Understanding Vehicle Turning Radius in Parking Areas
In blogs in the months past, we looked at a condition of an off-centered girder support post in the intermediary span between the sides of a deck. We discussed how it’s important that this particular post be off-centered, in the layout of most typical Washington DC row homes because space is relatively limited and without an offset, the post would actually block the access to the driveway and the garage. Although it seems like off-centering a intermediary post would compromise the structural capacity, generally, the span remaining at the wider area of the opening is still small enough that it maintains capacity.
The post thickness and height, associated footing, and the thickness and height of the girder are all parts of the analysis of the limitations to the opening size.
The geometry of parking can be a little bit complicated because there’s another level of consideration that should be factored into the overall spacing requirements. If a car is turning in from an alley that runs perpendicular to the driveway, the distance from the alley may require a wider amount of space available between the post underneath of the deck.
In certain circumstances, where a perpendicular alley runs directly into the area of the driveway, the turning radius and the associated wider amount of width may not be a requirement or a factor. That scenario is a bit rare though. In most cases, where there is a driveway behind a garage, built under a city row home, especially here in dc, the backyard spaces are normally relatively limited.
The amount of space required to make the turn, depends on the particular vehicle, but when entering a relatively thin driveway from a perpendicular angle, it might be tight. Therefore, the amount of width of the driveway required depends both on the turning radius of the vehicle and the width of the alley.
The geometric turning radius requirements become particularly important around the transition point between alley and driveway. A typical passenger vehicle requires a minimum outside turning radius of approximately 20-25 feet, but this assumes a set of optimal conditions without other impediments. In the tightly spaced urban layout of DC rowhouses / yards, drivers would often need to make multi-point turns, especially when the typical alley-to-driveway angle is 90 degrees. This maneuver typically requires a minimum driveway width of about 10-12 feet at the entrance point, with potential tapering or reduction possible at deck post locations.
The overall layout, configuration, and associated geometry becomes yet more complex when looking at the deck support posts. Posts should generally be set back a minimum of at least 3-4 feet from the driveway edge to allow for additional space to accommodate some simple space to maneuver tight turns. When posts need to be placed closer to the driveway edge, based on the deck configuration, protective bollards or wheel stops may be useful.
The height of vehicle space under the deck should also account for modern SUVs and larger vehicles, if the space is available, typically 7 feet or more is ample. Just to explain this clearly though, changing the height is rarely ever an option because that height is normally based on the available space between the existing grade and the existing building configuration. The height between the top of the garage floor and the top of the kitchen floor minus a small offset to lower the deck below the kitchen door threshold or floor height and the thickness of the deck assembly provides the remaining available space. There’s no easy way to change that or increase that.
Additionally, the driveway surface slope and grade transitions must be considered. An abrupt grade change between alley and driveway can make longer vehicles scrape the undercarriage, so a gradual transition layout is ideal. This often requires planning of deck post placement to avoid creating obstacles in these particular transition spaces. In cases where space is particularly tight, considerations might be made to allow for angled entry points or modified post spacing to optimize the available turning radius while maintaining structural integrity of the deck above.
The next picture shows a clear shot to pull or back into the garage through the area under tge deck, similar to what a vehicle and driver might have to interface when parking. Currently most people still drive themselves instead of using driverless technology. In the future it might not be as important, but at this point in time it would be almost always preferred to have a garage door as wide as possible and have as much clearance to make it easier to squeeze a vehicle into an otherwise relatively tight space. Especially when making a parking maneuver like this in reverse, the extra few inches might be greatly appreciated.
The next picture below shows a different but similar type of deck. In this case though, one of the big differences is that the deck did not extend above the area of the driveway and the parking garage. Instead, the house was wide enough that if the deck were to be left just at the side outside of the garage door head area, that could still be just large enough to function as intended.
Here though, you can see that the original builder has installed cross bracing to run between the deck posts. This is a much stronger form of bracing. Knee bracing is strong, but cross bracing like this has the same effect but to a much greater extent. However, in this particular case, unlike this scenario shown above, a pedestrian could not just walk between the post of the deck, here, they have to walk around those posts because the ex-bracing creates a barrier.
The next picture shows another deck that we looked at in a different blog article. Here you can see though that they did not even install the girder to extend all the way out to the part of the left where an extension of the deck has been added. This will create a problem. However, the deck post might be spaced just enough so that both the vehicle can fit and that the support load that they carry is evenly enough space to meet the capacity of both the girder and the posts.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us
