Introduction

When designing parking garages, live loads are a crucial factor. The requirements for live loads vary significantly depending on whether the garage is for passenger vehicles or for heavy vehicles, such as trucks and buses. In this blog, we will discuss how design live loads are determined and briefly mention the possible impact of electric vehicles on parking garage design.

Live Loads for Passenger Vehicle Garages

For garages built to accommodate passenger vehicles (those with a gross vehicle weight rating up to 10,000 pounds) — such as cars, SUVs, and pickup trucks, the 2024 edition of the International Building Code (2024 IBC) specifies a uniformly distributed minimum live load of 40 psf in Table 1607.1. This load is not permitted to be reduced by the live load reduction provisions of the code, except for vertical members supporting two or more floors, where it is permitted to be reduced by up to 20 percent. The same uniformly distributed minimum live load is also specified in ASCE 7-22 Table 4.3-1. Prior to ASCE 7-02, the uniformly distributed,live load for passenger vehicle garages used to be 50 psf, subject to the live load reduction provisions of the standard. In ASCE 7-02, it was decreased to 40 psf non-reducible load based on a study by Y. K. Wen of the University of Illinois at Urbana Champaign, published in 2001.

ASCE 7-02 Commentary Section C4.8.3 explained that:

• “…Compared with the design live load of 50 psf given in previous editions of ASCE 7, a 40 psf live load represents a 20% reduction but is still 33% higher than the 30 psf one would obtain were an area-based reduction to be applied to the 50 psf value for large bays.”

In addition, both the 2024 IBC and ASCE 7-22 include provisions for concentrated loads, as shown below. Structural members are required to be designed for the greater load effects produced by the uniformly distributed load and the concentrated load. Although concentrated loads generally do not govern the design, engineering judgment should be exercised to check if they might in specific situations.

• for garages restricted to passenger vehicles accommodating up to nine passengers, a single concentrated load of 3,000 lb acting on an area of 4.5 in. × 4.5 in. is to be considered. This represents the load caused by a jack when changing tires on a single passenger vehicle. The load is to be located so as to produce the maximum load effects in the structural member being considered. This may occasionally govern the design of garage slabs against punching shear. It may also govern the flexural design of a double-T section when the load is placed near the end of the flange.

• for mechanical parking structures without slab or deck that are used for storing passenger vehicles only, 2,250 lb of concentrated load per wheel is required to be applied.

Live Loads for Heavy Vehicle Garages

For garages that accommodate heavy vehicles (those with a gross vehicle weight rating of over 10,000 pounds), such as trucks and buses, fire trucks and emergency vehicles, and forklifts and movable equipment, live load requirements are provided in Section 1607.8
Structures or portions of structures where heavy vehicles are permitted to be driven must be designed using vehicular live loads in accordance with the locally adopted code for design and construction of roadways and bridges, such as AASHTO LRFD Bridge Design Specifications. Section 1607.8.1 of the 2024 IBC requires the effects of impact and fatigue to be consideredin these situations. However, for structures meant only for parking heavy vehicles, such as heavy vehicle garages, design for impact and fatigue is not required (see Section 1607.8.3). ASCE 7-22 Commentary Section C4.10.2 explains:

• …For heavier trucks and buses, the vehicular live loads in the AASHTO LRFD Bridge Design Specifications (AASHTO) are to be applied. The AASHTO provisions for fatigue and dynamic load allowance are not required to be applied as the vehicle speeds in garages and vehicle storage areas are much lower than typical highway speeds.

The Impact of Electric Vehicles on Live Loads for Parking Garages

The rise in popularity of electric vehicles (EVs)—EVs comprised over 10% of total passenger vehicle sales in the US in 2024—and the perception of their heavier weight has raised concerns about whether current garage live load requirements are sufficient. As discussed above, there are uniform live load and concentrated live load considerations. While sales are increasing, most estimate that EVs are less than 2% of all passenger vehicles on the road. Regarding the trends in overall passenger vehicle weights, an article in the July 2024 STRUCTURE magazine (https://www.structuremag.org/article/electrical-vehicles-and-parking-structures/) reviews recent trends and compares EV weights to the weight of a typical design vehicle. As the article notes, the overall shift to SUVs and pickups results in similar expected load magnitudes as caused by electric vehicles. There is ongoing research regarding parking garage live loads and the impact of electric vehicles. The Charles Pankow Foundation, along with several partners, has recently been awarded a new research grant aimed at understanding the impact of the increasing market penetration of electric vehicles.

A larger concern regarding EVs that has been raised is the possibility of fire due to thermal runaway of the batteries. Thermal runaway is an uncontrolled, self-heating process where the battery’s temperature rises rapidly and uncontrollably, potentially leading to fire or explosion. These fires have been found to be more challenging to extinguish than traditional internal combustion engine vehicle fires. Research by the Fire Protection Research Foundation found that EV fires tend to develop slower and not get as large, but they burn for much longer. It is likely to be several years before any change proposals resulting from the structural or fire research underway become mandatory building code requirements.