PEMB Uplift

[BSVBD]

I know there are several threads referring to this topic, but, i haven't seen any addressing the 0.75 i am about to inquire about.

I know i can only use 0.6D to resist. BUT... does IBC EQ 16-13 allow me to reduce the reported PEMB uplift by 0.75?

In my office, we've pondered how PEMB engineers design buildings with (seemingly) virtually no safety factor. Are there other factors i am not aware of to apply to the PEMB reported uplift reaction?

Thank you!

 

[mike20793]

No, I believe that is only one combination out of the many that need to be considered (the actual number depends on how many wind load cases the PEMB designer is submitting) and IBC Eq 16-15 would still control for uplift.

 

[Hokie93]

IBC 2012 Equation 16-13 should include the 0.75 factor on wind load provided you have at least two transient loads acting (wind and live load, wind and roof live load, etc.). For wind uplift, Equation 16-15 should govern. Equation 16-13 should govern for downward acting wind loads and could govern for thrust/shear. At any rate, you need to clearly understand if the PEMB supplier is providing load case or load combination results. Certainly it would be incorrect to double-dip on the load combination factors (that is, applying 0.75 to a wind load that has already been multiplied by 0.75).

 

[cal91]

PEMB = Pre-engineered metal building?

 

[cal91]

PEMB = Pre-engineered metal building?

Yes

 

[calvinandhobbes10]

I believe Metal Buildings get away with lean designs by relaxing wind drift limitations.
This is becoming more problematic as their use in office/church/retail/finished space is growing, since these structures utilize conventional finishes, often without deflection accommodation.

For foundation design, usually metal building guys will report all their load cases (20+ of them). Sometimes they will envelope these with max and min values, but rarely do I find their application of load combinations to be well clarified/accurate. Identify the controlling wind/seismic uplift load CASE as well as the dead+collateral cases (0.6dead+wind). Check if they are using ultimate or service wind (believe most still use ASCE 7-05 service). Then apply your IBC equations (0.75 won't apply to governing uplift). Whether to include collateral load (extra dead) in uplift resistance is engineering judgment. In my opinion the 0.6 factor is there to cover "artificial" dead load values such as this. Interior column footings will nearly always be controlled by uplift, same as Walmarts and other single-story warehouses. Use soil and SOG weight above to resist, and consider lowering your footing and increasing pedestal height to minimize concrete volume.

Metal Building anchor bolt design is another can of worms.