Codes Modern building codes have changes the "allowable stress"approach to a more reasonable "ultimate strength"design method. A building designer should use building codes only as a reference and as a guide,but not bible.Some code requirements are excessively conservative,others are not sufficient(especially for unusual designs).Codes Modern building codes have changes the “allowable stress” approach to a more reasonable “ultimate strength” design method. A building designer should use building codes only as a reference and as a guide, but not bible. Some code requirements are excessively conservative, others are not sufficient (especially for unusual designs)