What is a high rise?

How is a high-rise defined?

The International Building Code (IBC) defines a high-rise building as a building with an occupied floor located more than 75 feet above the lowest level of fire department vehicle access. This definition is primarily based on the height at which firefighting operations from the ground become less effective.

The measurement of 75 feet is typically taken from the lowest level of fire department vehicle access to the floor of the highest occupiable story. The significance of this height is that it presents unique challenges for fire safety, structural engineering, and evacuation procedures, which are distinct from those in lower buildings.

As usual, building codes are ever evolving and being updated, it’s always a good practice to refer to the latest version of the IBC or consult with a local building authority for the most current definitions and requirements.

Are there any exceptions?

Yes.

  • Massachusetts

    • Definition – A building more than 70 feet in height above grade plane.
    • Measured from – Average grade plane NOT lowest level of fire department access
    • Measured to – The roof structure NOT the lowest level of the highest occupied floor
    • Impact – Typically trigger high rise requirements 1-story sooner than the IBC definition
  • Chicago

    • Definition – A building more than 80 feet in building height.
    • Measured from – Average grade plane NOT lowest level of fire department access
    • Measured to – The mean elevation of the highest roof plane NOT the lowest level of the highest occupied floor
    • Impact – Can trigger high rise requirements 1-story sooner than the IBC definition
  • Michigan

    • Definition – A building with an occupied floor located more than 55 feet above the lowest level of fire department vehicle access.
    • Measured from – Lowest level of fire department access.  Same as IBC.
    • Measured to – Highest occupied floor.  Same as IBC.
    • Impact – Typically trigger high rise requirements 1-2 stories sooner than the IBC definition
  • Nevada

    • Definition – A building with an occupied floor located more than 55 feet above the lowest level of fire department vehicle access.
    • Measured from – Lowest level of fire department access.  Same as IBC.
    • Measured to – Highest occupied floor.  Same as IBC.
    • Impact – Typically trigger high rise requirements 1-2 stories sooner than the IBC definition

 

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What is a podium?

How is a podium defined by IBC?

As defined in Section 510.2:

A building shall be considered as separate and distinct buildings for the purpose of determining area limitations, continuity of fire walls, limitation of number of stories and type of construction where all of the following conditions are met:

  • The buildings are separated with a horizontal assembly having a fire-resistance rating of not less than 3 hours.
  • The building below the horizontal assembly is of Type IA construction.
  • Shaftstairwayramp and escalator enclosures through the horizontal assembly shall have not less than a 2-hour fire-resistance rating with opening protectives in accordance with Section 716.5.

    Exception: Where the enclosure walls below the horizontal assembly have not less than a 3-hour fire-resistance rating with opening protectives in accordance with Section 716.5, the enclosure walls extending above the horizontal assembly shall be permitted to have a 1-hour fire-resistance rating, provided:

    • The building above the horizontal assembly is not required to be of Type I construction;
    • The enclosure connects fewer than four stories; and
    • The enclosure opening protectives above the horizontal assembly have a fire protection rating of not less than 1 hour.
  • The building or buildings above the horizontal assembly shall be permitted to have multiple Group A occupancy uses, each with an occupant load of less 300, or Group B, M, R or S occupancies.
  • The building below the horizontal assembly shall be protected throughout by an approved automatic sprinkler system in accordance with Section 903.3.1.1, and shall be permitted to be any occupancy allowed by this code except Group H.
  • The maximum building height in feet shall not exceed the limits set forth in Section 504.3 for the building having the smaller allowable height as measured from the grade plane.

What is a podium in layman terms?

A podium is a “base” structure that serves as a fire-separated platform to build another building on top of.  The IBC has specific regulations and requirements for podium construction, mainly concerning fire safety and structural integrity. This style of construction is often used in mixed-use buildings where the lower floors (the podium) might contain commercial or parking spaces, while the upper floors are residential or office spaces. The podium design allows for a separation of uses and can also serve as a way to provide a larger footprint for the tower portion of the building without extending the entire building to the ground, thus allowing the integration of mixed uses vertically.

What are the major benefits?

  • Podium design offers several benefits, particularly when integrating parking or retail spaces below residential uses. This design approach is quite popular in urban settings where surface parking is not possible.
    • Allows the more cost effective wood construction to be used and maximized at residential spaces where bearing walls are naturally occurring
    • Naturally accommodates retail, parking, and amenity spaces below the podium where wall free space is desired.
    • Effective a maximizing the land-use the use to accommodate the tallest wood framed building allowed by code.  Essentially building the maximum height wood building on top of a pedestal, “the podium”.

What is a podium constructed with?

  • Podium is required to be non-combustible Type IA construction with a 3-hour fire rating.
    • Composite steel
      • Steel beams with a slab-on-deck
    • Post-tensioned concrete
      • Typically a flat slab
    • Conventional concrete
      • Typically a flat slab (but thicker than post-tensioned)

How tall can a LGMF building be?

What is the absolute tallest light-gage metal building allowed by IBC?

Standalone LGMF Building

LGMF buildings can be considered either Type I or Type II buildings as they are non-combustible construction.  From there, it is simply a matter of the required fire rating for each classification.

IBC Table 504.4 limits Type II buildings to 5-stories or Type IB buildings to 12 stories.  Refer to diagram above.

The tallest standalone LGMF building would be 12 stories and be limited to 180 ft (easily accomplished in 12 stories).

LGMF building on podium

Similar to a wood framed podium, IBC Section 510.2 allows the use of podium construction to build a LGMF building above a Type IA podium structure.

In this case, the limiting factor in the maximum height allowed for a Type IB building of 180ft.  This means we can put as many podium levels under the 12-story LGMF as we can fit to stay below the 180ft height limit.  See diagram above.  This typically means around 4-6 stories additional depending of floor-floor heights and the selected structural system.

Can a LGMF building be built on a podium?

Absolutely!

  • LGMF structural systems are heavier than wood framed structures, so expect more substantial transfer podiums.
  • Podium is required to be non-combustible Type IA construction with a 3-hour fire rating.
    • Composite steel
      • Steel beams with a slab-on-deck
    • Post-tensioned concrete
      • Typically a flat slab
    • Conventional concrete
      • Typically a flat slab (but thicker than post-tensioned)

Major benefits?

  • Provides an intermediate price point between wood framing and Type IA framing, steel and concrete
  • May be required for certain use/occupancies.  For instance, the I-2 use/occupancy may require non-combustible construction even at 2-4 story building.
  • Typically comes with lower insurance costs when compared to wood framed construction, due to the non-combustible nature.
  • Can help in meeting FM global requirements where wood framing cannot.

Things to consider?

        1. Grade can be your friend – the max building height is measured from average grade, so if you have a sloping site, average grade can be above the ground floor elevation.
        2. Use post-tensioned concrete for the podium levels.  PT provides the thinnest structural system to help maximize ceiling height.
        3. There are a variety of LGMF structural systems.  Some have shallower structural assemblies than others, but this typically has a tradeoff in terms of limited span lengths or weight of structure.
          1. Joist based systems – Ecospan, Hambro, etc
          2. Slab based systems – Epicore MSR, New Millenium Versa-dek, etc

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