Building Stability
- In addition to the design of the primary load-resisting system and the individual elements with respect to wind pressures, the entire structure must be checked for overturning.
- This phenomenon can be simply described by considering an empty cereal box located in front of a household fan.
- Without proper connection to the table, the box will act like a rigid body; overturning by rotating and falling about its far corner.
- Similarly, you need to ensure moment stability of the entire structure and those elements that are part of the LFRS, (e.g. shearwalls).
- According to '94 UBC 1619.1,
or '97 UBC 1621.1, the overturning moment (OM) must be £
2/3 of the resisting moment (RM) due to dead loads.
- But if h/b £ 0.5, h £ 60', and the entire structure OM accounts for both lateral and uplifting loads, then may use OM £ RM.
- Consider the simple model of our building in terms of the cereal box.
OM = Ph + U b/2
RM = D b/2
If OM ³ 2/3 RM, then hold corner A down with an anchor capable of resisting a tensile force of:
T = (OM - 2/3 RM) / b.
- In addition to checking the entire structure, you need to also check each shearwall as modeled below.
- If all shear walls are adequate wrt OM, surely the entire structure will be adequate.
- With other structural systems it is common practice to check overturning of the entire building, but with a conventional shearwall building the practice is to check each shearwall instead.
