HOWE TRUSS CONCEPT

The Howe Truss was designed by William Howe in 1840. In 1840 William Howe

patented the Howe truss, another truss that enjoyed widespread popularity. Howe based his

design on the limited stress analysis information available at that time, the first designer to do

so since previous trusses were unadaptable to analysis (Edwards 1976:156-157). The Howe

truss used metal vertical tension rods and timber diagonal compression members. This joint

use of metal and wood materials for bridge components, called a "combination truss," was a

significant transitional feature in the eventual development of an all-metal truss. The

popularity of the Howe truss resulted, in part, from its comparatively simple erection. The

Howe truss design eliminated the need for skilled carpenters to notch and peg wooden jointed

bridges by using threaded iron rods for verticals and simple junction boxes as connections

(Kemp and Anderson 1987:19). As bridge historian Eric DeLony wrote, "The Howe truss

may be the closest that wooden-bridge design ever came to perfection. For simplicity of

construction, rapidity of erection, and ease of replacing parts, it stands without rival" (DeLony

1994:11).

It used mostly wood in construction and was suitable for longer spans than the Pratt truss.

Therefore, it became very popular and was considered one of the best designs for railroad

bridges back in the day. Many Howe truss bridges exist in the North West United States,

where wood is plentiful.

Howe Truss in model bridges

The common truss designs, including the Howe, is that they were designed a long time

ago. They were designed when bridges needed to fill a specific role, and for the particular

resources that people had available. For instance, the Howe truss design used a lot of wood as

opposed to the Pratt which used more iron. This made the Howe popular earlier on when iron

was expensive to produce.

The Howe trussed used wooden beams for the diagonal members, which were in

compression. It used iron (and later steel) for the vertical members, which were in tension.

The Pratt truss was the opposite. Thus, because the diagonal members are longer, the Howe

truss used less of the more expensive iron material. It made good use of the cheap wood

which was readily available.

For model bridges, we typically only use wood. Our compression and tension

members are both made out of wood. If you wanted to be fancy, you could use string or metal

wire for the tension members. Nonetheless, in reality, the reasons why the Howe design

became popular are not applicable to model builders.

Howe truss

Howe truss illustrated - the diagonals are under compression under balanced loading

The relatively rare Howe truss, , includes vertical members and diagonals that slope up

towards the center, the opposite of the Pratt truss. In contrast to the Pratt Truss, the diagonal

web members are in compression and the vertical web members are in tension. . Examples

include Jay Bridge in Jay, New York, and Sandy Creek Covered Bridge in Jefferson

County, Missouri.

A howe truss at first appears similar to a Pratt truss, but the Howe diagonal web

members are inclined toward the center of the span to form A-shapes. The vertical members

are in tension while the diagonal members are in compression, exactly opposite the structure

of a Pratt truss. this design was common on early railroads. The three drawings show various

levels of detail. The thicker lines represent wood braces; the thinner lines are iron tension

rods. The Howe truss was patented as an improvement to the Long truss.

How the forces are spread out

Here are two diagrams showing how the forces are spread out when the Howe Truss is

under a load. The first shows the load being applied across the entire top of the bridge. The

second shows a localized load in the center of the bridge. In both cases the total load = 100.

Therefore, we can take the numbers as a percentage of the total load.

Howe Truss Center Load

Howe Truss Spread Load

Similar to all the major truss designs (Pratt, Warren, K Truss, and Howe), when the

load is centered on the bridge the forces are much greater on the internal truss members than

if the load is spread out along the top of the bridge. The same principle applies if the load was

coming from the bottom of the bridge. The diagrams shows the load applied to the top of the

bridge.

Using Bridge Designer program from JHU and plug-in selected design. Load the

design in the same way your bridge will be loaded as specific in the rules and guidelines were

given to build the bridge.

REFERENCES

http://garrettsbridges.com/design/howe-truss/