Hale Allen Jones The Institution of Structural Engineers East Anglian Branch Award for Structural Excellence Submission

Transcription

1 The Institution of Structural Engineers East Anglian Branch Award for Structural Excellence Submission Details of Project Location Jarrold Whitefriars Office Development Whitefriars, Norwich. Project Description New 3 Storey Office Building for Mills & Reeve Solicitors by Jarrold Property Ltd. Client Jarrold Property Ltd The Yarn Mill, Whitefriars, Norwich. Structural Engineer Matthew Byatt Hale Allen Jones 8 Chalk Hill House, 19 Rosary Road, Norwich. Architect LSI Architects The Old Drill Hall, 23a Cattle Market Street, Norwich. Contractor R G Carter Drayton, Norwich. 1 13/02/07

2 Jarrold Property - Offices 1, Whitefriars, Norwich Structural Engineering Design Statement In the autumn of 2000 we were asked to provide the Structural Engineering Service for the design of two new office buildings to be located on part of the site used by Jarrold Printing Works adjacent to Whitefriars and the River Wensum, and close to the existing, Grade 1 listed, Yarn Mill Building. From the outset costs were critical in determining whether or not the project would be financially viable for the Client, Jarrold Property Ltd. In addition the site was known to be of Archaeological interest with a medieval arch still standing at the front (south) of the site and a crypt within the existing building complex at the rear (north) of the site. Both of these structures were grade 2* listed monuments. In order to attempt to minimise costs, both direct and archaeological, we decided to look into the feasibility of maintaining some of the existing print works structure for re-use in the new development. In order to assess the worth of the existing structures we searched for the Building Control Records held by Norfolk County Archives in order to find the existing drawings for the St James Mill Factory Building, which stood over the proposed site for the new Office 1 building. We knew that this building was constructed in the early 1950 s, and that the original north end was of a heavy reinforced concrete construction. 2 13/02/07

3 View of Original St James Factory Buildings from Whitefriars Bridge Our investigations of the archives discovered several of the original building drawings, including the foundation general arrangements which had been marked up to As Built status by the Building Control Officer, including the dates and times of his inspections. The drawings encountered showed the north end of the building was originally designed for four storeys, although only two were subsequently constructed. The south end of the building was a much lighter single storey steel framed building supported off pre-cast concrete piled foundations. Armed with this information we agreed with the Architect that we should reconfigure the arrangement of the new building s columns in order to utilise the existing concrete pad foundations within the design. Once this principle was agreed, we commissioned site investigations to confirm the nature of the ground conditions, the existing ground floor slab construction and to excavate a trial pit to expose one of the existing reinforced concrete pad foundations, in order to confirm that the archive documentation was correct. The results of the archive searches were plotted over 3 13/02/07

4 the proposed structural grid (drawing 8268/100) and the new structure was designed to utilise the existing foundations, where appropriate, and to avoid clashes with the existing piled foundations at the south end of the building (see drawing 8268/101). The ground floor of the new office was to have raised access floors and as such consideration was given to maintaining and re-using the existing ground bearing floor slab, over as much of the building footprint as possible. The existing ground bearing slab was in relatively good condition and had been used predominantly for paper storage and printing operations over the last years. As such it had been loaded to a minimum of 18kN/m². Based upon this we concluded that we could re-use this slab, albeit with a reduced floor loading, and did not expect to experience any further significant consolidation of the underlying fill material. The site investigations confirmed both the thickness of the slab and the quality of the concrete. We then carried out calculations to confirm that the slab would be capable of supporting the non-loadbearing internal blockwork partitions. Copies of our calculations for the existing foundations and the existing ground floor slab are appended to this document. Once it had been decided to re-use existing foundations, the superstructure was designed such that any connections from the new steel columns to the existing reinforced concrete columns would be pin jointed. Where columns did need to have moment bases these were located at the south end of the building where new piled foundations were provided. 4 13/02/07

5 Demolition of St James Factory Building Note: The Crypt is immediately adjacent the R.C. Columns being Demolished R.C. Columns cut back at end of Demolition Contract 5 13/02/07

6 At the southern half of the building, the existing foundations were not suitable for reuse and 450mm diameter displacement augured piles were adopted, founded within the sand and gravels at relatively shallow depth (approx 4-5m). At the north of the building the existing reinforced concrete columns and foundations for the St James Mill factory were maintained below ground floor level, and the new steel columns were chemically anchored to the existing retained concrete structure. The elevational walls at this end of the building were then supported off mini-piled ground beams bridging over the existing factory s pad foundations. Ground Beam for External Wall Crypt in the background Note: To the right of the ground beam is the existing R.C. Column cut down for re-use, with Resin Anchor Bolts in place, and the saw cut edge of the existing retained ground bearing floor slab. 6 13/02/07

7 Existing R.C. Column being cut down to final height for support of new Steel Frame Above ground level Office 1 is a steel framed 3-storey masonry and glass clad building with a metal deck roof, supported upon cold rolled steel purlins. The elevational walls are formed from a combination of full height curtain walling (southern end) and cavity masonry, utilising a brickwork outer leaf and blockwork inner leaf. There is a plant room at 3 rd floor level over the main core area and a roof well to the north end of the roof housing mechanical engineering plant. Upper floors comprise traditional pre-stressed pre-cast concrete floor planks supported upon steel floor beams. The gaps between the pre-cast floor planks were grouted up, as there was to be no screed, and a raised access floor was laid over the top. In addition nominal reinforcement was grouted into the joints between the units over beam supports. Within the central core areas there is no raised access floor and the floor units were laid at a higher level to accommodate a 75mm cement sand screed. 7 13/02/07

8 View of Completed Office 1 Building from Whitefriars Bridge View of Office 1 and part of Office 2 taken from Front Entrance Courtyard Area 8 13/02/07

9 Lateral stability is provided by a combination of moment bases at the southern (glazed) end of the building and vertically braced bays and pinned base columns elsewhere. Moment connections between the beams and columns were also adopted at the north of the building where the building adjoins the medieval crypt, in order to allow the vertically braced bay to be interrupted between first and second floor levels. Horizontal bracing is provided within the roof and locally within some floor bays. Main Entrance Atrium Staircase with Structural Glass Balustrades 9 13/02/07

10 Internal View of 3 Storey Front Entrance Atrium 10 13/02/07

11 As works progressed on site a number of archaeological finds were encountered resulting in the discovery of the Whitefriars Monastery, which was burned down and destroyed by Henry VIII during the reformation. Several of the new piles and caps had to be redesigned and repositioned to avoid sections of historic wall often several feet thick and constructed from flint. The final foundation layout (drawing 8268/101) shows how much disruption to the works was caused by the Archaeology, at the south end of the building. At the north end of the building this was avoided completely by the fact that works did not have to be carried out below ground level, therefore alleviating the client from having to pay for additional Archaeological investigations, or the disruptions to the contract that any major discoveries would undoubtedly have had. Drawing 8268/100, 101, 107, 108 and 124 appended to this document show the substructure works to Office 1. In addition to the buried archaeology, at the north of the site immediately adjacent to the St James Factory building, is a medieval masonry crypt. The proposals for the Office 1 building incorporated the crypt into the design and included for a new mezzanine floor and roof to be constructed on top of the medieval arched structure. Due to the historical importance of the Crypt the uttermost care had to be undertaken within the design to avoid damage to the existing structure. In addition, regular meetings were undertaken with the Conservation Officer at each stage of the design and during the construction, in order to ensure his continued approval for the works, which was a requirement of the Listed Buildings Consent. However, in order to link the crypt with the new office building we had to form new piled foundations immediately against the old walls. Even with the relatively small diameter mini-piles, that were adopted as part of the original piling scheme, the level of vibrations resulting from driving these piles was found to be to high. We therefore instructed the use of augured cast insitu mini-piles to be used in the positions of highest risk to the crypt /02/07

12 Notwithstanding the level of care taken, some cracking did occur to the crypt s masonry arches during the excavation works for the ground beams immediately to the south of the crypt. In addition the east wall of the crypt was subjected to a minor knock during the demolition of the reinforced concrete St James Factory building resulting in some damage to the flint and masonry infill wall. As a precautionary measure, to ensure the continued stability of the structure, the crypt s arches were propped during the works, and the crypt was regularly monitored to ensure that there was no ongoing or progressive movement occurring. Internal View of Medieval Crypt At the end of the main building works, the monitoring results were assessed and it was determined that the cracks that had occurred during the foundation works, had found an equilibrium and appeared to be stable. At this point we carried out a detailed assessment of the condition of the crypt structure and subsequently proposed a solution of simple remedial works. These remedial works involved 12 13/02/07

13 discretely tying two of the main arches back to the masonry vaulted ceiling using Helifix stainless steel ties, and the other minor cracks were simply filled with lime mortar. This proposal fulfilled the requirements of the Conservation Architect and English Heritage who required that only minimal works should be carried out to the existing structure so as to not interfere with the way in which the structure behaves. With regard to the new roof over the crypt this was achieved using pinned structural steel columns, resin anchored down to a new reinforced concrete sill member, on top of the medieval masonry. The roof was braced horizontally and connected back to the main Office Building s Steel Frame to provide the overall stability. As for the floor loading imposed upon the crypt from the new mezzanine, this was in fact significantly less than it s use prior to the contract, which was as a store for old lead printing blocks, which previously formed part of the Jarrold Printing Museum. One of the many skeletons uncovered by the Archaeologist 13 13/02/07

14 Archaeologists Excavation at the rear (east) of the Crypt In addition to the above a new stair well was constructed immediately to the east of the crypt serving both the main office building and the crypt, and this proved to be one of the areas of highest archaeological density. The new piles, pile caps and concrete staircases all had to be constructed without destroying any of the encountered archaeology. This involved a complete re-design of the foundations and piles in this area both on plan and level. Drawings 8268/111 and 109 show how this was achieved /02/07

15 North elevation of Office 1 showing junction of Stairwell Core to the external wall of the Crypt 15 13/02/07

16 View on the northwest Corner of Office 1 at junction between main office building and the Crypt. The Client, Architect, Conservation Architect and English Heritage have all expressed their satisfaction of the outcome of the project /02/07