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AN APPRAISAL OF ACCIDENTAL DAMAGE DESIGN PROCEDURES FOR MASONRY BUILDINGS

J. A. KAVANAGH O'CONNOR KAVANAGH, Consulting Engineers. 25 Lower Mount Street, Dublin 2, Ireland.

ABSTRACT

The historical background to the development of current des ign procedures in Britain and Ireland is briefly presented. A general discussion of features of the procedures from the designer's viewpoint emphasises their empirical and arbitrary nature and indicates areas where further guidance for designers would be helpful. The low leveI of risk of occurrence is emphasised. Irish experience is discussed with particular reference to a recent accident.

INTRODUCTION

On 16th May 1968 a town gas explosion occurred in Flat 90 on the 18th fIoor of Ronan Point, a 22 storey block of flats in Canning Town, London. The block of flats was built using precast concrete floor slabs and large­pane I precast concrete wall units. The externaI wall panel to flat 90 was bIown out, the floor it supported then collapsed leading to progressive collapse of wall panels and floors in the corner of the building affected and the death of 4 people.

At the subsequent public enquiry, several matters having far-reaching implications for structural engineers amongst others were discussed. Some of these issues were not directly related to the explosion but arose out of the subsequent debate on structural engineering design procedures. After the report of the enquiry was published, public opinion in Britain called for controls to avoid the recurrence of such accidents and shortly afterwards the British BuiIding Regulations were amended making it mandatory to carry out additional design checks for alI new buildings over 4 storeys high. These design requirements were, it could be argued, chosen in a somewhat arbitrary way but they have become established as the basis for the approach now adopted generaIIy by structural engineers in Ireland, Britain and many other countries when considering possible accidental damage to a building.

As a direct result of the Ronan Point incident therefore, Structural Engineers have incorporated additional design procedures into their considerations when designing buildings. Buildings are now assessed for the effects of accidental damage with a view to preventing in the event of an accident damage which is disproportionate to it's cause.

DESIGN CONCEPTS

Two points can be made here about the Ronan Point accident and its subsequent impact on structural design procedures. Firstly, the construction involved large panel precast wall units which were not positively tied to the precast floors and which thus led to the "house of cards" type of progressive collapse; it is probable that no other form

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of construction would have performed as poorly, and yet the legacy of Ronan Point has been applied by public demand to alI forms of construction. Secondly, the plan area damaged in Ronan Point was below the permitted ar ea allowed in current British Building Regulations (i.e 15% of plan area) - although of course the dramatic impact of Ronan Point related to the vertical extent of collapse, not the horizontal. lf some relatively simple tying of the wall panels had been carried out, explicit design procedures may never have developed as they have done.

Following Ronan Point, the British statutory requirements for design of alI multi-storey buildings were stated in the form of three basic principIes which have underlain design practice since. These are :-

(i)

(ii)

(iii)

it can be assumed that accidental pressure will not exceed 34 kN/sq.m,

it can be assumed that no more than one structural member will fail as a direct result of an accident,

if an accident occurs which causes conditions no more severe than these two assumptions, then structural damage must be localised.

The established design procedures can be summarised as follows:

(a) each main structural element should be designed to withstand an accidental design load of 34 kN/sq. m (acting in any direction on it and on attached building components), i.e to be a "protected" member, or

(b) each structural element should be proved to be removable one at a time without causing more than localised damage, or

(c) vertical and horizontal ties should be provided according to empirical rules set out in the various design Codes for different construction materiaIs,

or combinations of these three options.

lt can be seen that options (a) and (b) derive directly from the three underlying principIes. Option (c) evolved subsequently and has become accepted as an empirical method of providing generally equivalent structural integrity and robustness without the need to prove by calculation that such is the case. lt has the added virtue of calling for increasing robustness as the number of storeys increases (up to 10 storeys).

These requirements were set out in relation to buildings of five or more storeys. Based on the notion that in the event of an explosion, damage could not be prevented from occurring to the floor where the incident happened (within a certain area), the floor under and the floor over, then little can be done by way of structural design to protect buildings up to three storeys high. Four storey bUl1dings are somewhat anomalous and appear to have been excluded from the main design requirements for pragmatic economic reasons.

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MASONRY CONSTRUCTION

For steel or reinforced concrete buildings the empirical tying rules have become the standard design approach adopted in most cases. Most R.C. or steel buildings will have little difficulty in satisfying the recommended tying forces - although precast concrete construction will not automatically be amenable to tying and the requirements for continuity of ties at connections must be borne in mind from the earliest design stages. The tying rules have proven to be a relatively simple pragmatic approach to providing robustness to buildings which are or can be easily made triorthogonally tensile i.e whose members and connections on each of three main axes of the building are inherently capable of resisting tensile force s .

Masonry structures however are not triorthogonally tensile because the masonry walls have little resistance to vertical tensile forces unless vertical reinforcement is introduced. This is awkward and expensive to do and has led to masonry designers giving preference to one or both of the other two approaches i.e design of "protected" members or "removal of member" analysis. It is regretable that in the case of masonry construction the empirical approach has never been modified or extended to take account of masonry 's inherent ability to arch and span in-plane. Discrete panels of masonry do not fail in the way that large panel precast walls might but rather portions falI away when support is removed and the remaining sections of walls i nteract with the floors and will arch, span and cantilever in-plane very effectively. Empirical rules that reflected these ca pabilities of masonry construction could be of much assistance to building designers.

In fact t he vertical ty ing in masonr y construction i s intended to per f orm a fundamentally different role to the vertical tying in other forms of construction. In the case of masonry, the vertical ties are intended to "clamp" the wall section between the floors above and below. The tying might be regarded as latent prestressing to be activated when the wall is subjected to lateral loading; it provides a clamping action where wall panels in the top 3 or 4 storeys do not have the necessary precompression loading from floors overhead to enable them to resist the horizontal design load of 34 kN/sq.m .

DISCUSSION OF DESIGN PROCEDURES

lt is worthwhile highlighting some general points in relat ion t o the design procedures: .

(i) buildings are not designed to resist the force of any explosion without collapsing. Rather, each member of the building is designed to resist a relatively large notional force, or the building is designed to allow any one structural member to be removed without causi ng the building to collapse. lf an accident causes more than one member to fai1 or if it causes equivalent static pressure greater than 34 kN/sq.m then the accident is outside the normal design parameters. The notional design force may be exceeded in an explosion, and certainly in an explosion not just one member would be subjected to exp10sive forces. Thus the basic ru1es themse1ves are i dealised and arbitrary and should only be regarded as ensuring a de~ree of robustness to a building that will serve it we11 in the event of an acciden t .

(H)

(iH)

(iv)

(v)

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the design rules allow for only one accident.

"localised" damage can be quite extensive if the requirements of the British Building Regulations or the Irish Proposed Building RegulationS [l] are used as criteria, as most designers would probably consider reasonable to do. As stated earlier, Ronan Point damage would have been acceptable in plan area.

The rules or procedures are not always capable of simple strict application and anomolies can often arise. It is the spirit rather than the letter of the design procedures that is important and in the last analysis design to resist accidental damage is to a great extent a matter of engineering judgement.

Tpe design measures are essentially to protect the occupants and other persons immediately following an accident: the factor of safety used in analysis is very low and the building itself may need extensive repair or even require to be demolished after the accident .

Neither the Irish nor the British design Codes [2] -[3 ] conta in recommendations in relation to venting of explosive pressures. While many features will determine the actual explosive pressure exerted on structural elements, a key factor is the type and size of pressure relieving panels. Windows and doors will fail at a relatively low pressure and the build-up of explosive pressure will thereby be relieved. A committee of the Institution of Engineers of Ireland[4] ha s recommended that venting panels of area= 0.1 sq. m per cU.m of compartment be provided although this requirement has not been incorporated in the proposed Building Regulations. This area of vent panel is not always provided easily in practice but designers should be aware of the significance of pressure relieving panels.

Other questions will occur to a designer when applying the Code recommendations. Why for instance is it normally assumed that alI floors over are loaded with one-third their design live load in situations where this is a benign assumption i . e where it assists pre-compression and thus lateral load resistance? And when allowing for debris loading should impact loading rather than just static loading not be catered for? These questions merely remind us of the arbitrary nature of the design procedures.

The question of continuity and anchoring of vertical ties is somewhat contentious. Some authorities (e.g see ref. 5) commend discontinuity of ties at each floor leveI, the ties being staggered and anchored into the floors. Others suggest discontinuity of ties but without staggering. The British and Irish Standards appear to be implying vertical ties should be continuous (Clause 37.4 : "Ties should be fully anchored at each end and at each floor leveI .. ... ").

Precast floor units without any projecting reinforcement (e .g. extruded prestressed units) are widely used in Ireland and elsewhere. Tying these units together and into the main structure is particularly problematical. Guidance on stability tying and continuity of ties is given in BS 8110 [6] . While these rules provide some guidance they are not readily applicable to masonry construction. For instance in the case of an externaI support they require anchorage of the internaI ties to the

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support of the precast floor using steel links, presumably because in R.C. framed construction the support beam will normally contain the peripheral tie. In masonry construction the floor support (i.e the wall) will not necessarily contain the peripheral tie. More detailed guidance regarding tie continuity in masonry building with precast floors would be warmly welcomed, particularly with reference to situations where the precast floor units themselves are not easily used to provide the horizontal tying. Where vertical tying is being used, the detailing must be appropriate to develop the clamping action on the wall between floors as commented on earlier.

A favoured approach amongst Irish designers is probably Option 2 in Table 12 of I.S. 325 and BS 5628, where the wall panels are proven to be "protected" members. In many cases reliance is placed on Table 10 "Factor k" which can permit up to a four-fold increase in lateral pressure if the wall panel is well buttressed by return walls. It should be noted that to use the k factor the wall panel must be continuous and tied into the buttressing return walls and any crack control joints should be carefully arranged and detailed to avoid reducing the assumed structural integrity of the panel. Similarly, badly placed control joints could have a deleterious effect on other design assumptions. Steps should also be taken to ensure that movement joints are not incorporated without the structural engineer's knowledge e.g. by the Architect or Contractor.

RISK LEVELS

To provide quantified data for appralslng the appropriatness of the design requirements contained in the British Building Regulations, the Building Research Establishment carried out a survey of accidental structural damage to non-industrial buildings throughout Britain [7]. A detailed report presented and analysed data on the frequencies of occurrence of accidental loads of different severities and deduced average annual risks. Accidental explosions were found to be the principIe cause of damage with impacts by road vehicles second in importance.

It was found that on average about 5 explosions of piped gas, 3 of liquefied gas and 2.5 vehicle impacts were reported per week in Britain. However if attention is confined to significant incidents, the total number was reduced to an average of 1.63 significant incidents being reported per week. A significant incident would be for instance an explosion that would destory a small bungalow or, in larger structures, would destroy light cladding and windows, damage stud partitions and blow doors off hinges.

By far the largest number of explosions recorded was of piped gas. Together with explosions of liquefied (cylinder) gas they accounted for at least 87% of the total and they alone accounted for all very se vere explosions.

Current structural design procedures are based on assumed explosion pressur es of 34 kN/sq.m acting on any structural element and on attached building components. Estimates were made of the frequency with which an equivalent

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static pressure of this magnitude is 1ike1y to be reached and it was estimated as being once per year in Britain. The 1ike1y frequency of accidenta1 explosions giving equivalent static pressures of 17 kN/sq.m was estimated at five per year. Average equivalent static pressure was assumed to be 75 per cent of the mean of the maximum and minimum estimates of dynamic peak pressure.

The BRE report concluded that current structural design procedures are reasonable for a risk leveI of about one serious partial col1apse of a building of five or more stories once every 25 years. It further recommended that there should be no re1axation of requirements for buildings without piped gas supply because of the risk of gas 1eaking into a building from outside and the higher risk which would be incurred if cylinders of 1iquefied gas were substituted.

It has been estimated [8] that the risk of fatality resulting from structural failure amounts to approximately 1 in 10,000,000 annually. The risk arising from structural failure, primary or consequential, resulting from an accident wou1d be a sub-set of this, and the risk of a fatality resulting from disproportionate damage (i.e non-Iocalised damage) following an accident would be a sub-set of that again. This is not stated here with the intention or wish to encourage complacency, but rather with a view to keeping matters in perspective.

IRISH EXPERIENCE

In general Irish design practice closely follows practice in Britain. Irish Standard IS 325 Part 1:1986 [2] is a modified version of BS 5628 [3]. The main differences in respect of accidental damage are that in IS 325 the 1ength of wal1 defined as being a loadbearing element is the same for an externaI as for an internaI wal1, and there is an extra requirement whereby at externaI corners formed by two walls, a "loadbearing element" must be considered which inc1udes a 1ength of each wall.

Recently there has been much pub1ic and media interest in accidental damage design procedures because of an accident at Rag1an House, Dublin where two gas explosions occurred on the 1st January 1987 partly demolishing the 5 storey block of apartments and causing two fata1ities. The building was constructed using precast f100r units supported on loadbearing m?sonry wa11s. The official report commissioned by the Minister for Energy presented evidence in relation to the cause of the exp1osions and their effects, drew conc1usions and presented recommendations [9].

This report and the public presentation of and reaction to its findings were such as to cause the engineering profession to establish a joint committee of structural engineers nominated by the Institution of Engineers of Ire1and, the Association of Consulting Engineers of Ireland and the Institution of Structural Engineers (Republic of Ireland Branch). This Joint Committee studied in detai1 the evidence presented in the official report on the accident insofar as it re1ated to the design of buildings to prevent disproportionate damage [10]. The accident was the result of an unlikely combination of circumstances: a major fracture of the externaI gas main,

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the particular sub-soil conditions, the apartments not being occupied for several days over the holiday period, the calm weather conditions prior to the explosion restricting ventilation and creating the most favourable conditions for the accumulation of an explosive mixture within the flats, etc .. Two explosions occurred within a short period and it would appear that as a result of the first explosion, possibly six or more lengths of wall as well as floor units were demolished. The Joint Committee pointed out in it's report that such primary damage would not be resisted by any building designed using the normal procedures and the committee considered that the collapse that occurred was not disproportionate to the original cause. The committee concluded that the official report presented no evidence to indicate that the accidental damage provisions of the current structural engineering design codes are in need of amendment.

It has been widely suggested in the media that the building had collapsed because it contained no structural frame. The committee rejected this suggestion and pointed out that properly designed and constructed load­bearing masonry construction is perfectly acceptable for high rise buildings.

Regarding the explosive pressures, the official report stated that in the case of the critical first explosion the maximum (peak) overpressure was estimated to be of the order of 35 kN/sq.m. A different assessment of the evidence [11] concluded that "there was potential for the overpressure to reach 40-50 kN/sq.m".

The incident was a reminder that the structural design procedures are based on acceptance of a risk that a more sev.ere accident might occur than is reflected by the normal design procedures. Structural design procedures are formulated within a specific risk climate. Since Irish Design Standards are essentially in line with British Standards the Joint committee's report emphasised that there is an overiding need that the risk of occurrence of gas explosions and other accidents be brought to a level generally in line with internationally accepted standards of safety. The official report on the Raglan House incident clearly highlighted deficiencies in relation to the safety of the gas s upply system. This was endorsed by the Task Force set up by the Government subsequently [12]. A programme of upgrading of the gas network has been commenced and assurances have been given that this is proceeding apace and is ahead of target.

A further outcome of the incident is that the Irish Building Regulations will most probably incorporate a requirement that sub-floor voids in buildings must be vented.

APPRAISAL DF EXISTING BUILDINGS

In line with the recommend ations of the official report on the Raglan House incident, the Irish Government set up a Task Force to identify and advise in relation to potentially vulnerable buildings and situations. This Task Force requested the Joint Committee of the Irish professional engineering bodies referred to above to provide it with advice in relation to design and appraisal of multi-storey buildings with respect to accidental damage . The committee in it's report for the Task Force [13] pointed out the very low risk to life from disproportionate structural damage following an

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accident. The committee stated that existing buildings should generally be accepted unless an appraisal shows special cause for concern. This finding was in line with the recommendation published by the Institution of Structural Engineers in RP!68!03 [14] .

The committee's report gives general guidance on structural appraisal of an exis ting building for resistance to accidental damage. However it is pointed out that such appraisals must of necessity involve a large degree of engineering judgement. If a building is found by an appraising engineer to have poor robustness or resistance to accidental overload, then a series of recommendations are presented. Most of these recommendations relate to control ling the risk of occurrence of a gas explosion. Structurally upgrading partially or fully to modern standards was found to be an unlikely option. Two case studies were carried out which indicated that bringing an existing building up to modern standards can be very expensive - of the order of half of present day construction costs .

CONCLUSION

Engineering design and construction, like alI other human activities, involves a risk of unwanted occurrences. The leveI of risk which is acceptable is influenced by many considerations. In Ireland following the partial collapse of a block of apartments resulting from a town gas explosion there has been and continues to be public concern and misunderstanding of what the normal accidental damage design procedures cater for and what leveI of risk acceptance they reflect. Ultimately society tells engineers what safety standards it wants. But it behoves engineers to do alI they can to explain the general parameters and the background to the methods used i n design and to try to ensure that relative risks and cost benefits are not lost sight of.

REFERENCES

1. Proposed Building Regulations, Dept. of the Environment 1983.

2. IS 325, Part 1: 1986: Code of Practice for the use of masonry.

3. BS 5628, Part 1: 1978: Code of Practice for use of masonry: Structural use of unreinforced masonry.

4. "Report on Accidental Damage to Buildings": LE.L, October 1972, revised March 1974.

5. "Accidental Damage Robustness and Stability"; J. Morton, BDA, May 1985.

6. BS 8110 Parts 1 & 2: 1985: Structural use of concrete: Code of practice for design and construction and special circumstances.

7. Structural Damage in Buildings caused by gaseous explosions and other accidental loadings 1971 - 1977: survey by BRE published by British Department of the Environment.

8. "Structural Safety", J.W. De Courcy, Paper read at Seminar on New Building Regulations, Dublin 1982, I.I.R.S .•

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9. "Report on the Investigation of the Explosion at Raglan Rouse, Ballsbridge, Dublin-1st January 1987"; Cremer & Warner, Department of Energy, March 1987.

10. "An Appraisal of the Official Reports on the Raglan Rouse and Dolphin Rouse Incidents by Cremer and Warner Ltd. with regard to aspects relating to structural engineering design of buildings to prevent disproportionate damage in the event of an accident"; A.C.E.I ., May 1987.

11. "An Appraisal of the Reports by Cremer and Warner Ltd. on the Gas Explosions at Raglan Rouse and Dolphin Rouse"; A.C.E.I., July 1987.

12. "Report of the Task Force on Multi-Storey Buildings";August 1987.

13. "An overview of Structural Design and Construction since 1960 with particular reference to Accidental Damage"; I.E.I.!A.C.E.I., July 1987.

14. RP!68!03: "Guidance on the Design of Domestic Accommodation in Loadbearing Brickwork and Blockwork to avoid Collapse following an internaI explosion" I. Struct. E., 1969.