By Leslie Jo Hurwitz, PE, Brian Fisher, PE, and Carl Schneeman, PE
Recent events, public perception, and the fact that electric vehicles (EVs) are heavier than gas powered (ICE) vehicles have caused many to question whether the current building code minimum design requirements are adequate to accommodate increasing EV sales. How this will affect minimum required design live loads depends on factors such as electric vehicle sales trends, the weights of the vehicles themselves, and the different loading scenarios.
Today and Future Trends
Today, approximately 260 million vehicles are on the road and roughly 15.6 million passenger vehicles were sold in 2023. Roughly half of 2023 sales were crossover and SUV type vehicles, with the balance largely split between traditional cars and pickups. Trends show that car and van sales are steadily declining while the proportion of pickup sales have been relatively stable over the past decade.
So, how do EVs fit in the mix?
The auto industry is undoubtedly going electric; however, it will take quite a while for EVs to become the typical, much less dominant, passenger vehicle on the road. For comparison, in 2023 approximately 1.2 million electric passenger vehicles were sold, representing 7.9% of annual sales, per Argonne Laboratories data. Please note that the term EV herein includes only fully electric vehicles. Table 1 indicates Walker Consultant’s projections of BASE case (expected trend) and HIGH case (accelerated adoption) for electric vehicle sales and the corresponding percentage of such vehicles on the road. Both projections are based on a consensus of auto industry consultant projections. The on-the-road projections are based on a model developed by Walker that considers industry projections of total passenger vehicle sales through 2030, population growth, as well as historic vehicle scrappage rates to determine total vehicles and EVs on the road.
Sales projections of EVs as a percentage of all vehicles sold is expected to increase as more vehicle choices are offered, prices decrease, and technology improves; however, the actual EVs on the road as a percent of all cars on the road is increasing at a much slower rate. In 2040, the estimated sales range from 55% – 85% but the percentage of EVs on the road is only projected to be 23% – 42%.
Vehicle Weight and the Design Vehicle
Vehicle Weight
Walker has been tabulating vehicle size data since 1983 for cars, with light trucks (crossovers, SUVs, pickups and vans) added in 1996. Adjustments were made in 2016 to account for the percentage of pickups and vans that are classified as heavy duty and used for commercial purposes rather than personal transportation. This annual analysis uses sales data reported by Automotive News for individual makes and models, which is correlated to size, height, and weight data per the manufacturer websites. Historically, the primary focus of this analysis has been to study the footprint of vehicles to determine parking dimensions; however, the weight and height of vehicles have become items of interest while crossovers and SUVs have become more prevalent on the road. Overall, vehicles today have gotten wider by 3 inches, generally stayed less than 6 feet tall, and still have lengths appropriate (or slightly less) for 18-foot-long stalls as compared to vehicles of the 1980s.
When it comes to the weight of passenger vehicles, 70% to 75% of all cars sold each year in the U.S. since 2001 including EVs, have curb weights between 3,000 pounds and 5,000 pounds. Overall, we have observed a reduction in vehicles of less than 3,000 and 4,000 pounds (predominantly cars), and an increase in vehicles that are 4,000 to 5,000 pounds, reflecting the increase in crossover sales. But, as demonstrated in Figure 1, the percentage of all passenger vehicles that are over 6,000 pounds has remained less than 5% since 1996.
What Is a Design Vehicle?
The “Design Vehicle” is commonly used in traffic engineering to design roadways and intersections throughout the world. The concept has been applied to parking to define the composite vehicle on which parking geometrics are based. It is assumed that all cars using a parking structure will be the Design Vehicle and it is defined as the 85th percentile vehicle on the road from smallest (0%) to largest (100%).
By assuming all cars parking in a structure are the Design Vehicle for parking layout purposes, 84% of the vehicles actually parked in the structure will likely be smaller and have a more comfortable experience parking than the 14% of the vehicles that are larger than the design vehicle. In fact, the probability of having a parking space occupied by a vehicle larger than the Design Vehicle parked on both sides of it and across the aisle is 15% multiplied by 15% multiplied by 15%, or 0.33%. Very unlikely!
By studying annual sales, not the specific number of vehicles on the road, we are able to annually monitor vehicle size trends. Therefore, the Design Vehicle is not changed each year, but rather the analysis enables identification as to when a trend has been repeated for several years. We can then be reasonably confident that a lasting change has occurred. Since the average age of vehicles on the road is 12 years, the Design Vehicle on the road will not reach revised dimensions for 5 to 7 years. While some may argue that the average vehicle has gotten larger, the sales data definitively identifies that the 85th percentile vehicle footprint has not changed more than a few inches in length or width in the last decade. This same sales data suggests that the footprint of EVs and conventional vehicles is also similar.
Are EVs Heavier Than Conventional Vehicles?
But what about weight? Are EVs heavier? Based on Walker’s tracking of vehicle sales and properties, yes, generally they are. Typical electric vehicles do appear to have a curb weight greater than their traditional counterpart. Over the last decade, the 50th percentile electric vehicle has a weight of approximately 4,400 pounds whereas the same percentile conventional vehicle has a weight of approximately 3,700 pounds. As an example, the 2023 Volvo XC40 EV has an approximate curb weight of 4,780 pounds compared to 3,970 pounds for its conventionally-powered all-wheel drive variant. But as we look at larger vehicles a different trend emerges from the data.
Design Vehicle Weight
When considering a design vehicle curb weight, the 85th percentile weight vehicle in 2022 had a curb weight of just over 5,000 pounds. The 85th percentile weight between 1996 and 2018 varied between 4,400 to 4,900 pounds, but since then has been slightly increasing on an annual basis. The 85th percentile curb weight of vehicles over the past five years is roughly 5,000 pounds, whereas over the last 10 years was 4,900 pounds. Therefore, it is reasonable to identify the Design Vehicle curb weight of passenger vehicles is currently 5,000 pounds.
Considering electric vehicles, study of sales data from 2011 through 2023 yields information relative to their specific design vehicle weight. Over the prior 11 years, the 85th percentile electric vehicle weight is 4,861 pounds, represented by the Audi Q4 e-tron. Over the prior 5 years, the 85th percentile electric vehicle weight is 4,877 pounds represented by the VW ID4. Note that these figures are very near that of the Design Vehicle curb weight for all vehicles.
Yes, a small number of EV models with curb weights of 7,000 pounds and up to the 9,063 pound Hummer EV have been introduced, but they sell in small numbers, or exist above the 85th percentile point. In fact, the Hummer is not a “light duty passenger vehicle,” but rather a “heavy duty” truck. In its earlier 1992 to 2006 ICE form, Hummers were never more than a fraction of 1% of cars on the road.
Figure 2 shows the trends in the 85th percentile weight of EVs as well as the total passenger vehicle sales since the first EVs were sold in 2011.
Live Loads
A Brief History
Before the International Building Code (IBC) was first published in 2000, three general building codes were used throughout the U.S. that are considered ‘model’ or ‘legacy’ codes. Those codes were: The BOCA National Building Code primarily used in the northeast, the Standard Building Code (SBC) generally used in the southeast, and the Uniform Building Code (UBC) that was used in the rest of the of the U.S. Over time, the IBC has been adopted in most, if not all, areas of the country. Much of the design load criteria contained in IBC and in ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, are applicable to the determination of parking garage live loads.
A paper by Y.K. Wen and G.L. Yeo, “Design Live Loads for Passenger Cars Parking Garages” in Journal of Structural Engineering,” published in March of 2001,provided a better understanding of live loads in parking structures and recommended an appropriate value for design.
Looking at the maximum load effects on beams and columns due to vehicle loads over the anticipated life expectancy of a parking structure, Wen and Yeo determined the axial loads and beam midspan moments using the design live load of 50 pounds per square foot (psf) would be two to three times greater than floor members would actually experience. They also concluded that dynamic amplification resulting from moving vehicles would not increase the prescribed design load unless it is a speed ramp with slope greater than 10-degrees. Their final conclusion recommended a design live load of 40 psf with no allowance for area-based reductions. This recommendation did not apply to special purpose or densely parked structures especially when there would be no variety in the vehicle type parked.
About a year later, the March 2002 Concrete International magazine included an article by Javed Malik who was concerned with the increase in SUVs on the road and argued for point and uniform live loads in parking structure design to be significantly increased suggesting the wheel point loads be increased by 50% and the uniform live loads doubled.
His analysis was subject to an extremely conservative and rare building framing system for parking structure construction. The typical construction materials and framing systems for parking structures at the time was, and still is, precast concrete or cast-in-place post-tensioned concrete systems.
A rebuttal white paper by Mary Smith, PE, and Tony Chrest, PE, of Walker Consultants, reviewed the vehicle trends at the time and analyzed the assumptions made by Malik for maximum wheel loads, axle distribution, vehicle curb weights, and gross vehicle weight ranges (GVWR). The use of the 85th percentile vehicle in the range of lightest to heaviest curb weight was proposed for the first time to be used as the Design Vehicle in the determination of uniform loads since at the time, the practice of using the 85th percentile vehicle was the standard for parking geometrics and commonly used in traffic engineering.
Using the Design Vehicle, Smith and Chrest analyzed loading conditions for various parking structure concrete and steel framing systems as shown in Figures 3-5.
Next, they considered heavier vehicle density stacked parking such as rental car storage or valet parking.
Consistent with Wen and Yeo, Smith and Chrest’s analysis found that in spans greater than fifteen feet, uniform loads were all less than 40 psf. Shorter span conditions and some uncommon high vehicle density conditions were slightly greater than 40 psf.
Today, the IBC/ASCE 7 still specifies a 40 psf design live load for parking structures.
What Happens When Electric Vehicles Are Concentrated in One Area?
Walker intentionally checked for the live load when an entire area is parked with EVs due to the local requirements to design 20% or more of the parking spaces to be EV Ready. This is increasingly leading to EVs comprising all the parking stalls in one area. Using the theories of analysis used by Wen and Yo and Smith and Chrest, the uniform load is evaluated over typical bay sizes that are based on framing systems commonly utilized in parking structures in the U.S. today as shown in Figures 4 through 5.
To start, Wen and Yeo’s study found that while dynamic effects from moving vehicles can increase the anticipated load, “It would be reasonable to conclude that dynamic amplification due to moving vehicles would not cause the equivalent uniformly distributed load (EUDL) to be more than the case of fully parked two-way traffic bays with vehicles waiting in the aisles.” In short, it was determined that with moving vehicles in a parking structure, the suggested dynamic amplification factor is independent of vehicle weight since the shallow slopes in which vehicles interact with floors does not transmit appreciable vertical load to the structure. As is consistent with generally accepted practice, when closely spaced concentrated wheel loads are distributed at a 45-degree angle from the direction of span they quickly overlap other wheel loads, allowing the load to be judged as a uniformly distributed load.
Following the Smith and Chrest philosophy of using the Design Vehicle for the determination of uniform loads, Walker used the 85th percentile electric Design Vehicle to establish the anticipated loading on structural members. Following this approach, all parking spaces in a structural bay were assumed to be occupied by electric Design Vehicles at curb weight plus cargo and that a string of electric Design Vehicles loaded to their gross vehicle weight rating (GVWR) are simultaneously placed each way along the drive aisle. This approach is a conservative estimation of uniform live load in a bay since the likelihood that all the vehicles in an area are all an 85th percentile Design Vehicle simultaneously traversing the drive aisles is extremely small. Additionally, for most public parking facilities serving visitors, commuters, or shoppers the odds of all the vehicles in the drive aisle being loaded to the full GVWR are even less likely.
Let’s now check the uniform load for the electric Design Vehicle created by the scenario in Table 2.
Assumptions:
- Parking Geometrics: 8 ft. 6 in. stalls @ 90 degrees on 60 ft. modules
- Assume Electric DV at 6908 lbs. are driving both ways down the aisle
- Assume Electric DV at 5858 lbs. are parked in every stall
The authors then considered what the equivalent uniform distributed load would be for the framing systems commonly employed in parking structures in the U.S. today:
- Precast Garage.
- Post-tensioned, cast-in-place concrete with one way slabs and beams.
- Conventionally reinforced concrete with 30” x 30” column grid.
- Steel beams and column with P/C double tees.
- Steel beams and columns with a cast-in-place post-tensioned slab.
The calculated equivalent uniform loading with an electric Design Vehicle for each system is shown in Table 3.
As can be seen, none of the typical parking structure framing systems have an equivalent uniform load exceeding 40 psf when considering the electric Design Vehicle. This should be no surprise as the design vehicles weights between conventional and electric vehicles are quite similar.
Conclusions and Recommendations
Despite all the discussion surrounding electric vehicle weights exceeding the weight of their internal combustion engine counterparts, Walker’s data base of vehicle sales and weight indicates that the electric Design Vehicle is of similar curb weight than the overall passenger vehicle mix sold in recent years. As such, the overall 85th percentile weight vehicle is most appropriate for live load calculations. Combined with Wen and Yeo’s finding that maximum load effects on beams and columns using a uniform load of 50 psf would be two to three times greater than what they expected the members to actually experience, the current IBC and ASCE 7 prescribed live load of 40 psf for passenger vehicle garages is viewed as an accurate representation of expected conditions for the current information available on EV’s and their presence on the road.
Structures that have experienced deterioration represent a very different structural concern, one that is not specific to electric vehicles or internal combustion vehicles. Aged or neglected parking structures that have experienced deterioration may need to be assessed structurally to ensure that they have a sufficient strength for the loads in which they are intended to support. If the available strength is determined to be inadequate, then it is only prudent to repair, strengthen, or restrict loads to such a deteriorated structure. However, this is not due to electric vehicles, but rather the overall shift to SUVs and pickups, as compared to 20 years ago.
How vehicle weight will change remains to be seen as battery technology is still in its infancy and could change to be lighter yet more efficient and achieve longer ranges; but it is viewed by the authors as unlikely that trends of ever-heavier cars, crossovers, and pickups that meet consumers vehicle range expectations will continue. Further, is it premature to consider an increase in design loads on the speculative basis of vehicle weights 25 years in the future if/when EVs are finally approaching 50% of the vehicles on the road?
As engineers, it is important to consider that realistic, rational, and safe loads for the design of facilities that are to be used by the public. However, finding a prudent balance for reasonable design is also necessary so that designs may not be viewed as unreasonably conservative and undermine public trust. Further, judicious designs are needed more than ever to foster sustainable designs and reduction of costly construction materials. Continual monitoring of data will help ensure that the evolution of code load standards reflects actual load scenarios combined with rational safety factors rather than on public perception. ■
References
- Gosh, S.K., Ph.D. Design Live Loads for Parking Structure Deck. PCI Journal, Jul-Aug 2005, 140-142.
- Malik, Javed, Sport Utility Vehicles and the Design of Parking Garages. Concrete International, Mar 1 2002, 67-71.
- Smith, Mary, PE, and Chrest, Anthony, PE. SUV’s and Parking Structure: Is there a weight problem?. Walker Parking Consultants, 2002
- Wen, Y.K, and Yeo, G.L.. Design Live Loads for Passenger Cars Parking Garages. Journal of Structural Engineering, Mar 2001 280-289