Significance of Importance Factor in Wind and Seismic Calcs

What kind of structure?

Long vessel column

It should be specified in the code to which your structural engineers work.

It puts more design load on important structures, structures whose survival is important in the event of a disastrous event, such as power stations, hospitals, communication centres etc. and lower values on barns and uninhabited structures.

For more details, see my post #2 of CR4 Thread Calculating Wind Resistance.

For Seismic considerations, you will also want to take particular note of ASCE 7-05.

Equipment designed per ASCE Chapter 13.

Look for "Component Importance Factor" regarding seismic anchoring and support for equipment. This also directly relates to the excellent wind-loading analysis provided in that linked post by AHG. The Architect determines the building Occupancy Category (I,II,III, or IV). The structural/civil engineer determines the Site Class (A-F per Table 1613.5.2 definitions) and the physical location requires reference to the USGS or equivalent map for the Short Period Spectral Response. This will let you check Table 1613.5.6 (1) for the Seismic Design Category.

Basically, equipment with an Ip = 1.5 in a Seismic Design Category C or D situation requires certification and proper seismic installation for proper support/anchoring (E and F not on that chart, described separately).

Per Section 13.1.3 the component Importance Factor will be assigned either 1.0 or 1.5. The Ip of 1.5 is assigned if any of the following apply:

1. The component is required to function for life-safety purposes after and earthquake, including fire protection sprinkler systems.
2. The component contains hazardous materials.
3. The component is in or attached to an Occupancy Categor IV structure and it is needed for continued operation of the facility or its failure could impair the continued operation of the facility.

All other components are assigned a component Ip of 1.0

A few 'gotcha's:
Require siesmic hangers and supports for non-essential systems if they happen to be installed above another system with an Ip = 1.5, such as looking at the effect of plumbing piping, ductwork, or conduit routed above or around gas and fire-sprinkler piping.
Rooftop equipment (as in my area of Illinois) cannot typically be installed on a factory 'curb' but must be on a siesmic frame anchored to the structure, with the equipment secured to that frame.
Oh...and the 'assembly (equipment on frame) must be certified; see 1708.5:
- actual test on a shake table by 3-dimensional shock tests, by analytical using dynamic characteristics and forces, use of experience data (exact products survived a logged event), or by 'more rigorous analysis.'
Equipment must be Labeled, installation must be done correctly per tested configuration, and the final construction must be Inspected.

For wind, of course, all construction occupancies in all areas apply - equipment designed per ASCE Chapter 6, MWFRS. Main issue is to never allow anything on the roof to rely on its 'weight' or attachment to the roof deck, it must always be anchored directly through the roofing to the structure below, period, if you wish to ensure no liability from the old lady who just had some misc. piece of equipment land on the hood of her car at the mall.

It doesn't matter, if he is designing a structure to operate under seismic conditions he needs to retain a licensed civil engineer who understands the imporatance factor for buildings. It really isn't all that complicated, but if that type of design is necessary you are legal obligated to have the design performed by a competent licensed engineer. Failing that, well then a little time in jail or a few thousand dollar fine from the jurisdiction leveed against your company will probably change their opinion about using properly educated staff to do the task at hand.

I have another question pertaining this. from ASCE 7-05 code, why even from the same occupancy (lets say occupancy III), the value of importance factor is different for seismic (I=1.25) and wind (I=1.15)? please help me