Location: UK
Client: Totty Construction
This case study relates to a Victorian building called Crispin House in Leeds. The developer put together a £7m plan to convert the building, which was a former orthopedic factory, into 76 apartments and penthouses.
The existing 25m high building had four floors and a basement and was of red brick construction with limestone lintels, corbels and ornate details. The building is situated on a tapering plot of land between two streets, Trafalgar Street and New York Road. The internal structure consists of steel beams built into pockets in the brickwork piers, spanning onto cast iron columns along the centre line of the building, with timber floors. The building was heavily glazed and the footpaths outside sloped steeply on both elevations.
The refurbishment of the building introduced more floors within the existing building and extended the building upwards above the current roof level. The existing internal steel frame did not suit the new usage and so was replaced with a new one. In addition the basement was to be deepened to provide underground car parking facilities. This was to be achieved by staged underpinning the existing foundations. The two 65m long elevations of Crispin House, one along Trafalgar Street and the other New York Road were to be restrained with an external façade retention system.
Design Development Process:
The information required to design the façade retention came from a number of sources. Electronic copies of the existing structure drawings were used together with results of condition surveys and verticality surveys as well as detailed specifications. Meetings with the contractor and engineer identified site specific requirements such as access points, cranage capacity and programme constraints. RMD Kwikform offered their expertise in this specialist field by advising on the standard practices, options available and guidelines for design. Outline designs were produced and the final design was developed considering the design parameters covered in the next section.
Design:
The loading to be applied to the façade retention frame come from two main sources, the wind load and lateral load due to out-of-plumb of the retained façade. The wind loading was calculated in accordance with BS5975, and CP3 Chapter 5. (since superseded by BS6399 Pt2) Crispin House is situated in the city centre of Leeds and was surrounded by buildings of a similar height of 20 to 25m. This enabled appropriate wind pressures to be established for the design.
Detailed surveys of the building at each floor level enabled the wall thickness to be established. The verticality survey and condition survey showed that the facade was in good condition and at better than 1:67 out of verticality. This enabled the percentage of brickwork dead weight acting as a de-stabilising force to be reduced to from 2.5% to 1.5% of the façade weight.
Support to walls is provided by walers both inside and outside effectively sandwiching the wall. These walers are secured with ties through the window openings and positioned to avoid new and existing floors whilst still providing adequate lateral restraint to the façade.
Due to the basement deepening works we needed to avoid surcharge on the footpath adjacent to the building. This was achieved by using the self weight of the façade. The inner leg loads were transferred to the facade via a beam introduced into the height of the tower. This beam projected into the building through the windows and was connected to shear plates bolted to the inside face of the façade. To standardise the steel frame connecting to the building the beams were made to enter the building all at the same level. This was further complicated by the 3m slope of the footpath outside on which the towers were founded. It was overcome by tailoring the make-up of the tower beneath the beam and having a standard tower configuration above the beam. This enabled a good proportion of the towers to be a standardised design which facilitated a modular erection process.
RMD Kwikform worked with the engineer in the design and detailing of the foundations providing design loads for each leg position. The engineers basic design was then modified to suit the street slope of the site by stepping the upper surface of the foundation. The kentledge on the outer leg was installed on top of the low level stepped strip footing. The anchorage system to the footing was designed by RMD Kwikform.
Pedestrian access was maintained throughout the contract by the introduction of RMD Kwikform's unique Megashor Half Portal frames at the base of the tower system. These frames are specifically designed to enable the bottom of the façaderetention towers to be left clear of bracing and can be spaced apart with Megashor to suit the width of tower used.
It was established that the party wall required restraint. This was achieved by continuing walers around the party wall having flying shores, knee braces, across the corners thus transferring loads back to the main retention system. This meant that internal towers were avoided thus leaving the basement clear for deepening and underpinning works.
The development of the inside of Crispin House left a section of the original structure untouched. The main façade retention system was already installed when it was discovered that a large concrete roof beam needed extensive concrete repairs. The beam needed to be propped down to the basement for the duration of the repairs.It also transpired that the existing steel and timber flooring was left unsupported at the demolition line once the load bearing walls and stairwells were removed. Further more the removal of the stairwells and load bearing walls left the façade unsupported beyond the extent of the current façade retention system.
A single system was designed to accommodate all of these issues and took the form of a a braced frame across the entire 13m width of the building. The frame had header beams at roof level to support the concrete beam during repairs. The Megashor verticals had brackets fixed to them which picked up the unsupported steel frame. The ledgers had superslims suspended on ties which cradled the unsupported timber floors. Finally the whole frame was braced across the width of the building and connected to walers extended from the external façade retention.
It took around three weeks to develop the design which was ultimately represented by 11 drawings. The project used 72 tonnes of Megashor, 46 tonnes of Superslim Soldiers to restrain the two elevations amounting to 2420m2 of facade.
