# Can someone explain Section / Table 1604.5?



## Code Neophyte (Jun 9, 2014)

I've never understood the implication of Section 1604.5:

Occupancy Category.  Buildings shall be assigned an occupancy category in accordance with Table 1604.5.

Ok, so we do that -- now what?  What does that Occupancy Category classification drive within the code?  I don't recall ever seeing that, let's say, for a Occupancy Category IV building - maybe an Emergency Operations Center - that the design loads specified in Chapter 16 are to be modified by an increased safety factor?  Or that Type V construction is prohibited?

I'm sure it's obvious, but what am I missing?


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## cda (Jun 9, 2014)

Not an engineer but slept on a train last night,

Have you read the commentary

This section requires classification of the occupancy category of any building in accordance with the nature of occupancy as described in Table 1604.5. The occupancy category serves as a threshold for a variety of code provisions related to earthquake, flood, snow and wind loads. Particularly noteworthy are the importance factors that are used in the calculation of design earthquake, snow and wind loads. The value of the importance factor generally increases with the importance of the facility. Structures assigned greater importance factors must be designed for larger forces. The result is a more robust structure that would be less likely to sustain damage under the same conditions than a structure with a lower importance factor. The intent is to enhance a structure's performance based upon its use or the need to remain in operation during and after a design event.

The impact of a higher occupancy category classification is not limited to increasing the design loads. Compared to Occupancy Category I, II or III, for instance, an Occupancy Category IV classification can lead to a higher seismic design category classification that can, in turn, require more stringent seismic detailing and limitations on the seismic-force-resisting system. This can also affect the seismic design requirements for architectural, mechanical and electrical components and systems.


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## cda (Jun 9, 2014)

The occupancy category determined in this table generally increases with the importance of the facility, which relates to the availability of the facility after an emergency, and the consequence of a structural failure on human life. The categories range from Occupancy Category I, which represents the lowest hazard to life, through Occupancy Category IV, which encompasses essential facilities.

Occupancy Category IV: These are buildings that are considered to be essential in that their continuous use is needed, particularly in response to disasters. Fire, rescue and police stations, and emergency vehicle garages must remain operational during and after major events, such as earthquakes, floods or hurricanes. The phrase "designated as essential facilities" refers to designation by the building official that certain facilities are required for emergency response or disaster recovery. This provides jurisdictions the latitude to identify specific facilities that should be considered essential in responding to various types of emergencies. These could include structures that would not otherwise be included in this occupancy category. This designation would only be made with consideration of broader public policy, as well as emergency preparedness planning within the jurisdiction in question. The reasons for including facilities, such as hospitals, fire stations, police stations, emergency response operations centers, etc., should be self-evident. Some items warranting additional discussion are as follows:

• Designated emergency shelters and designated emergency response facilities. These items repeat the term "designated," which is referring to designation by the building official that the facilities have been identified as necessary for sheltering evacuees or responding to emergencies (see discussion of "designated" above). For example, an elementary school having an occupant load of 275 would typically be considered an Occupancy Category III facility. If that school is designated as an emergency shelter, then the school will be considered an Occupancy Category IV building.

• Facilities supplying emergency backup power for Occupancy Category IV. A power-generating station or other utility (such as a natural gas facility) is to be classified as Occupancy Category IV only if the facility serves an emergency backup function for an Occupancy Category IV building, such as a fire station or police station. Otherwise, the power-generating station or utility should be classified as Occupancy Category III.

• Structures with highly toxic materials in excess of the quantities allowed in Table 307.1(2). This applies only to "Highly toxic materials" (see definition in Section 307.2), which are covered in the second row of Table 307.1(2). That table lists the maximum allowable quantities per control area of materials posing a health hazard. Since the use of control areas is permitted by Table 307.1(2), it is reasonable to recognize the control area for the purpose of making this occupancy category determination. In other words, this would apply only to occupancies classified as Group H-4 based on the quantities of highly toxic material. Recognizing control areas means the occupancy classification could be changed from a high-hazard occupancy, Group H-4, to the appropriate classification by adding either fire-resistance-rated walls or floor-ceiling assemblies in order to divide the building into a number of smaller "control areas." Also note that a Group H-4 occupancy classification could be based on exceeding the quantities permitted for toxics or corrosives [see Table 307.1(2)], but the presence of those materials would not affect the assessment of the facility as Occupancy Category IV.

Occupancy Category III: Occupancy Category III buildings include those occupancies that have relatively large numbers of occupants because of the overall size of the building. They also include uses that pose an elevated life-safety hazard to the occupants, such as public assembly, schools or colleges. In addition, Occupancy Category III includes uses where the occupant's ability to respond to an emergency is either restricted, such as in jails, or otherwise impaired, such as in nursing homes housing patients that require skilled nursing care. A discussion of some of the specific table listings follows:

• Buildings and other structures with a primary occupancy that is public assembly with an occupant load greater than 300. Public assembly occupancies meeting this criterion will typically be classified as Group A in Chapter 3. The wording requires agreement on the determination that a building's "primary occupancy" is in fact public assembly. This could be as simple as verifying that the portion of the building housing the public assembly occupancy is more than 50 percent of the total building area.

• Group I-2 occupancies with an occupant load of 50 or more resident patients but not having surgery or emergency treatment facilities. This category applies to health care facilities with at least 50 resident patients. The term "resident patient" is not defined or used elsewhere in the code, but would seem to refer to locations where those patients receive around-the-clock (24-hour) care as opposed to ambulatory surgery centers or outpatient units. This table entry covers facilities where patients have difficulty responding to an emergency or are incapable of self-preservation.

• Buildings having an occupant load greater than 5,000. Uses that pose elevated life-safety concerns, such as public assembly uses, schools and health care facilities, are covered elsewhere and have a much lower threshold based on the number of occupants. This table entry covers buildings that are large enough to have more than 5,000 occupants, providing added protection for the occupants of larger structures whatever the use happens to be. In order to determine occupant load, the methods outlined in Section 1004 are normally used. Chapter 10 sets forth standards that provide a reasonably conservative number of occupants for all spaces, and while actual loads are commonly less than the design amount, it is not unusual in the life of a space in a building to have periods when high actual occupant loads exist. Because there is no clear rationale that connects the occupant load used to calculate minimum means of egress requirements to the risks associated with structural design, Note a provides some reasonable adjustment to this determination by permitting the use of net floor area. It provides a more reasonable approach for occupancies, such as office, mercantile and residential, that are required to base occupant load on gross area—an area that includes corridors, stairways, elevators, closets, accessory areas, structural walls and columns, etc.

• Power-generating stations, potable water treatment facilities, wastewater treatment and other public utilities not included in Occupancy Category IV. A failure and subsequent shutdown of these types of facilities would not pose an immediate threat to life safety. These infrastructure items are considered Occupancy Category III because of the impact an extended disruption in service can have on the public.

• Buildings not included in Occupancy Category IV containing sufficient quantities of toxic or explosive substances that would be dangerous to the public if released. Buildings included under Occupancy Category IV would be those containing quantities of highly toxic materials that exceed the permitted quantity


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## Mark K (Jun 10, 2014)

In the latest code editions this is referred to as risk category.


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## Code Neophyte (Jun 11, 2014)

So am I correct that this classification basically means nothing to the code official, with there being no references to the classification anywhere else in the code?


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## Frank (Jun 11, 2014)

They mean a great deal to the code official in seeing the the structural engineer and architec tprovided proper design.

See IBC 2009 tables 1613.5.6,  These occupancy catagories in conjuction with the short period and 1-second response accellerations determine the seismic design catagories.

Also used in 1609.1.2 for opening protection exceptions


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## mtlogcabin (Jun 11, 2014)

Code Neophyte said:
			
		

> So am I correct that this classification basically means nothing to the code official, with there being no references to the classification anywhere else in the code?


The term "Risk category" is found 59 times in the IBC. Below are a few examples. We pay particular attention to it due to the seismic zone we are in. There are also special inspection requirements based on the "risk category" of a structure.

HURRICANE-PRONE REGIONS. Areas vulnerable to hurricanes defined as:

1.	The U. S. Atlantic Ocean and Gulf of Mexico coasts where the ultimate design wind speed, Vult, for Risk Category buildings is greater than 115 mph (51.4 m/s); and

RISK CATEGORY. A categorization of buildings and other structures for determination of flood, wind, snow, ice and earthquake loads based on the risk associated with unacceptable performance.

403.2.3 Structural integrity of interior exit stairways and elevator hoistway enclosures.

For high-rise buildings of Risk Category III or IV in accordance with Section 1604.5, and for all buildings that are more than 420 feet (128 000 mm) in building height, enclosures for interior exit stairways and elevator hoistway enclosures shall comply with Sections 403.2.3.1 through 403.2.3.4.

2308

6.	The use of the provisions for conventional light-frame construction in this section shall not be permitted for Risk Category IV buildings assigned to Seismic Design Category B, C, D, E or F.

3408.4 Seismic.

When a change of occupancy results in a structure being reclassified to a higher risk category, the structure shall conform to the seismic requirements for a new structure of the higher risk category.


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## georgia plans exam (Jun 11, 2014)

Also, buildings assigned a risk category of III or IV require special inspections for fire-resistant penetrations and joints per 2012 IBC Section 1705.16. GPE


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## allis_ch (Jun 11, 2014)

Notice where it refers you to ACSE 7 in numerous locations.

ASCE 7 is where you find the structural formulas.  The category influences the Importance factor "I" which occurs in most of the basic formulas.

In general, the higher importance building categories (with higher risk associated- such as emergency services, or assembly buildings with large numbers of occupants) have increased safety factors required in their design.  (Commonly required to a 115% of the "normal" design loads).

Whereas, private farm buildings (with lesser risk) can be allowed to be designed for only 90% of the "normal design" in a number of instances.


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## allis_ch (Jun 11, 2014)

In Most cases, I will equal 1.0 meaning that most buildings are designed for the "standard importance".


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## Code Neophyte (Jan 13, 2017)

So to (maybe) revive this topic and ask the question in a more specific way:  Would the Occupancy Category or Risk Category in some way prohibit a fire station from being constructed as a II-B basic metal building (post-and-beam steel structure with standing seam metal skin)?  It would seem that the intent of these provisions would be to construct a critical facility like a fire station as a Type I building - even in a low seismic risk area or non-hurricane-prone area.


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## mtlogcabin (Jan 13, 2017)

No it would not prohibit it specifically. The metal building manufacture may need additional bracing, install a heavier gauge steel skin with larger fasteners or spaced closer together and the foundation engineer may require more concrete to hold it down in a high wind area.


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## ADAguy (Jan 13, 2017)

So Neo,

This is why the code is a performance code (based on occupancies) with prescriptive (specific) limitations.


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