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FOR ENGINEERS - COMMON PITFALLS

FOR ENGINEERS - COMMON PITFALLS

Summary:
  • Project designers often find their structure squeezed between ADA slope regulations, the 100 year flood event or other clearance mandates.
  • American Disabilities Act may impact your project significantly in other ways.
  • This industry expresses bridge arch, i.e. camber, as a percentage of length equals the rise at the center.
  • Stream loads need to be investigated if local hydrology is not known.
  • Skewed ends, freezing water and snow can be the cause of some surprises.
The Squeeze: Because the American Disabilities Act (ADA) limits a trail surface or ramp to a maximum 5% slope, (or in some circumstances 8.3%,) it can become problematic, or just plain expensive, to raise the travel surface enough to clear rail road tracks, roadways and flood events. Excel Bridge recommends you be alert for this common problem. One fact that is not always apparent is that these bridges can easily accommodate unequal elevations at the abutments. Just specify "dead load camber only," i.e a flat bridge, and specify the amount of drop from one end to the other.

Below deck dimensions: Pony and through trusses can greatly diminish the "top deck to low steel" dimension over a girder type bridge, but there are limits. Let Excel help you determine this measurement based on the particulars of your specification. Some generalizations are:

The "U" section configuration is only possible with spans under 70-100 feet. Top deck to low steel 11 1/4" to 15".

The "H" section must be used for longer spans, heavier loadings or to limit height or railings. Top deck to low steel dimensions may be 2' to 7'.

Here is a typical abutment plan and elevation view. The "step height" shown must vary with bridge particulars, please call for assistance.
Bridge Arch: The pre-fabricated steel bridge industry defines "camber" as the amount of rise at the center of a radius divided by the length of the bridge. The easiest way to translate this expression into deck slope is to lay out the radius and measure the slope at the extreme ends. You will find that a 1.2% camber yields 5% slopes at bridge ends, and 2% camber yields a 8.3% slope. Most engineers and architects seem to opine that if a project is not required to meet ADA, a 2.5% camber is the most attractive and walkable choice, and rarely will a bridge be cambered over 5%. A quick calculation will show that the arch of the bridge is very little help to the designer to clear a certain required elevation.

  • Dead load camber only


  • 1.2% camber (i.e., 5% slope)


  • 2% camber (i.e., 8% slope)
Note that in trusses, the camber is not a structural element. For a flat bridge just specify "dead load" camber, and Excel will arch the bridge enough to offset it's own weight.

Stream Load: Be sure this issue is investigated. The designer has three choices when crossing water:
  1. Raise the abutments enough to clear the flood waters.
  2. Have the bridge designed to withstand flood forces, if the resulting back up or water flow diversion is acceptable. Generally stream flow of up to 12 fps can be accommodated, but less as the bridges become longer.
  3. Design the bridge as a break-a-way. Most engineers specify the bridge manufacturer to provide a massive cable attachment bracket under the bridge at the end floor beam. Sometimes bridges that are washed off the abutments are simply set back in place. But damage or complete destruction is likely as well. The theory behind the cable is to prevent the bridge from hurting someone as it tumbles down stream, and maybe lessen the damage to the bridge.
American Disabilities Act: In addition to the "Bridge Arch" discussion above, a project designer needs to consider several other bridge and ramp issues. If there are no reasonable alternative routes for the disabled, ADA law may apply:
  • If a bridge has unequal abutment elevations, effectively making it a ramp, it should be specified as "dead load" camber, i.e. flat.

  • If the ramp effect or camber causes a slope over 5%, toe plates and grabbable pipe handrails must be included. If the 5-8.3% slope continues further than 30', a 5' level landing much be designed into the bridge deck. ADA does not allow slopes over 8.33%.

  • Many times ramps for highway overpasses are made partly or completely of earthwork and concrete. But sometimes it is cheaper or faster to use ADA bridges for the upper sections. Please call and we can help you with this determination.

  • For further discussion of this topic, click here.
Snow Loads: In mountainous areas, many National Parks have observed a multiplied load due to snow cohesion when a bridge is buried in snow. During Spring melt the bridges are crushed, not because they could not withstand the snow directly above, but because, as it settled in mass, the snow applied load from much of the snow adjoining the column of snow over the bridge. Some parks now specify 250 psf up to 450 psf as uniform live loads.

Freeze damage: Years ago it was thought that tiny pin holes in the welds or self-tapping screws would not admit significant moisture, but field observations have regularly reported swelled tubes due to freezing. Specifying drain holes has become the norm.

Skewed ends: Unlike beam bridges, truss bridges are more complicated on a skew. If your project is using a truss bridge, and requires one or more bridges to have a skewed end, in the plan view, be aware that it will add several thousand dollars or more for each skew, and sometime cause other difficulties. When possible, often a better solution is to skew the abutment bearing shelf(s) instead of the bridge ends.

Longevity: Wood bridges usually do not last as long as steel, usually require piers, and the railings require regular maintenance.

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