CIVIL ENGINEERING · STRUCTURAL TRUSSES

Pratt Truss vs Howe Truss: types, advantages and disadvantages

Everything about the Pratt truss and the Howe truss in one place — definition, how each one works, types, advantages and disadvantages, real-world uses, whether they are safe, and how to identify which one you’re looking at.

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Tension member Compression member

Toggle to compare how the same downward load travels through a Pratt truss versus a Howe truss.

Fig. 04 — Context

Why Compare the Pratt Truss and the Howe Truss?

Students and engineers compare these two truss types because they are the clearest teaching example of how member orientation controls force type. Understanding why one pattern became the modern default (Pratt) while the other became mostly historical (Howe) teaches a transferable lesson: in structural design, the cheapest, lightest solution usually keeps the longest, most highly loaded members in tension rather than compression, because tension members do not need to resist buckling.

Same basic geometry, opposite diagonal direction — an easy visual comparison
Directly illustrates the tension-vs-compression design trade-off
Both are still tested topics in civil and structural engineering courses
Both still appear in real bridges, roofs, and heritage structures today

Fig. 05 — Classification

Types of Truss Structures (Where Pratt and Howe Fit In)

Pratt and Howe are two members of a larger family of truss types used in bridges, roofs, and towers. Knowing the family helps place both patterns in context:

Pratt truss — diagonals in tension, verticals in compression; the modern standard for steel spans
Howe truss — diagonals in compression, verticals in tension; common in timber roof and historic bridge construction
Warren truss — no verticals, only alternating diagonals forming equilateral triangles
K-truss — diagonals split into a “K” shape to shorten unsupported member length on very deep trusses
Baltimore & Pennsylvania trusses — Pratt variants with extra sub-diagonals for long-span railway bridges
Fink truss — a Pratt-derived roof truss optimized for long-span, low-slope roofs
Bowstring truss — curved top chord combined with Pratt-style web members

Among all of these, the Pratt truss and Howe truss remain the two most frequently taught because every other type is essentially a variation on their diagonal-and-vertical logic.

Fig. 06 — Mechanics

How a Pratt Truss and a Howe Truss Actually Work

How the Pratt truss carries load

Under a typical downward (gravity) load, a Pratt truss bends slightly like a beam: the top chord goes into compression and the bottom chord goes into tension, exactly like the top and bottom of any loaded beam. Because the diagonals run from the outer bottom chord up to the inner top chord, the geometry forces every diagonal into tension, while the short vertical posts push together and go into compression. Use the animated diagram at the top of this page (toggle “Pratt Truss”) to watch the blue tension diagonals and orange compression verticals highlight together.

How the Howe truss carries load

A Howe truss uses the exact same rectangular panel, but the diagonals are flipped so they run from the outer top chord down to the inner bottom chord. That flip reverses the force pattern: the diagonals now go into compression and the verticals go into tension. Historically this was convenient because carpenters could cut the diagonals from solid timber (strong in compression) and use simple threaded iron rods for the verticals (strong in tension, and easy to tighten to pre-stress the truss).

Engineer’s note A simple memory trick: in a Pratt truss the diagonals point like a “P” pinched toward the middle (tension); in a Howe truss the diagonals point outward like they are “heading Home” toward the supports (compression).

Fig. 07 — Data table

Pratt Truss vs Howe Truss: Full Comparison Table

Feature	Pratt Truss	Howe Truss
Patented	1844, Thomas & Caleb Pratt	1840, William Howe
Diagonal direction	Slopes toward center	Slopes away from center (toward supports)
Diagonal force	Tension	Compression
Vertical force	Compression	Tension
Ideal diagonal material	Slender steel rods / eye-bars	Timber or thick steel section
Material efficiency (steel, long span)	High	Lower
Common era	Modern steel bridges	19th-century timber & combination bridges
Typical use today	Steel highway & railway bridges, roof trusses	Timber roof trusses, heritage/covered bridges
Buckling risk of diagonals	Low (in tension)	Higher (in compression)
Relative cost for long spans	Lower	Higher

Fig. 08 — Practical guide

How To Identify a Pratt Truss vs a Howe Truss (Step-by-Step)

Here is a simple, reliable how-to method for telling the two apart just by looking at the structure:

Locate the center of the span. Stand back and find the midpoint between the two end supports.
Trace the diagonal members. Follow any diagonal bar from the top chord down to the bottom chord.
Check the slope direction. Diagonals sloping down and inward, toward the center, indicate a Pratt truss. Diagonals sloping outward, toward the supports, indicate a Howe truss.
Confirm with the vertical members. In a Pratt truss the verticals are usually the heavier compression members; in a Howe truss the verticals are often thin steel tension rods, sometimes with visible turnbuckles.

Fig. 09 — Pros & cons

Advantages and Disadvantages

Pratt Truss — Advantages

Long diagonals only need to resist tension, so they can be slender and light
More material-efficient for long steel spans, lowering cost
Lower risk of diagonal buckling under load
Simple, well-understood analysis and fabrication

Pratt Truss — Disadvantages

Vertical members must be braced against buckling in tall trusses
Less naturally suited to timber construction than Howe
Standard form can need extra sub-panels (Baltimore/K-truss) for very long spans

Howe Truss — Advantages

Well suited to timber diagonals paired with iron/steel tension rods
Tension rods can be tightened with turnbuckles to pre-stress the frame
Simple and economical for short-to-medium timber roof spans

Howe Truss — Disadvantages

Diagonals in compression need thicker sections, adding weight and cost
Higher risk of buckling on long steel spans
Largely replaced by the Pratt truss for modern steel bridges

Fig. 10 — Applications

Common Uses of Pratt and Howe Trusses

Pratt truss uses Steel highway and railway bridges, industrial building roofs, crane booms, and long-span pedestrian bridges, wherever a light, tension-efficient diagonal pattern reduces steel tonnage.

Howe truss uses Timber roof trusses for houses, barns, churches and warehouses, and historic covered bridges, wherever wood diagonals in compression can be combined with steel tension rods.

Fig. 11 — Safety

Is a Pratt Truss or Howe Truss Safe?

Yes — both truss types are safe when they are designed, fabricated, and maintained correctly. Safety in either pattern depends on the same fundamentals of structural engineering, not on which diagonal direction was chosen:

Loads (dead load, live load, wind, snow, seismic) are calculated correctly for the intended use
Members and connections are sized with an appropriate factor of safety
Materials are protected from corrosion (steel) or rot and insect damage (timber)
The structure receives regular inspection and maintenance over its service life

Historical truss bridge failures — Pratt or Howe — are almost always traced to overloading beyond the original design rating, deferred maintenance, corrosion, or a poorly designed connection, rather than to any inherent flaw in the truss pattern itself.

Fig. 12 — Decision guide

Which One Should You Choose?

For a new steel bridge or long-span roof, the Pratt truss (or one of its modern variants) is almost always the more economical choice because it keeps the longest members in tension. For a timber roof structure or a restoration of a historic covered bridge, the Howe truss is often the appropriate and authentic choice, since it was specifically developed around the strengths of wood and iron rod construction.

Fig. 13 — Frequently Asked Questions

Pratt Truss vs Howe Truss — FAQ

What is the main difference between a Pratt truss and a Howe truss?

The main difference is the direction of the diagonal members and which force they carry. In a Pratt truss the diagonals slope toward the center and carry tension while the verticals carry compression. In a Howe truss the diagonals slope away from the center and carry compression while the verticals carry tension.

Which is stronger, Pratt truss or Howe truss?

Neither is inherently stronger — strength depends on span, material, and design load. The Pratt truss is generally more material-efficient in steel because its long diagonals only resist tension, while the Howe truss needs heavier diagonals to resist compression and buckling.

Why is the Pratt truss more common in modern bridges?

Putting the long diagonal members in tension lets engineers use slender steel rods or eye-bars instead of thick compression members, cutting material cost and weight for the same span, which is why the Pratt pattern became the modern default.

What is a Howe truss used for today?

The Howe truss is still common in timber roof trusses for houses, barns, and warehouses, where diagonal members are cut from wood that performs well in compression, combined with steel tension rods for the verticals.

Are Pratt and Howe trusses safe for bridges and roofs?

Both truss types are safe when sized by a qualified structural engineer using correct design loads, safety factors, connection details, and routine maintenance. Age, corrosion, overloading, and lack of inspection — not the truss pattern — are the usual causes of failure.

Can a Pratt truss and a Howe truss be combined?

Yes. A hybrid lattice truss can place diagonals in both directions within each panel so members share both tension and compression, adding redundancy for very long or heavily loaded spans.

What are the disadvantages of a Howe truss?

A Howe truss needs bulkier diagonal members to resist compression and buckling, which adds material and weight as the span grows, making it less economical than a Pratt truss for long steel spans.

How do you identify a Pratt truss vs a Howe truss visually?

Look at the diagonal members. If they angle downward toward the center of the span, it is a Pratt truss. If they angle outward toward the supports, it is a Howe truss.