the ASCE/SEI 7 combination. The second excep tion in ASCE/SEI 2.3.6 states that in ASCE/SEI combination 6, S must be taken as eitherthe flat roof snow load, pf , or the sloped roof snow load, ps. This essentially means that thebalanced snow load defined in ASCE/SEI 7.3 for flat roofs and in ASCE/SEI 7.4 for sloped02 Fanella Chapter 02 p005-030.indd 826/07/18 7:15 PM

Load Combinations  9roofs can be used in ASCE/SEI combination 6. Note that S in ASCE/SEI combinations 2 and4 is defined in the same way (see exception 2 in ASCE/SEI 2.3.1). Drift loads and unbal anced snow loads are covered by ASCE/SEI combination 3.According to IBC 1605.2.1, the load combinations of ASCE/SEI 2.3.2 are to be used whereflood loads, Fa, must be considered in design (flood loads are determined by Chapter 5 ofASCE/SEI 7 and are covered in Chapter 7 of this publication). In particular, the followingmodifications are to be made: V Zones or Coastal A Zones1.0W in IBC Equations 16-4 and 16-6 must be replaced by 1.0W 2.0Fa Noncoastal A Zones1.0W in IBC Equations 16-4 and 16-6 must be replaced by 0.5W 1.0FaDefinitions of Coastal High Hazard Areas (V Zones) and Coastal A Zones are given in ASCE/SEI 5.2 (see Chapter 7 of this publication).The load factors on Fa are based on a statistical analysis of flood loads associated with hydro staticpressures, pressures due to steady overland flow, and hydrodynamic pressures due to waves, allof which are specified in ASCE/SEI 5.4.In cases where self-straining loads, T, must be considered, their effects in combination with otherloads are to be determined by ASCE/SEI 2.3.4 (IBC 1605.2.1). Instead of calculating self-strain ingeffects based on upper bound values of this variable like other load effects, the most probableeffect expected at any arbitrary point in time is used. More information, including load combina tions that should be considered in design, is given in ASCE/SEI C2.3.4.IBC 1605.2.1 requires that the load combinations of ASCE/SEI 2.3.3 be used where atmosphericice loads must be considered in design. The following modifications to the load combinationsmust be made when a structure is subjected to atmospheric ice and wind-on-ice loads (atmo spheric and wind-on-ice loads are determined by Chapter 10 of ASCE/SEI 7; see IBC 1614 andChapter 4 of this publication): 0.5(Lr or S or R) in ASCE/SEI combination 2 (IBC Equation 16-2) must be replaced by 0.2Di 0.5S 1.0W 0.5(Lr or S or R) in ASCE/SEI combination 4 (IBC Equation 16-4) must be replacedby Di Wi 0.5S 1.0W in ASCE/SEI combination 5 (IBC Equation 16-6) must be replaced by Di Wi 1.0W L 0.5(Lr or S or R) in ASCE/SEI combination 4 (IBC Equation 16-4) must bereplaced by DiSee ASCE/SEI C2.3.3 for more information on the load factors used in these equations.ASCE/SEI 2.3.5 provides information on how to develop strength design load criteria where noinformation on loads or load combinations is given in ASCE/SEI 7 or where performance-baseddesign in accordance with ASCE/SEI is being utilized. Detailed information on how todevelop such load criteria that is consistent with the methodology used in ASCE/SEI 7 can befound in ASCE/SEI C2.3.5.02 Fanella Chapter 02 p005-030.indd 926/07/18 7:15 PM

10  Chapter 22.4Load Combinations Using Allowable Stress Design2.4.1 OverviewThe basic load combinations where allowable stress design (working stress design) is used aregiven in IBC 1605.3. A set of basic load combinations is given in IBC 1605.3.1, and a set ofalternative basic load combinations is given in IBC 1605.3.2. Both sets are examined below.2.4.2 Basic Load CombinationsThe basic load combinations of IBC 1605.3.1 are summarized in Table 2.3. A factor of 0.75 isapplied where these combinations include more than one variable load because the probability islow that two or more of the variable loads will reach their maximum valu

Equation 16 ­7 where it counteracts the effects from uplift due to seismic load effects, E. Because the wind load effects, W, can be present when the tank is either full or empty, F is not incorpo­ rated in IBC Equation 16­6; that is, the maximum effects occur when F is set equal to zero. Two exceptions are given in IBC 1605.2.