44. - The Design of Brickwork Against Gas Explosions*

by H. C. AOAMS

Department ofthe ElIviromnent, Lo"don SWI

A BSTRACT

The paper discusses lhe aceidental damage concept and possible callses of abnormal loading, alld describes lhe nafure of exp/osion damage to brickwork. Design requirements are considered in relation to lhe Building (Fiflh Amelldmenl) Regula/iolls and lo lhe possibilily of pro l'idillg I'enling relie! The 'allernalil'e palh' bridging solution is lhe preferred design method aI present.

Le Calcul des Maçonneries en Briques Devant Résister aux Explosions de Gaz

Die Konstruktion von Ziegelmaue,"­werk, das Gasexplosionen widersteht

Der Aufsalz diskuliert die Vorslel­lungen über den Unfallschaden 1I11d môgliche Ursachen aujJerordeJ1llicher Belaslungen. Ferner beschreibt er die Natllr VOII Explosionsschaden an Ziegelmauerwerk. KOllslruk I;ons­Erfordernisse sind in bezug auf die Bau- Verordmmg (Füllfle abgeiinderle Fassullg) und die Schaf!ung besserer Entlüf tungs- Vorriclulmgen belrachlet. Die gegenwarlig bevorzugte Kon­struktiofismelhode isl die des "Ersatz­weges" mittels f reitragellder Balken.

La 110lion d'effondremenl aeeiden/elle el les causes possibles de charge QlIDf"male som examinées. La na/tire des effondrements aux maçonneries en briques prol1oqllées par les ex­plosions est décrite. Les exigences concernant te ca/cul des maçonneries S0111 trail ées en relation avec les réglements relatifs à la constrllctioll et avec la possibilité d'assurer lll1

dispositif de sécurilé. La solulion aCluellemel11 préférée est décrile.

J. INTRODUCTION On the morning following the Ronan Point collapse the front page of a popular newspaper carried a photograph with the sim pie caption 'Why, Why, Why?' A good question? It has certainly kept the construction industry occupied since aod caused a rather uncornfortable reappraisal of ali building methods and materiais. The newspaper rnay have reflected public reaction accurately, the public failh in lhe engineer's abilily to achieve structural safety being consistent with its clear noo­acceptance of failures of this type.

2. THE 'ACCIDENTAL DAMAGE CONCEPT'

The concepl lhat slructures should resist loads other than lhe ' normal' ones due to gravity and wind is nol really new. The old conventional methods of construction incorporated this concept to some extent in their empirical design rules, and also beca use of the massive nature of many of their non-structural elements. Sub­sequently increased knowledge and improved design methods and materiais have properly allowed a reduction in the structural framework, while lhe non-structural elements have been pared down to meet more c10sely their prime functions. This process has rendered the resulting structures progressively more vulnerable to the inadvertent load. There is no longer a ' hidden bonus.'

The piped gas explosion is lopical, but abnormal loading may also oceur from other causes. Some examples, which are difficult to quanlify, are:

Explosions due to bottled gas or petrol or similar vapour Fire Vehic1e impacl Earth slip Water erosion to faundation Failure of one pile in a group Improper loading by user Material fault.

An abihty to sustain one type of accidental damage should give a si milar performance with others. Ali abnormal loads will occur only rarely- indeed, so do lhe maximum design gravity and wind loads in most cases. However, only gravity loads are sustained for periods and a reduced load factor is thus justified for accidental loads. It is of course already in use for wind loading and is consistent with the ' hmit state' philosophy.

3. THE NATURE OF EXPLOSION DAMAGE TO BRICKWORK

There is a considerable hislory of gas explosions in brick buildings, nearly ali of which are of domestic type with timber floors and roofs. These explosions occur quite frequently bul provide little useful design data beca use lhe brickwork is lightly loaded and inadequately tied by the timber fioors and roofs.

Normal construction for buildings of five ar more storeys would have reinforced concrete floors either precast ar in siltl. While some forms of precast units might provide little ' tie' stability, it is not difficult to improve this by properly designed steel in an in silu joint. The ability of in sirll construction to provide tie action is inherent. E ither type must be superior to the timber fioor.

The maximum pressure which can be reached in a gas explosion in a closed compartment is known to be about 100 Ibf/in 2. Actual pressures occurring in practice are considerably lower, firstly because the gas/air mixture is not the optimum and only partially fill s the compartment and secondly because venting provides some relief.

What pressures are reached has yel to be fully establish­ed by full-scale testing, but it seems clear that pressures can be and have been sufficient to remove brickwork panels of the lype and thickness in common use.

In 1969 there was a gas explosion in Copenhagen involving a block of three-storey fiats of brick

*The author's own views and not necessaril y the offieial policy or the Department.

273

274 The Design of Brickwork Against Gas Explosions

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9 WALLS REMOVE0 BY EXPlO$ICN

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FIG URE I- Copenhagen 1969: fioor plan.

construction with in s;tu concrete floors. Figure I shows the plan_ The externai waUs were solid, 360 mm (14 in.) , with 160-mm (5-in.) internai cross and spine walls. The windows were double glazed and had an area of 22 % of the waU or 10·5 % of the ftoor area. This is rather less than the usual average and is also less than that in the B.Ceram.R.A. tests.' These tests produced bowing and cracking of a solid 9-in . wall panel of roughly the same dimensions as lhe Copenhagen ' explosion' room wall , but witbout a window, at a pressure ar between 14 and 16Ibf/in2. lt must be observed that the B.Ceram.R.A. panel was fully restrained at aU four edges withiri a concrete bunker and lhe weakening effect af the window must be conjecturaI.

In lhe B.Ceram.R.A. test on room 4 lhe window was 'ejected bodily' with a peak pressure below 2 Ibf/in2, leaving lhe II-Ül. cavity externai and 4!-in. partition waUs undamaged. Perhaps the 14-in. solid wall panel with window at Copcnhagen may have had lhe same order of strength as the B.Ceram.R.A. 9-in. homogeneous walI panel, but the restraint conditions were different, and it is not to be expected that the explosion pressure would be as high as in the B.Ceram.R.A. test (about 15 Ibf/ in2). The length of wall removed was slightly less than Ihat specified in the Fifth Amendment2

(2'25 x height) but was limited by the window openings.

4. DESIGN REQUIREMENTS

4.1 Regulations

Following the report of the Ronan Poin! Tribuna l, the Ministry of Housing & Local Government published Circular 62/68, which was of a stop-gap nature and referred to large pane I precast concrete construction only. It was followed by three publications from the Institution of Structural Engineers which were essentially extended guidance 'footnotes' to the 62/68 circular bu! the third J dealt specifically with domestic accommodation in load-bearing brickwork.

The Fifth Amendment to the Building Regulations succeeded Circular 62/68 and became law on April I, 1970. In essentials it specifies functional requirements

designed to minimize and restriet the local damage resulting from accident and extends the scope of the 62/68 Circular to ali types of construction over four storeys high. It allows alternative approaches of wbich the first is a 'bridging' requirement following the notional loss of a load-bearing member and requiring an ability in the remaining structure to bridge the resultant gap. The alternative requires structural members which cannot be allowed to be removed to be capable of withstanding a load of 5 Ibf/ in2 acting on their own area and that of contiguous elemenls. Reductions in applied and wind loads are permitted and the resultant total load must not exceed 95 % of the collapse load. A 'deemed to satisfy' provision specifies the maximum fioor area wbich may be affected by the local structural failure.

The object of the 5th Amendment could be sum­marized as to specify broad functional requirements which when mel by the designe r, will provi de buildings with the ability to resist the random accidentalload. It is drafted so as to cover ali types of constructiol1 with a minimum cost penalty whilst leaving freedom of solution to the designer.

4.2 Vcnting Relief

The provision of a 'safely valve' relief element in a structure is a concept which must appeal to a designe r providing for the effects of an explosion.

Some tests on venling relief have been carried out in Sweden,4 and in this country by the Gas Counci l 5

on simulated gas ovens. Based on these tests a nd a correlation with small-scale experiments with ducts 6 the following relalionship has been suggested by RASBASH 7

for stoicheiometric propanejair mixtures.

where

P., = 1·5P. x O-5K _ . ( I)

p", = maximum pressure wit hin the compartment reached in the explosion (lbf/ in2);

Pv = pressure at which vent covering is removed ; Ac . .

K=-= venung ratiO; A.

Ac= smallest cross-sectional area of the compart­ment (ft2) ;

A.= area of the vent (ft2).

Some extrapolation is jnvolved but the relation is stated to be valid for compartments up to 2000 fP where maximum: minimum dimensions do not exceed 3: 1 and where values of K are between I and 5. The value of P. should also no! exceed I Ibf/ in2. To be effective a vent relief must nol only act (i.e. open or brcak) quickly at minimum pressure, but must also be as light as possible. The coefficienl of K makes some allowance for lurbulence due to furni ture, but serious turbulence might produce higher pressures. The pressure developed is proportional to the fundamental burning velocity of the gas, so tha! with town gas, maximum pressures could be two to three times higher.

The time in which lhe vent cover must be accelerated clear to permit escape of the gas has to be related to the time scale of the explosion and the time required for the structural members to develop their strain and resultant stress under the explosion loading. The latter figure will be half the natural period amplitude of the member.

H. C. Adams 275 Typical structural members will have natural periods between 0·1 and 0·03 s. A gas explosion where venting relief is eITective in keeping pressure down lo a few pounds per square inch would have a duration of about 0·3 s and struclural members would clearly altain their full stress. Without venting relief the duration of pressure would be considerably longer.

In his paper RASBASH 7 concludes that vent relief panels should weigh not more than 5 Ibf/ft 2 and open under apressure not exceeding I Ibf/in '- The weight criterion can be mel, but difficulties can arise with lhe relief pressure since lhe panel must withstand wind loads, which can be high for cladding, particularly near the corners of buildings. Rasbash also calls for 'back relief' vents which should open at apressure af nOl more than 0·25Ibf/in2. This is hardly practical for panels exposed to high winds.

Venting relief would seem to hold considerable promise in dealing with explosion loading particularly for load·bearing panel type structures, which include brickwork. The Iimited tests 50 far carried out seem encouraging, but adequate data for design can only come from a series af comprehensive full· scale tests. It should also be noted that the Fifth Amendment does not recogllize venting relief, as indeed it could hardly do in the present 'state Df lhe art'.

4.3 Addilional Non-Slruelural Preeautions

Both town and natural gas are lighter than air and so can be removed by appropriate natural ventilation. However, in praclice there must rernain the danger af stagnant pockets occurring. This could also occur with mechanical ventilation and reliance 011 removal af gas by ventilation as a first line of defence is not justified.

The volumes involved are large and preliminary studies indicate that the additional heating costs associated with frequent air changes are uneconomic. A gas content in the air of 5 % is the lower limit of flammability though the explosion pressures produced are much lower when more ar less than the stoicheio­melric proportions are present. Nevertheless discreel ventilation of danger spots appears to be good practice. An instance would be an internai compartment (cup­board) housing a gas heater, a common arrangement today. Gas alarms able to detect dangerous con­centrations of gas are another passible line af defence, but their cost and dependence on effective maintenance are disadvantages in many cases.

4.4 Design Basis

In general it would appear that unreinforced brickwork cannot be designed to wilhstand the specified 5·lbf/in2

loading without adequate preload and thal the 'alter­native path' bridging sol ution is the appropriate design method. 'Bridging' or cantilever action over the damaged portion, once determined, presents no unusual design features, especially as the load factor required is low.

HASELTINE and THOMAS 8- 10 have provided useful comment and guidance on the Institution of Structural Engineers document,3 including some typical specimen calculations. Both paper and document were, however, published prior to the Building (Fifth Amendment)

Regulations although essentially there is no change in principie.

In particular it is clear that in load·bearing wall st ructures the plan form adopted is of first importance together with the 't ie' ability of the floors or beams. The multi-cell structu re, with \Valls in both directions at frequent intervals buttressing each other, is the easy solution but will have obvious functional and cost penalties for many buildings.

The desirable 'cell ' concept, however, with wall lengths broken by buttresses cao slill be attained with the common cross-wall construction by means of a spine wall or, in appropriate cases, 'block' buUresses formed by stair or service cores.

Gable walls provide a special case for ali construction systems except some framed types. For brick con­struction two sol utions are feasible. Either the end bay may be designed to cantilever using the composite action of walls and floor, or the gable end above the damage may be carried on 'storey post' columns. Such columns oeed to be designed for the 5·lbf/io2 horizontal load. This should present little difficulty on the columo

. area itself, but the oeed to prevent transfer of such loading from the adjacent wall by discontinuity will be c1ear.

Looking further ahead , once the eITects of venting relief have been established by full-scale tests, this will provide another and possibly better means of overcoming the explosion problem. It would no doubt need to be coupled with a dynantic 'pressure pulse' analysis of the structure under the 'vented' cxplosion pressure. ALEX­

ANDE R and HAMBLY 11 have proposed a melhod though they emphasize that this must remain qualitative pending, once again, the results of practical research.

Experience 50 far indicates lhat the additional costs involved will be small and may well become negligible as designers become familiar with the problem and a further incentive to architect/engineer co-operation is thus provided .

REFERENCES I. AsTBURY, N. F. and others, Gas Explosions in Load~bearing

Brick Structures. B.Ceram.R.A. Spec. Publ. 68. Stoke-on­Trenr, British Ceramic Research Association, 1970.

2. MINISTRY OF HOUSING & LOCAL GOVERNMENT, The Building (5th Amcndment) Regulations, 1970.

3. INSTITUTION OF STRUcrURAL ENG1NEERS, Guidance on lhe Design of Domestic Accommodalion in Loadbearing Brickwork and Blockwork to A void CoJlapse Following ao InternaI Explosion. Document RP/68/0J.

4. Komitten for Explosions fõrsôk. Slutrapport. Bromma 1957. Stockholm, Apri11958.

5. CUBÜAGE, P. A. and SIMMONDS, W. A., Ao Invesligation of Explosion Reliefs for Industrial Drying Ovens. Gas Council Research Communications GC 23, 1955; and GC 24, 1957.

6. RASBASH, D. J. and ROOOWSK I, Z. W., Relief of Explosions in Duct Systems. Proc. Symposium on Chemical Process Ha zards. Inst. Chem. Eng., 1960. pp. 58~68.

7. RASBASH , D. J., The Relief of Gas and Vapour Explosions in Domestic Structures. Inst. Struct. Eng. Srrucl. Engr., October 1969.

8. HASELTINE, B. A. and THOMAS, K., Loadbearing Brickwork­Design for Accidental Forces . C.P.T.B. Tech. NOle 2, (6), July t969.

9. HASELTlNE, B. A. and THOMAS, K. , B.D.A. Tech. NOle I, (I), 1970.

10. HASELTINE, B. A. and others, R.D.A. TecI!. Note I, (2), 1970. 11. ALEXANDER, S. J. and HAMBLY, E. c., The Design ofStructures

10 Withstand Gaseous Explosions. COllcre/e, Feb. and March 1970. pp. 62- 65 and 107- 116.