Jettying is a Medieval technique used in timber framed buildings in which the second story protrudes beyond the walls of the first story floorspace below. This was a rather interesting way to make the most of available resources. It is sometimes referred to as 'garrison' with reference given to military or defensive functionality.

The use of jettying in structural design began in the Medieval Era, sometime after 1000 AD. In time, it became a prominent architectural characteristic during Medieval times, and especially of England's Tudor Era (1485-1603). This was a time in which space was at a premium within town centers, as travel was still dangerous and expensive. It would eventually spread across the Atlantic to be used in many Colonial American towns.

Structural Advantages

There are a number of advantages gained from this type of design. It increased the floorspace of upper floors, making the most of available land, as well as providing some cover from the elements next to the building.

But the structural effects on floor beams were just as important. In a normally framed structure, where the upper stories have the same footprint as the lower ones, bending-moment forces are zero at the ends of floor beams. In this case, loading is arranged in such a way as to maximize bending-moment forces and deflection at the center of the floor span.

There are two primary considerations when determining the size of beams. One is the strength under loading, the other is serviceability. That is, how much does the beam deflect under loading? The best way to reduce deflection in a beam is to increase the depth of the beam. This uses more space and adds quite a bit of cost and construction difficulty for timber framed structures in the Medieval Era.

Within a structure that utilizes jettying, negative moment is created at the ends of floor beams, causing them to bend upward under the weight of the floors above. In this case, maximum bending moment and deflection are reduced. This allows for the use of much more shallow beams, reducing cost and increasing floorspace.

This phenomenon created unusual and distinct architecture within European civilization. Small buildings throughout the continent can be noticed with upper stories protruding beyond the footprint of the floors below. This created more street space around buildings in towns, and created outdoor covered storage in the rural areas that could be used for storing firewood.

Depression Era Bridges

This technique was also used in the construction of bridges in times of limited resources. End sections of bridges could be designed to overhang piers. These overhangs would have no foundation support at the very end of the bridge, thus subtracting from moment forces, and reducing needed design strength.

This also increased the total length of the bridge without adding the complexity and cost of additional foundations. Many bridges were constructed for the US Forestry Service from the 1930's into the 40's using this unique design.

Originally published March 6, 2021
Researched and Written by: Thomas Acreman

Sources:
  Surveys of London Houses by John Schofield
  The book of buildings by Richard Reid
  ASCE Standard 7-05 by ASCE/SEI
  University of Warwick on Internal Jettying
  Manual of Steel Construction by AISC
     Beam overhang one support uniform load
     Beam overhang one support concentrated load
     Simple beam uniform load