Once upon a time on a former job east of Seattle, I agreed to manage the site construction of an 800-acre project that was permitted for clearing and grading but stalled due to lack of access. The catch was, before erosion control work and clearing could proceed, a fish-bearing stream running the length of the project had to be crossed. At the time, there was no other way to ingress the property. Permanent bridge access had been designed and approved however the local jurisdiction wouldn’t issue a construction permit. Each day that the bridge permit was delayed, financial and strategic penalties increased for the owner. With no relief in sight, I searched for an alternative.

Further downstream and partially hidden by blackberry bushes, there existed an abandoned dirt crossing. Years ago, a large culvert had been filled over with what could only be described as “suspicious material.” With plenty of headroom, the old steel pipe was still in good condition however the fill’s integrity was another issue. Whether or not it consisted of compacted structural material was never in question. Plainly, it didn’t. We were on our own.

After conversing with our contractor and a can-do geotechnical consultant, we decided to clear the approaches and proof-roll the fill. Beginning with a pickup truck, we gradually worked up in weight to a point where we felt comfortable blading the fill with a small dozer and proof-rolling it with a loaded solo dump truck. Confident that the fill would support slow-moving highway-rated vehicles, we rented a pair of twenty-foot, 1-inch steel plates and laid them flat down the centerline of the fill. The plates were bedded into fabric and butt welded, end-to-end. We tack-welded diamond steel grates along the running surface and installed tire bumps along each side. Our crossing could only be used one way but we were in business. As a bonus, this crossing allowed us to save significant dollars by being able to construct the permanent four-lane bridge from both sides of the stream after the permit was issued – six months later.

Had this old fill not been available, how else could have we installed temporary access? Here’s a list of possibilities. We could have used:
• Log Stringers
• A pre-engineered Timber Bridge
• Railroad Flatcars
• Precast Reinforced Concrete Slabs
• A Modular or one-piece Steel Bridge

Though each of the above bridge-types differ greatly, they are all used to accomplish the same goal and thus can have a role in the scheme of land development. To determine which temporary bridge alternatives might work best in your situation, consider:
• Cost
• Location and span
• Acceptable width
• Expected loading
• Local materials available
• Bridge delivery lead time
• Offsite access requirements
• Available onsite working area
• Environmental constraints and ease of gaining permits
• Desire to move or reuse the bridge later, or turn it into a future project amenity

Bearing soils
In order of importance, cost reigns supreme. If installing a temporary bridge will allow you to construct in better weather, maintain schedule, and keep your contractors onsite, combine this gross value with any costs saved in being able to arrest or reduce your window for borrowing money. If you plan to reuse the bridge, build it into a future trail system, or utilize it for emergency access, stack the value of these benefits onto your analysis. Then compare this amount against the cost of installing the temporary bridge.

Expected loading entails deciding whether or not you want your bridge to accommodate highway or off-highway loading. Remember that off-highway construction vehicles can range significantly in size and weight. Once you commit to a design loading and deck geometry, you’ll have to stick with it. As with any bridge, you must regulate and protect temporary structures from being overloaded or abused. Approaches from pavement to bridge locations must be built study enough to handle delivery traffic and at the same time, drain properly so budget accordingly.

Lead time is another important consideration. How soon can the bridge be designed and shipped? What road conditions and minimal curve radiuses will be needed to guarantee truck delivery? Will overhead wires have to be dropped?

When assessing lead time, consider that you may have to schedule the following in order:
1) Evaluating and deciding if a temporary structure will be used. If so, what type?
2) Meeting onsite with an engineer, contractor, and/ or bridge salesperson.
3) Meeting with local authorities to discuss requirements for
approval.
4) Performing soils work and/ or environmental studies.
5) Performing site surveying and topographic work. (A one-foot contour interval is recommended for bridges)
6) Scheduling design, engineering and submittals.
7) Permit time.
8) Time for drawing up contracts, delivering the goods, assembly, and construction subcontracting.

Installation
As a function of planning, you’ll have to consider abutments. Bridge abutments can be as simple as logs and timber sills or they can be constructed of poured reinforced concrete. Precast reinforced concrete abutments work well at remote locations, where access to the far side of the span is limited, or where there is too much risk associated with pumping concrete over water.

Span is also a key driver that will help narrow your alternatives. Though they are getting scarcer to find, long, straight, and sturdy logs make fine log stringer bridges. If you can get them, today’s logs are less likely to provide the span and strength required for heavy duty bridging. Timber aside, few engineers know how to design, let alone construct, a log stringer bridge so most owners will pass on this option. Timber beam and modular steel bridge kits are an attractive alternative. Manufacturers of these versatile products offer many options and any unit can be customized. Kits require plenty of ground for sorting and assembly as well as ample area for crane operations. Railroad flatcars are another alternative to consider. Flatcars can be a cheap way to go depending on availability, condition, and span. Up to 90 feet in length, they vary in width up to 10 plus or minus feet. Flatcars are narrow and will only allow one-way vehicular traffic. Unlike prefabricated bridge options, flatcars offer a superstructure only. To be used for bridging, they require the addition of decking, curbs, and timber joists. Consider overall cost when comparing temporary bridge options.

In general, the longer the span, the larger the area needed for assembly and lifting. Longer spans also equate to higher crane costs. In cases involving shorter spans, a pair of stout excavators may adequately handle lifting and placing a temporary bridge. If a crane is required, the need for access and maneuverability cannot be overstated. In extreme situations where area for siting a crane doesn’t exist, heavy-lift helicopters can be used to lift bridge decks or components into position. Another option for lifting and placing bridge decks involves the use of skyline logging techniques. The right operator can perform miracles with cables and winches. Factor in your limitations before committing to any type of temporary bridge.   

You can lease or purchase temporary bridge structures. In addition to preparing for staging, bridge assembly, and lifting, a complete plan should include designs for erosion control, clearing, grading, surface drainage, approaches, and gate security. As a rule, the further that abutments are located back from steep slopes, sensitive areas, and water, the easier it will be to procure permits. In environmentally challenged terrain, factor in how much additional bridge you can afford to ensure gaining timely approvals.  If all else fails and you get to the end of your rope waiting for permits, walk the drainage in search of blackberry bushes. You may end up pleasantly surprised. SLDT