SEISMIC STRENGTHENING ANO UPGRADIHG OF MASONRY BUILOINGS IN

NEW ZEALANO .

J M LEUCHARS Oirector Smith Leuchars Limited, Auckland, New Zealand

ABSTRACT New Zealand has a large stock of masonry buildings which were built before the development of modern seismic design principies. These buildings are now subject to review by local and national Government bodies and many have become subject to demolition or strengthening orders.

The paper describes the strengthening techniques used and discusses the benefits and shortfalls of each. This includes the choice of structural form, system and materiaIs to be used.

A series of brief case studies illustrating the strengthening techniques and the economic aspects of each project is given.

1 STRUCTURAL FORM

1.1 Stiffness

A stre ng-thening system which has a stiffness less than that of the; b uilding being strengthened cannot be expected to take seismic loads until the existing structure has degraded to a point where it has compatable stiffness. If the intention is to protect a building of historic importance this often defeats the purpose of the strengthening. If the strengthening system is markedly less stiff, the amount of damage occuring before the strengthening system starts to work may be major and be little short of collapsc .

1.2 Oistribution

The distribution of lateral load resisting elements to take both direct and torsional loads is important. Unfortunately many older buildings have poor structural plans and large torsional problems become evident. (See Case 1) The rapid drop in stiffness o f unreinforced masonry can accelerate the torsional attack and subsequent failure. It is often necessary to provide totally new e lements to improve the torsional characteristics of a building.

1.3 Addition

One technique which has been used on a number of occasions is to provi de a new extension to the existing building. This e xtension may house much of the required lateral strength for the total building as well as providing additional floor area. Often t he overall economics of a project can be improved by this technique (See Case 2). The binding in of more than one building into one structural system can often make the best use of element s in each individual building (See Case 1).

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1.4 Intrusion

On many occasions a strengthening system is required which is not an obivious addition to the original building. The strengthen­ing system must be disguised or concea1ed in such a way as to be virtually invisible.

This is often impossibleand some sacrifices must be made. In structures such as churches a choice is often necessary as to whether the exterior or the interior is to be disfigured by the strengthening processo

1.5 Disruption

The presence or otherwise of the building's tenants during strengthening operations often effects both the structural form and material type. It may not be possible to gain access to particular areas long enbugh to build major elements. Damage and disruption to existing building services and operations may be a major design constraint.

The programming of the sequence of the operations with tenant movements requires major consideration at the design stage. The development of a project criticaI path programme is an important design tool. Constraints applied by the programme may lead to a change in construction sequence which also changes materiaIs and formo

2 CHOICE OF MATERIAL

The choice of strengthening material will depend on the programme availability of materiaIs and applicators, strengthening leveI, stiffness compatibality requirements, disruption and damage caused by construction and the type of structure being strengthened.

2.1 Structural Steel

This material has the major advantage of being a totally dry processo There is no possibility of water damage to the building during erection of the steelwork. In most cases building authorities waive fire proofing requirements for structural steel work required for seismic reasons

The material is relatively easily connected component to component, but is not always easily connected to the originill structure. Most structural steel systems use a small numberof high1y stressed components. This is often not compatable with the strength of the existing materiaIs.

2.2 Concrete

Concrete is a material which can be easily formed or sprayed in position where it can be married up to irregular and difficultly shaped existing systems. Its major draw-back is that it is a wet process which can cause considerable disruption, noise and possibly damage. As most concrete systems are widely distributed they are often easier to connect intO' the existing structure. The new concrete

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system can be built giving adequate tolerance allowances to new and old construction.

2.3 Timber

Timber strengthening systems are generally only sufficient for relatively small buildings. The major problem in timber systems is obtaining joints of adequa te strength. This applies to connect ions between the timber components as well as to the masonry .

3 LATERAL LOAD SYSTEMS

3.1 Structural Steel

The commonest structural steel solution is to provide steel soldier posts along masonry walls to take face loads. The lateral loads are then taken by steel cross bracing .

The major drawbacks of this system is the deteriorating stiffness of steel crossbraces in the post yield range. The original design lateral l oad level needs to be high to reduce this effect . Even with large crossbracing sections a matching of stiffnesses to the orig inal building is often difficult.

The use of compression braces partly solves this provided plastic buckling can be controlled.

~vhen the original building has a steel frame the addition of steel crossbraces is often an economical solution. In most steel strengthening solutions some sacrifices in aesthetics are required as the steel tends to intrude on the internal spaces.

3.2 Reinforced Concrete Frames

New reinforced concrete frames can be used particular ly in bu ild­ings which have long masonry walls in one direction but no lateral strength in the other. This condition is typical of rows of warehouses or low rise shops. The addition of concrete frames on the street frontage and at intervals down the building prov i des a good level of se ismic resistance. The flexibility of the frames is not a problem as they act perpendicular to any wall systems.

3.3 Concrete Shear Walls

Concrete shear walls are probably the best form of structural system as they have a compatable stiffness to masonry walls and deep membered frames. The choice of either cast insitu or sprayed concrete walls will depend on the loadings applied to the wall. If high densities of steel or full ductile behaviour is required (Case 4), then cast-insitu concrete is prefered. Th e availability of spray concrete applicators and their quality should be invest­i gated before any decision is made. Reference 1 gives a descrip­tion of current sprayed concrete techniques in New Zealand.

Case 3 is a good example of the application of sprayed concrete.

A new method ) that of plastering both frames of a wall with fibre

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reinforced cement has been investigated by the New Zealand Ministry of Works with good results (Ref. 2).

One of the drawbacks of facing an existing wall with a new concrete skin is that the process will destroy any additional details on the wall. In Case 1 plaster casts were taken of orna te scotias so that they could be reproduced on the face of the new walls.

3.4 Timber Shear Walls

Timber shear walls of nailed plywood panels is an efficient and relatively cheap system particularly when numbers of cross walls are available to act as bracing walls. This system was used in the top floor of Scots College (Case 3) where heavier masonry cross walls were removed and the lightweight plywood faced walls were used in their stead. The strength of the plywood panels is not nearly as high as a steel or concrete system but is often sufficient in low rise buildings or at the top of higher buildings.

3.5 Prestressing of Masonry

The technique calls for drilling through masonry walls and columns, placing prestressing cables or bars and grouting them up. The stressing of the cables provides a normal force on a wall which increases both the flexural and shear strength of the elemento The leveI of prestress must be carefully controlled as the masonry must not be overstressed. It is often difficult to stop the spread of the prestressing forces into neighbouring elements. A check of the prestressing forces after a period of time often shows that the effects of creep and dispersion of load has markedly reduced the residual prestress.

4 DIAPHRAGM SYSTEMS

The most common form of floor in masonry buildings is timber joists on steel or timber beams. Most strengthening systems include a steel tension cross bracing either above or below the joists. It should be noted that the stiffness of a cross braced diaphragm is much lower than most strengthened wall systems. The resulting structure should be analysed as a flexible non-rigid diaphragm. Generally loads will be distributed to the nearest wall and will not then be redistributed to mor e distant stiffer walls.

Solid timber diaphragms using plywood sheets or even thin steel sheets provide a stiffer structure but if a fully rigid diaphragm is required then a concrete skin cast over the existing floor may provide this.

5 CONSEQUENTIAL WORK

In most cases although the initial trigger toupgrade a building may have been its lack of seismic strength, it is usually a minor component of the total project costs. The upgrading of fire and egress provisions, plumbing and drainage, internaI

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finishes and building services generally take the major share of the budget .

6 CASE STUDIES

CASE I NORWICH /NORFOLK HOUSES, FEATHERSTON STREET , WELL I NGTON (NOW BENEFICIAL HOUSE )

1.1 Or i ginal Building s

Two buildings were built on the site .

The Norwich house building was bu i lt in 1923 . This comprised a five storey reinforced concre te building with concrete walls on its Nor th and West facad e s. The South and East facades were made up of re in forced c oncrete frames and spandrel infills. The f l oors were concrete and d e sig n e d f o r warehouse loadings.

Th e Norfolk building built in 1 93 4 h a d two concrete suspended floors and a concre te roof. Th e Nor f olk building was corbe lled off t he Norwich building on its South side and had a brick bound­ary wall on its North facade . The East facade consisted of a concrete frame with a concrete spandrel faced wi th brick. The West facade was a punctured concre t e wall .

Th e original drawings showed substantia1 reinforcing in the slabs , beams and columns of the Norfolk buiiding and the slabs and columns of the Norwich building . Ali concrete wall s were lightly re i n ­forced a s were al i spandrels . The foundat i on consisted of heavy pads dug down to the orig i nal beach levei some 4 . 5m below ground levei .

1 . 2 Structural Solutions

The two buildings as separa t e entities had serious seismic short ­comings. The Norwich building while having quite substantia l walls on two facades was very poor in the torsionai mode . The Norfolk building had virtually no seismic capacity in the East ­West direction and moderate capacity in the North - South direction . The structural solution to the probl em wa s to join the two build­ing s with the beams see Fig . 1 so that they would act as one unit, then provi de new or supplimentary walls on the Nor t h wa ll of Norfolk and the North and West wal l of the Norw i ch .

~

! r TIE BEAMS J

J . SPRAYEO WALLS

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TIE BE AM J Fig . 1

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NORWICH NORFOLK 1355

The two North wa lls then provided r e sistance to torsion loads. The torsion load ing was calculated using the curre nt NZS 4203 equation and proved to be quite onerous. An additional shear wall was added to the Norwich building to take torsion loads occuring above leve l 3.

The buildings were designed to meet Government accommodation standards. Th e loading was defined at C = .15, S = 1.6, M = 1.0, R = 1.0, I = 1.0.

The new shear walls were built using sprayed concrete against the existing walls.

Problems were e ncountered ln the drilling of holes through exist­ing columns to take shear wall r e inforcing . The vertical column bars were often up to 75 ~n out of position and this made drilling very much a hit and miss thing . The column hoops were sometimes encountered one on top of the other probably having been displaced during the original concrete pours.

The Wellington City Council required that the two buildings would b e capable of standing alone if the other was demolished . Consequently both buildings were also designed to take 2/3 of the 1965 code loading separately.

1.3 Architectural Solution

The Architects decided to modernise the facades by enlarging the window openings and reglazing the whole building. A canopy was added along both street frontages and this helped to make the buildings read together. A penthous e was adde d on top of the concrete roof of the Norwich Building.

1 .4 Construction

The buildingwork was finished in mid 1981 and took only 40 we e ks to complete. The Contractor was given possession of the whole site and could therefore work on all floor levels at once. This accelerated the construction rate.

1.5 Cost

The total projec t cost wa s $ 1 .7 5m of which 25 % was structural work. This p r ovide d 358m 2 of new pen thouse s tructure and strengthened and upgrade d 2800m 2 of new ex isting buildings.

CASE 11 WELLINGTON WORKIN GMENS CLUB

2 . 1 Original Building s

The two or i gin a l build i ngs were built in the 1 900 's. The f irst building , to the North had a concrete f ir st floor, a timber second f loor and roof. All walls were of unre in forced brick. The Southern building had timbe r f irst a nd second floors and again all walls were of unreinforced brick work. The building s had bee n

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notified as Class A seismic ris k s by the Wellington City Council.

11. 2 Structural Solution

Th e strengthe ning work was separated into two stage s. Stage 1 was the strengthening of the No rthern building and Stage 2 was the Southern building . Stage 1 has been completed but Stag e 2 has not yet been started .

Th e Nor thern building was strengthened by the addition of new concrete frames o n the East and Nor t h facades infilled with rein­forced blockwork (S ee Fig. 2). This was then connected to the original building by infilling t he exis ting lig ht well with new concrete slab s at t he first and second floors . The brick walls on t he West and So uth facade s we re cross braced with structural steel or had gunited wall s laid u p to them . The brick walls were held i nto the building by stee l so ld i er s connected to the floor dia­phragm wh ich in turn were strengthened by the a ddition of steel ti e s on the underside of t he joists .

t---,- NEW BUILDINGS

SPRAYED WALL

___ ~-'---------+-CO NCR ETE BLOC K INFI LLS IN NEW CONCRETE FRAMES

---r---++-STEEL SOLDI ERS TYPICAL

WELLlNGTON WORKING MENS CLUB Pig. 2

11.3 Architectural Solution

Th e Architect d i d not want to alte r the main facade f ac ing Cuba Street . While the facade i s not particular l y brilliant it is a good example of the period wh i ch is becoming rarer in the main Hell i ngton street s . The i nterior of the building was opened up to provide la r ge open spaces for function s and indoor sports . No additional means of egress were required but a considerable amount of fire - proofing was added t o ceilings and structure.

11.4 Construct i on

The building wo r k was started December 1979 and was completed December 1 980 . The Contractor had possession of the bui l ding from first floor up but had to screen off areas of the shops below to allow access for construction . The club continued to function in the Southern building throughou t the construction períod o Some prob l ems were encountered in the waterproofi ng of the fi rst floor to allow the shops to continue operating below. The Eng i nee rs were called upon to provide a levei of project man agmen t ex tending further than would be the case in most other projects.

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11. 5 Costs

The total project cost was $ 550,000 of which 50% could be de scr­ibed as structural work. This provide d 600 m2 of new space and strengthened and upgraded 1650 m2 of existing building.

CASE 111 SCOTS COLLEGE, MAIN SCHOOL BUILDING, WELLINGTON

111.1 The Original Building

The original building was built in 1918. It ccnsists of ground, f irst and second floors of timber infilled wit~ 4" of concrete for sound proofing. The walls are either s o lid or cavity brick with reinforced concrete bands at foundation floor and roof leveIs. The brick veneers were tied to the inner skin with mild steel ties at a spacing of 9 per square yard. These were inspected where visable and were found to be in good condition. The original clay tiles were replaced with lightweight decramastic tiles after the Wahine Storm of 1968. This was to prove helpful to the final seismic designo The building fabric was in a very sound condition but the interior finishes had been allowed to deteriorate over the last five years as a decision on the building's future had to be made.

1%3 AdÓlhon' not 5trzngthem0

Sproyzd Gur>fe Wolls

SCOTS COLLEGE Fig. 3

III.2 Structural Solution

The design cri teria for strengthening was determined partly by the source of finance and partly by engineering logic. The building had to comply with NZS 4203 wherever possible to qualify for government mortgage finance.

The structural solution was to spray up to 150 mm thick gunite walls against ali crick walls which were to remain. The number of internai brick walls was reduced by demolition at each floor leveI and replaced by lightweight construction. This removal of walls helpe d free up the internaI spaces to provide class rooms

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which complied with Department of Education standards. The build­ing mass was also reduced and the resulting seismic and foundation loads accordingly.

Steel strapping was added to the underside of floors to assist the timber diaphragms. The resulting structure resembled an egg crate form with no walls being loaded excessively . The walls were designed as shear walls of limited ductility, using the draft of Reference 3 .

111.3 Architectural Solution

No changes were made to the exterior facade or to the main entrance. Ali internai finishes were to normál Department of Education standard. A new enclosed stair was added at the Eastern end which also provided a link to the existing science block.

111.4 Construction

Construction was scheduled to start on the 26 January 1981 and was finished by 7 June in time for the start of the second termo The Contractors were given a completely vacant building for the whole construction period . The school was relocated into other existing buildings.

111.5 Costs

The total projec t cost was $ 460,000. This provided a new stair and access structure of 70 m2 and a strengthened and upgraded building of 1000 m2 •

CASE IV AMP BRANCH OFFICE, WELLINGTON

IV.1 Original Building

The original building, designed by Clere & Clere, was built in the late 1920's. The building has a basement, ground, mezzanine and six further floors. The floors are bui1t around a centre lightwell with a further light well on the Eastern side. The bui1ding was built and still functions as the New Zealand Branch head office of the AMP Society. The decision to upgrade the building was made after an extensive investigation of the alternatives open to the Society such as shifting to another office building, rebuilding on the same site and eight alternative upgrading schemes.

The original structure and heavy stone facades

has a riveted steel frame, concrete floors, on the South and East facades.

IV.2 Structural Solution

The structure is strengthened by the addition of new concrete shear walls see Fig. 4. These correct the very poor torsional characteristics of the building and also provide a strengthening system which matches the stiffness of the remaining original structure.

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~~~--L '9ht Ar-w /. 5proyWall

o ----D--- --

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CUSTOMHOUSE OJAY

AMP SOCIETY- HEAD OFF ICE WELLlNGTON Fig. 4

Extensive inve stigations were made into the original building . These showed a number of variations with respect to the orig inal drawings. The stone facades were supposed to be backed with ureinforced brick infills. These were in f ac t backed by reinforced concrete infills. While the amount of stee l reinforc ­ing used was minimal it greatly aided the strengthening of the facades by simply tying these back to the steel frame.

The two lightwells were infilled with new concrete floors.

IV.3 Architectural Solution

The original architectural solution was to upgrade the central lightwell as an open garden area at each floor. However, the economic r e ality of simply infilling these f inally prevailed.

The entry space in the ground floor was to b e restored to some thing approaching its original grandeur . Unfortunately inve stigations into the conditions of the original coffered ceiling showed that this had been completely ruined af ter a modernisation in the 1960' s.

The refurbishment included the installation of full air condition­ing, sprinklers and new electrics throughout. The basement was turned into a car parking garage for 32 cars as we ll as accommod­ating some of the items of the planto

Paraplegic and goods access to alI floors is provided by a new lift in the North West corner.

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IV.4 Construction

Construction was started in April 1982. The construction sequence allowed for the Contractor to have pos s es sion of the basement, roof and two other floors at any one time. The Cont­ractor sequentially handed over the lower of the construction floors to allow . the tenant to move out of the next one above .

The new upgraded building allowed the total AMP Society staff to be accommodated and still l eave one floor free for lease. Before the upgrading the AMP occupied parts of two other buildings in addition to the bra~ch building.

I V.5 Cost

The final contract price was $ 7.7m . architectural trades made up 25 % each sprinklers, lifts, electrical and air remaining 50 %.

The structura1 and of the total cost whi1e conditioning made up the

The total gross are a of the c ompleted building is 11744 m2

compared with the orig inal 10142 m2 •

5 REFERENCES

1 Leuchars J M. The use of Sprayed Concrete in the Strength­ening of Earthquake Risk Bu ildings . 7th IBMaC 1985.

2 Hutchinson D L Yong PMF McKenzie GHF . "Laboratory Testing of a Variety of Strengthening Solutions for Brick Masonry Wa1l Panels" 8th World Conference on Earthquake Engineering 1984.

3 NZS 3101 1982 Design of Concrete Structures.

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