Physics professor David Kohlstedt explains the key forces in play: If the bolts are too loose, a strong wind could push over a post. If the bolts are too tight, the plates may be prevented from slipping and bolts may thus fail to snap when it should. Moreover, bolts that are too loose or too tight may prevent slip systems from performing another critical function: slowing down the impacting vehicle. Many slip systems also rely on installers properly torquing connector bolts. Impact load diagram from Roadside Design Guide, 2011, by the American Association of State Highway and Transportation Officials, Washington, D.C. These may not work properly if hills are involved or if a given crash is more complex. If hit at an unexpected angle, they may not break as planned. Even omnidirectional (non-angled) systems reflect a built-in assumption of relatively horizontal impacts. They can work along the sides of divided highways but are problematic in traffic medians between lanes. Still, slip base systems have their limitations. Inclined slip bases, for instance, work best if the direction of impact is known or can at least be assumed. They can also let cars zip under two-post signs with fewer complications (as shown below). Slip bases paired with with hinged upper connections can also help preserve infrastructure, like telephone lines running along the tops of utility poles (as shown above). The design of these inclined slip bases is optimized for hits from an assumed direction of impact. Instead of simply sheering sideways, angled-base posts are actually launched up into the air, causing them to pass over the impacting vehicle when hit straight on. The specific slip system shown above involves tilting the joined plates at angles (generally 10 to 20 degrees relative to the ground). The joints are sometimes hidden beneath street-level base covers but are often clearly visible (even if they go unnoticed by most): Inclined slip bases in Boston by Tom Parent The critical plate-to-plate connections underpinning slip systems can be subtle. After a crash, these systems also make subsequent infrastructure repair easier as well: in many cases, a new upper post can simply be bolted onto the undamaged base post below. This allows the above-ground piece get knocked out of the way while the vehicle passes over its underground counterpart.ĭuring a crash, breakaway systems like these can greatly reduce damage to cars and their occupants, helping save the lives of drivers and passengers.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |