Draft

Chapter 9

DESIGN PHILOSOPHIES of ACI

and AISC CODES

9.1 Safety Provisions

1 Structuresandstructuralmembersmustalways be designedto carrysomereserve loadabove

whatis expectedundernormaluse.Thisis to account for

Variability in Resistance:Theactualstrengths(resistance)of structuralelements willdif-

ferfromthoseassumedby thedesignerdueto:

1. Variability in thestrengthof thematerial(greatervariability in concretestrength

thanin steelstrength).

2. Dierencesbetweentheactualdimensionsandthosespeci ed(mostlyin placement

of steelrebarsin R/C).

3. Eectof simplifyingassumptionsmadein thederivationof certainformulas.

Variability in Loadings: Allloadingsarevariable. Thereis a greatervariationin thelive

loadsthanin thedeadloads. Sometypes of loadingsareverydicultto quantify(wind,

earthquakes).

Consequencesof Failure:Theconsequenceof a structuralcomponent failuremustbe care-

fullyassessed. Thecollapseof a beamis likelyto causea localizedfailure.Alternatively

thefailureof a columnis likelyto triggerthefailureof thewholestructure.Alternatively,

thefailureof certaincomponents canbe precededby warnings(such as excessive defor-

mation),whereasotheraresuddenandcatastrophic.Finally, if noredistributionof load

is possible(aswouldbe thecasein a staticallydeterminatestructure),a highersafety

factormustbe adopted.

2 Thepurposeof safety provisionsis to limit theprobability of failureandyet permit

economicalstructures.

3 Thefollowingitemsmustbe consideredin determiningsafety provisions:

1. Seriousnessof a failure,eitherto humansor goods.