Studying portal braces
Tuesday, February 09, 2010
We all know the situation. You find a truss bridge that is well-preserved, except for one little detail: the plaques are missing, leaving only dangling mounting brackets to taunt you. Meanwhile, county records are spotty or non-existent, destroyed by the Great Courthouse Fire of Aught-Something.
How do you track down the history of the bridge? If you're lucky, the bridge has a distinctive feature that ties it to a particular company (like the 'X' scrollwork used by the Canton Bridge Co. above the portal). Otherwise, the design of the bridge will likely give a vague idea about the time frame, but probably not much else.
For through truss bridges, however, the arrangement of the portal bracing can yield important clues about the bridge's origin and builder.
Take, for example, the Silver Bridge in Arkansas. County records show that the bridge was built in 1929-30, which seems wrong for a pin-connected truss.
It's not immediately obvious, but one of the Silver Bridge's three spans is different from the rest. Not only is it slightly longer, but it has more elaborate portal bracing featuring a lattice on top of curved knee braces.
The other two spans have a simple "A-frame" brace style.
It seems clear that the two sets of spans have different origins and were built at different times. Research by Randall Houp reveals that the bridge was first built in 1912 with three spans. Flooding in 1927 washed away two of the spans, which were then replaced by new pinned through trusses in 1929 by the Lakeside Bridge Company. It's not clear why Lakeside chose to use pin-connected spans (technology which had been obsolete since at least 1920). Perhaps the spans were surplus from elsewhere? I can just imagine the bridge company salesman telling the county officials, "We've got a great clearance price on these discontinued models. Everything must go!"
The different portal braces on this bridge tell the story. Even without confirmation from the county records, it's clear that this bridge had a complex history.
With this in mind, I would like to find a way to classify and study the different styles of portal braces used on metal bridges. Hopefully, by collecting information on the more common configurations, it will be possible to date (to some extent) those bridges that don't have plaques or records of any kind. For instance, it would be extremely helpful to find the oldest examples of A-frame portals so that we could make a statement like, "A-frame portals were never built before 1892" (or whatever the case may be).
A 1904 book, A text-book on roofs and bridges, is one of the few sources I've been able to find about portal bracing.
It explains, "The corresponding end posts of the two vertical trusses are connected by overhead bracing... The function of this portal bracing, as it is called, is both to stiffen the end posts against vibrations and to assist them in transferring the wind pressure from the upper lateral system to the supports of the bridge. Its connections with the end posts are extended as low as the headroom will allow, and the form of the bracing is determined to some extent by the depth available below the portal strut..."
So, let's take a look at some of the portal configurations. To keep my sanity, I've limited my searching to just Missouri bridges. Other states may have wildly different designs, but the basic styles can be found nationwide.
This straightforward design seems to be the most common among one-lane through trusses, especially those built during the 1900s and 1910s.
The 1904 textbook describes the A-frame as "the simplest and most economical of bracing" when enough headroom is available.
Here's an example of a footbridge where the A-frame brace serves as the only connection between the upper chords:
A variation on the A-frame is to add additional members to create the shape of an 'M'. The zigzag diagonal members resemble a Warren truss. In some ways we could say that the portal bracing is a truss-within-a-truss.
For wider bridges, especially those on state highways, a simple 'W' pattern was employed.
The standard style used by the Missouri highway department featured V-laced members arranged in a Warren truss-like form. You could say that this was a truss-within-a-truss-within-a-truss.
Thanks to their low clearance, the 'W' portal braces were frequently damaged by oversized trucks. In these cases, Missouri would sometimes retrofit the bridge with a simple beam allowing for more headroom. That's what happened to this bridge on Old US 40:
Missouri continued to build through truss bridges well into the 1950s, but these later bridges used a single beam for the portal bracing.
The W-style portals could be taken to the next level by adding additional members. The double-intersection Warren style is probably the second most common configuration for one-lane trusses, behind the A-frame configuration.
It could have curved knee braces:
Or stright knee braces at a 45° angle:
Or knee braces built with a solid, rounded plate:
Or knee braces that are incorpoated into the lattice:
More complicated lattices
From a double-intersection lattice, it's only a short leap to reach a "triple-intersection" design, such as this Wrought Iron Bridge Co. creation:
The next step is a quadruple-intersection lattice.
At this point it doesn't really make sense to keep counting the intersections, as there really is no limit to how many members can be incorporated into a lattice. Here's a particularly elaborate portal for a rail bridge over the Missouri River:
Before the widespread adoption of Parker and Pennsylvania trusses, it was necessary to build really tall Pratt or Whipple trusses to span great distances. These tall trusses required significant portal braces, typically featuring a stack of different lattices, beams, and X-braces.
Here's an example from 1885:
This 1886 bridge by the King Bridge Company featured a lattice, X-brace, another lattice, and fancy knee braces.
You've probably seen this portal brace before: the silhouette is at the bottom of every page on this website.
We've seen A, M, and W designs, but there's one letter of the alphabet left: the 'X'. A single X-frame is a typical design for long-span truss bridges.
One 'X' might be stacked on top of another.
Modern welded trusses often incorporate an 'X' design, but without the lower strut to allow for more headroom.
Here are two cantilevered trusses side-by-side, built at different times periods. Notice how the portal design evolved between the two.
Here are some other interesting portal designs from Missouri:
Among all U.S. bridges, the Wells Street Bridge in Fort Wayne, Indiana, has one of the most elaborate and ornate portals. The details are amazing:
The study of portal braces may seem trivial -- and perhaps it is -- but this aspect of bridge design warrants more research to help fill in the gaps for those bridges that are otherwise undocumented.