How to Complete a Bridge TS&L Study in Under an Hour

A Type, Size, and Location (TS&L) study is one of the most important — and most time-consuming — deliverables in early bridge project development. Done manually, it can take a senior engineer one to two days to assemble: site data gathering, span arrangement calculations, girder selection from published tables, cost estimating from unit price databases, formatting the report.

This guide walks through the full process of a TS&L study, explains what each step requires, and shows where the manual process can be compressed without sacrificing technical rigor.

What Is a TS&L Study?

A TS&L study (sometimes called a "bridge type study" or "structure type study") is a preliminary engineering document that answers three questions:

• Type: What structural system should this bridge use? (Precast concrete girder, steel plate girder, box girder, arch, etc.)
• Size: How long will each span be? How wide is the deck? How deep are the girders?
• Location: Where are the piers and abutments? Are there hydraulic, utility, or geotechnical constraints that affect placement?

The study typically evaluates two to four alternatives, provides a cost estimate for each, and recommends one alternative to carry into final design. State DOTs usually require a TS&L report before authorizing PS&E (Plans, Specifications, and Estimates) funding.

The Seven Inputs You Need Before You Start

Gathering these before sitting down saves significant rework:

Step 1: Establish the Span Arrangement

The span arrangement — number of spans and individual span lengths — is the first major decision in a type study. It controls:

• Total girder count and depth (longer spans need deeper, heavier girders)
• Number of piers (more piers = higher substructure cost, shorter spans)
• Whether a span crosses a hydraulic hazard zone in a single clear span

The practical starting point for most highway bridges is:

• Under 100 ft total: Single span, precast concrete girder (WF36–WF54), no piers
• 100–250 ft: Two or three spans, precast concrete or steel plate girder, balanced span arrangement
• 250–600 ft: Three to five spans, likely steel plate girder for longer spans, or spliced precast for medium spans
• Over 600 ft: Cable-stayed, tied arch, or segmental box — outside the scope of most preliminary TS&L tools

For a balanced multi-span arrangement, the rule of thumb is that end spans should be approximately 75–80% of the interior span length to equalize dead load deflection and minimize uplift at interior piers.

Step 2: Select the Girder Type

For spans under approximately 175 ft, precast prestressed concrete wide-flange girders are typically the most economical choice in most US regions. Steel plate girders become competitive above 130–150 ft and are preferred when depth is constrained (shallow superstructure required) or when very long spans are unavoidable.

Standard Wide-Flange (WF) Girder Span Ranges

Step 3: Size the Deck and Transverse Layout

Girder spacing is the key transverse dimension. It drives:

• Deck slab design (spans between girders)
• Number of girder lines (total cost)
• Live load distribution factors per AASHTO LRFD Section 4

Typical girder spacing ranges from 6 ft to 12 ft on-center. Wider spacing reduces the number of girders and saves cost — until the deck slab becomes the controlling cost element (typically above 10–11 ft spacing for an 8-inch deck). The minimum number of girder lines for a standard two-lane bridge (28–32 ft deck) is typically four to five.

Step 4: Develop the Cost Estimate

A TS&L cost estimate is a Class D estimate — it is expected to be within ±20–30% of final bid price. The components to price are:

Apply a regional multiplier for your state — costs in the Pacific Northwest or Northeast can run 20–35% higher than the national base; costs in the Southeast or Mountain West can run 10–20% lower. State DOT cost data reports (published annually by most DOTs) are the best source for current regional unit costs.

Step 5: Evaluate Constructability and Site Constraints

Every TS&L alternative should be screened against practical constructability factors that can eliminate an otherwise acceptable structural type:

• Overhead clearance: Can long precast girders be delivered to the site? Rural roads with low railroad underpasses or tight curves may limit maximum girder length (typically 130–140 ft without special haul permits).
• Crane access: Does the site allow erection cranes alongside the structure? Water crossings, steep embankments, and live traffic may require a falsework-free erection method.
• Stage construction: Does the existing bridge need to stay open during construction? Staged replacement requires the new structure to fit within a portion of the footprint.
• Scour and hydraulics: Does pier placement require deep foundations or costly scour countermeasures?
• Seismic: Does the seismic design category trigger special detailing, isolation, or capacity design that makes one alternative significantly more expensive?

Step 6: Write the Recommendation

The TS&L report ends with a recommendation of one alternative to carry into final design. A strong recommendation includes:

1. A brief statement of why the preferred alternative was selected (cost, constructability, risk, aesthetics, local preference)
2. A summary comparison table of all alternatives (span arrangement, bridge type, estimated cost, key risks)
3. A statement of the cost estimate basis and accuracy class
4. Any assumptions that could materially change the recommendation (e.g., "assumes spread footing foundations are feasible — geotechnical investigation required")

How Long Should a TS&L Study Take?

Manual process: 12–24 hours of engineering time for a 2–3 span highway bridge, excluding project management and document formatting.

With Bridge Copilot: Under 60 minutes for the core engineering computations. The engineer's time shifts from calculation to judgment — reviewing the generated alternatives, applying site-specific knowledge that the tool cannot have, and writing the narrative recommendation.

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