On Howe Trusses Work [yet again]

The benefit of u-frame action

In previous posts we have looked at different forms of truss and how they work. We started with an intrigue about the underlying logic of the Howe Truss and moved on to look at the intricacies of various other forms. Something that may not have been obvious, perhaps until part way through the post immediately prior to this one, is that we have thus far existed in a 2D world, we have not yet looked in the third dimension and considered what happens to trusses out of plane.

The place where this is most obviously important is the top chord. We have discovered already that the top chord is in compression and that failure by compressive buckling is proportional to the square of a member’s length. In the plane of a truss its effective length is relatively short due to the position of the internal chords connected along its length.

Out of plane there is of course no restraint from and therefore the effective length of the top chord is the full length of the truss. The top chord is almost certainly going to buckle. This is a pretty big issue to have overlooked. Fortunately there are several solutions available to solve this particular problem.

In almost all cases trusses come in pairs, for example one either side of a bridge deck. Our first option is therefore to take benefit from the bridge deck, which can be attached to the top chord of both. Since the deck is relatively stiff in plane, sometimes it may even be braced, it will have the capacity to prevent the top truss chords from displacing laterally. They are therefore unable to buckle.

The trouble with this solution is that it is rarely viable in practise. If, for example, our bridge were to span over a motorway, and the deck were located on the truss top chords, one of two scenarios would occur. Either the bottom of the truss would project down into the road below or the whole truss would need to be lifted up into the air meaning that much larger ramps would be needed to get traffic up unto the bridge.

For this reason a better solution is to place the bridge deck on the bottom chords. This, however, reintroduces the problem of top chord buckling. If the trusses are tall enough the possibility exists to introduce a horizontal truss between the two top chords. The purpose of this truss is to resist the out-of-plane loading that results from top chord buckling. It does so by the same means that the vertical trusses resist the loads to which they are subject.The two vertical and one horizontal truss exist in a symbiotic relationship, because the vertical ones can equally resist buckling action in the horizontal truss.

It is also worth noting that if our hypothetical bridge structure is outside it will also be subjected to the full force of the wind and our horizontal truss can, in combination with the bridge deck, be used to resist the wind.

A problem arises when the vertical trusses are not tall enough to permit passage below the horizontal truss. You would not wish to duck as you passed along the bridge. It would of course be possible to increase their height, but this seems an inelegant solution and a waste of material if the trusses need not be so tall. A different approach is needed.

This is the perfect opportunity for u-frame action. U-frame action requires continuity between the vertical chords in the two bridge trusses. This is achieved with horizontal members that connect them beneath the bridge deck. If the connections are stiff enough to resist bending forces then a series of rigid u-shaped structures are formed along the length of the bridge. If the top truss chords then try to move laterally and buckle their vertical chords are able to provide resistance by cantilevering from the horizontal member below the bridge deck. This is known as u-frame action. In this way the bridge trusses remain stable without the need for a horizontal truss. It is a neat solution, which is not obvious to the casual observer, and makes for an elegant bridge with unobstructed views.