The Historical Context For The Salisbury Cathedral Spire

2.1 The historical problems

The spire and tower of Salisbury Cathedral have presented problems for builders, clerks of the works, architects and engineers through the centuries. These problems appeared from the very start when the decision was made to construct a stone tower and spire of unprecedented height on a supporting structure at the great crossing that from all available evidence was never intended to support a tower of any significant height. The consequences of this decision have impacted almost every structural component from the supporting foundations to the top of the spire. The impacts include the following:

• In the original cathedral structure, minimal foundations under the great crossing piers have visibly settled, especially at the south-west pier, resulting in an overall 'lean' to the tower and spire to the southwest.

• The crossing piers have a visible bow, a situation that is believed to have prompted the installation of the strainer arches at the great crossing.

• Buttressing, much of which appears to have been added as part of the tower construction, was probably installed in response to outward thrust from the added tower weight that caused shifting of tri-forium piers, cracking of walls and vaulting, and deflections of transept piers that can still be seen in the Cathedral today.

• The base of the tower up to just above the main roof ridges is a comparatively light arcaded structure with comparatively thin two-foot thick outer walls with multiple penetrations. Reinforcing of this level has been implemented on several occasions over the centuries and includes iron framework ties, infilled stair towers and wall openings, and stiffening walls between the arcade columns and the outer walls.

• Extended periods of neglect have resulted in failure of iron reinforcing at the base of the spire, resulting in stone damage and the need for a reworking or replacement of the iron reinforcing and major restoration of damaged masonry on at least two occasions.

• The spire itself has suffered frequent damage from storms and lightning, resulting in cracks and displaced units in the masonry and, in at least one instance, fire damage to the internal timber framing. There is also a visible bend twelve meters from the tip of the spire in the southwest direction that is of unknown origin.

The structural behavior and conditions behind these problems have been considered by caretakers of the cathedral fabric over the centuries and solutions have been implemented. For early work prior to the 17th century, there is little or no written documentation and the history must be conjectured primarily by an assessment of the surviving physical built structure. Salisbury Cathedral, though, is fortunate to have a comparatively extensive documentation of its history since the 17th century that provides insight into the conditions noted and the solutions implemented to maintain the structure through the past 350 years. Two documents in particular are landmarks in the history of English building archaeology and conservation. The first of these is Sir Christopher Wren's assessment on the state of the Cathedral prepared for Dr. Seth Ward, Bishop of Salisbury, in 1668. This document is considered to be the first reasonably comprehensive evaluation of a historic medieval ecclesiastical building in England. The second document is titled A Series of particular and useful Observations, Made with great Diligence and Care, upon that Admirable Structure, the Cathedral Church of Salisbury, written by Francis Price, the Clerk of the Works for the Cathedral from 1738 to 1753. Not only did Price describe the building in words, but he also produced detailed drawings based on careful measurements, providing at the time a unique snapshot of the Cathedral at that specific point in time.

2.2 The structural systems supporting the base of the tower

The concerns with the foundations and supporting pillars at the great crossing of Salisbury Cathedral have been the topic of extensive discussion, conjecture and concern over the years. Some of the conjecture has been based on incomplete evidence or concern extrapolated from experience at other medieval cathedrals. Until the 1980's, when Peter Taylor was able to make specific measurements of the pier foundations at the main crossing, the actual foundation sizes were unknown quantities that many had presumed were insufficient. Measurements taken in the 1980's by Peter Taylor have been used to evaluate the footing situation and using soil mechanics theory developed in the twentieth century have for the first time been able to provide a theoretical basis for explaining the marginal adequacy of the footings under these piers. Taylor notes that the historic concern with the delicacy of the situation has perhaps saved the spire and tower from disaster, as this current theoretical explanation highlights the importance of maintaining the confining soils around the foundations, a contribution that would have been disrupted by any attempt to excavate and enlarge the foundations. (Taylor, 1988, p. 3)

Similarly, the bow of the pillars at the great crossing has been the topic of much discussion over the centuries. The bow and associated cracking and displacement of arcade piers at the triforium level are generally believed to have originated when the tower and spire were added to the original tower base. Much of the engineering that can be found within the main body of the Cathedral to counteract forces imposed by the weight of the tower and spire, including scissors arches at the east crossing and 112 buttresses at the arcade and triforium levels (Price, 1753, p. 35), were believed to have been added during or shortly after construction of the tower and spire in the early 14th century. The exception to this are the east-west strainer arches at the great crossing, which appear to have been installed approximately 100 years after the spire addition. The contribution of these strainer arches, which are supposedly restraining through compression bearing, has been called into question over the years based on visible gaps between the arches and the crossing piers. (Price, 1753, p.53) Strengthening measures have been recommended as recently as the late twentieth century to stiffen the crossing pillars and increase the capability to resist buckling forces, but no specific deficiency has been identified (except the visible bow). The current approach is to monitor the situation to ascertain if the bow is a manifestation of an active condition, or an adaptation of the building structure to stresses from loads imposed in the past that have found a steady state of equilibrium.

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