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LOADING CRITERIA FOR SUBSTATION STRUCTURES

57

TABLE 3-17. Ultimate Strength Design Cases and Load Faetors Load Cases

Case Case Case Case

1 2 3 4

Load Factors and Combinations

1.1 1.1 1.1 1.1

0 0 0 0

+ 1.2 Whw + 0.75 SC + 1.1 T w + 1.2 Iwh!" + 1.2 W1lrll/ + 0.75 SC + 1.1 Tw + 1.0 SC + 1.1 T w + 1.25 E (or EFS)lrE + 0.75 SC + 1.1 T w

"The impürtance factor für ice is applied to the thickness. "The importance factor für wind with ice, IFI\\" is 1.0 with wind speeds from Figllres 3-3a to f. Notes: Replace load factor 1.1 on 0 with 0.9 for cases in which dead load is COllnted on to resist other applied loads. Fm allowable stress design, the load factors should equal 1.0.

This diseussion addresses the parameters that influenee the seleetion of the reeommended load faetors for substation struetures presented in this manual. The load faetors should be used with all materialbased design speeifieations in eonjunetion with the nominal resistanee faetors. Table 3-17 shows suggested design load eases, eombinations, and minimum load faetors to be use for substation struetures. The individual load eomponents are the following: • • • •

• • •

• •

0 = strueture and wire dead load; W = extreme wind load (F, from Eq. 3-1, without Im); W1 = wind load in eombination with iee; Iw = iee load in eombination with wind; E = earthquake load (FE, from Eq. 3-10, without hw); EFS = earthquake load reaetions from first support imposed on the remainder of the strueture (without IFE ); Tw = horizontal wire tension for the appropriate wind and temperature condition; SC = short-eircuit load; and I F = importallCe faetors (IFW, Irl' I FI % and irE)'

The general format of the load ease equations is the combination of load faetors times the ealculated loads, such as 1.1 x dead load. Load faetors are used to accowlt for the uneertainties in the estimation of the loads. A load faetor of 1.0 does not imply that the strueture is not strueturally reliable.