Kevin Fay outlines the construction challenges in recreating the historical roof of St Mel’s Cathedral, Longford while Jonathan Macauley focuses on the engineering aspects of the traditional queen posts trusses
Civil

 

Authors: Kevin Fay, BSc (Eng) Dip Eng MIEI, contracts director, Gem Purcell Ltd and Jonathan Macauley, BEng CEng MIStructE MIEI, director, Design ID

On 20 October 2014, the authors discussed the restoration of St Mel’s – one of the largest conservation projects currently under way in Western Europe – at an event jointly hosted by the Heritage Society and Structures and Construction Division, in association with the Institution of Structural Engineers, Republic of Ireland Branch. Click here to view the entire webcast presentation.

The Cathedral Church of St Mel is the cathedral church of the Diocese of Ardagh and Clonmacnoise, located in the town of Longford. St Mel’s Cathedral was devastated by a fire on the morning of 25 December 2009. By the end of that day, all that was standing was the exterior walls, portico and campanile. The roof had burned through and collapsed into the crypt. After much investigation, a fire in an original brick lined chimney to the rear of the cathedral was established as being the most likely cause of the fire.

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St Mel’s original roof

After pre-qualifications, interview and a competitive tender process, Gem Purcell Ltd (GPL) was selected as the preferred contractor to restore and rebuild the landmark St Mel’s Cathedral.

Gem Purcell Ltd is a joint venture partnership which brings together over 55 years of collective experience in conservation and restoration of landmark public buildings, including Tullamore Courthouse, Dublin Castle, Government Building and the Department of An Taoiseach, Sligo Courthouse, National Art Gallery, Castletown House, Pearse Museum, Rathmines College and Limerick Courthouse.

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St Mel’s roof – January 2010

Gem Purcell Ltd is committed to the continued restoration of St Mel’s Cathedral, noted as being the largest ecumenical restoration project in Western Europe, to its former architectural presence whilst protecting and reflecting the heritage of the building.

Roof construction challenges


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Plan of St Mel’s at eaves level

Unlike the structure of a new building, the existing masonry structure of St Mel’s Cathedral has a number of variances, all of which had to be taken into account in the setting out and roof overall design in order to achieve the necessary geometrical and technical specification requirements. The following existing variances had to be considered:

  • Varying height eaves level along the length of the nave on gridlines E and F;
  • Varying height eaves level from east to west at each gridline across the width of the nave;
  • The nave walls along gridlines E and F are not parallel and vary in width along the length of the nave, thus resulting in varying span widths for the queen post trusses;
  • There are original vertical slots formed on the inner face of the nave wall to locate the truss haunches. These vertical slots are not aligned across the nave and are out of alignment by over 400mm in some locations. This means that the trusses when installed will not be parallel and all purlins will be of varying length.

Coupled with the above, two fixed parameters had to be achieved:

  • A maximum pitch of 25° to a minimum pitch of 22.5° for the Blue Bangor roof slates;
  • A fixed horizontal ridge line.

Gem Purcell Ltd initially looked at installing the roof trusses horizontally across the nave, however as the east and west vary in height by on average 90mm (the west being higher), this presented problems with cutting into the new coping stones fitted at eaves level to match the original and also affecting the roof pitch outside the parameters stipulated above. To achieve a solution that addressed both of these fixed parameters, the trusses (bottom chord) had to be installed off level by 90mm across the nave.

When assessing the roof pitch and ridge height, GPL reviewed the maximum and minimum widths along the nave. The maximum clear span dimension occurred on gridline 5 and the ridge-line height was established at this gridline, having the longest common rafter length. This ridge line will was kept constant along the length of the remaining nave. As the ridge line is constant and the eaves level vary at each gridline, the roof rafters pitch also has to vary at each location.

To accommodate this minimal varying pitch along the nave, the timber purlins were raised or lowered as required at each gridline by packing them with site surveyed hardwood timber packers. Gem Joinery, the truss manufacturers and installers had to keep the top of the purlin and the top of the principal truss rafter practically flush at the highest location. This occurs on gridline 16. The lowest situation is on gridline 5. From our assessment using geometry and site checks, the purlin will vary in height by approximately 50mm from the lowest to highest locations on the nave roof.

To ensure that the pitch of the roof rafters never fall below 22.5° after the truss has been rotated by 90mm, the queen post trusses were manufactured by Gem Joinery with a roof pitch of 23°. We have also indicated the typical roof cross sections at both gridlines 5 and 15, being the narrowest and widest widths along the length of the nave.

The setting out philosophy adopted by GPL in conjunction with Gem Joinery and PCE allowed the detail structural analysis to be carried out by Design ID. The final roof characteristics had a constant ridgeline level, a queen post truss with a principal rafter pitch of 23° and installed roof rafter pitches varying from 22.9° to 23.5°.

Queen post trusses and primary roof structure


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Typical queen post to nave indicating haunch supports

Design ID Consulting Ltd was first contacted in February 2013 by GEM Joinery as structural engineers with experience in designing traditional timber structures – GEM Joinery having been appointed by Gem Purcell Ltd to manufacture and install the timber roof structure to St Mel’s Cathedral. It was a fortuitous contact, as Design ID had the previous day witnessed the installation of virtually identical queen post trusses they designed at Larne Market Yard, Co Antrim. St Mel’s Cathedral offered a great opportunity for Design ID to build upon its experience and to assist GEM Joinery and Gem Purcell Ltd with this important element of the restoration works.

The initial design and concept for the primary timber roof structure was prepared by Punch Consulting Engineers (PCE), the consultant engineers for the project. They worked in conjunction with Fitzgerald Kavanagh & Partners, the project architects to develop a scheme design for the overall roof which met with the requirements of both the client and also An Bord Pleanála in providing a timber roof structure using the same construction methods to that of the original roof.

The main roof structure consists of:

  • 600x900mm Penrhyn County Grade Blue Bangor natural slates on
  • 35x50mm treated timber counter battens on
  • 22x50mm treated timber battens on
  • Tyvek Supro breathable under-laying membrane
  • 18mm marine plywood on
  • C16 150x44mm treated timber common rafters at 400mm centres on
  • C18 200x150mm treated timber purlins at varying centres on
  • Traditional timber queen post trusses.

The primary roof structure consists of 13 queen post trusses along the length of the nave with a bottom chord length of approximately 13.5m supporting timber purlins, which in turn support the common rafters and roof build-up. King post trusses span the transepts and finally lean-to trusses span the side aisles.

The trusses are manufactured from a combination of Douglas fir, German spruce and a glue laminated bottom chord, as allowed by An Bord Pleanála. The trusses were designed to support a service gantry and significant ceiling loads in addition to the dead, live and wind loads applied to the roof pitch.

The sequence of load application was of particular importance as the haunch members had the potential to exert a significant outward thrust onto the side walls of the upper nave which was not deemed to be acceptable or in accordance with the original design prepared by Punch Consulting Engineers.

PCE’s adopted solution was to design the trusses to support dead (excluding ceiling), live and wind loads without the need for the haunch members and only install the haunch members once all the above loads had been applied. Subsequently, the ceiling support structure and plasterwork was added. A sequential approach to the analysis and design was adopted to prove the theory could work.

Joint design and apse ceiling structure


As with most timber structures and in particular traditional timber construction, the focus of the design was creating joints that met the current requirements of Eurocode 5 along with other current standards and expectations and also that met the An Bord Pleanála requirements of using traditional methods of mortise and tenon joints, cotter, gib, stirrups and pins etc.

The key joint is the principle rafter to tie beam mortise and tenon joint. With compression forces of up to 243kN being transferred though this joint, the challenge was to develop a joint using the minimal amount of steel and in keeping with the original design. The most important aspect of this joint is the shoulder rebate as this transfers the axial load via direct bearing into bottom chord.

The final design of this joint consisted of four M20 grade 8.8 threaded bar housed internally within the mortise and tenon joint. In addition, a 10mm stainless steel stirrup strap was connected externally around the joint as per a traditional queen post detail. The through bolts were fixed through this stirrup on the bottom boom where it was notched into the bottom chord (see attached details).

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3D modelled view of the apse ceiling structure

The apse ceiling support structure spans from the semi-circular stone walls onto the second queen post roof truss. The principle elements were seven arched radial trusses, supporting four lines of purlins in turn supporting the infill rafters. A lot of design emphasis was placed on creating a 3D analysis model to produce the most efficient timber solution. The second reason for creating a 3D model was to undertake a clash detection exercise, to ensure the proposed solution suited the finished roof geometry, especially as the existing walls were non-symmetrical and required some creative work by the plastering team to disguise the irregularities!

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Plan details of apse truss layout with in plane timber bracing

The radial trusses exerted a significant thrust onto the mid-span of the supporting queen post truss that was had to be transferred into the side walls by a system of in plane timber bracing.

Installation

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Nave roof truss installation

In order to protect the remaining structure after the fire and to prevent excessive moisture ingress into the building fabric, a steel cladded temporary roof had been placed on St Mel’s Cathedral. At the time of the new roof structure installation, this temporary roof covering had to remain in place to protect the completed lime plaster works to all areas below. GPL in conjunction with Gem Joinery devised an installation sequence.

Using the newly installed solid blue limestone copings as a runway, a timber shuttle track system was placed on top to take a specially designed steel trolley that would support and guide the queen post trusses into position. The queen post trusses were lowered with a tower crane through an opening formed in the temporary roof. Each truss was individually manoeuvred by hand into position. See photographic sequence below

Substantial completion of St Mel’s Cathedral is expected for Christmas 2014 with midnight mass being celebrated on Christmas Eve, five years after the devastating fire that almost destroyed this historic listed building.

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  Authors: Kevin Fay, BSc (Eng) Dip Eng MIEI, contracts director, Gem Purcell Ltd and Jonathan Macauley, BEng CEng MIStructE MIEI, director, Design ID On 20 October 2014, the authors discussed the restoration of St Mel’s – one of the largest conservation projects currently under way in Western Europe – at...