BS 5628: The Structural Use 01 Masonry:Par11 : Unreinforced Masonry

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May 1985Amended and ReprintedNovember 1996

Accidental damagerobustness& stability

Prepared by

J. Morton BSe PhD CEng MICE MlnstM

Based on independent presentations given at BOA seminars by J . O. A. Korff BSe CEngFIStruetE MICE · R. J . M. Sutherland BA FICE FISTRuetE FIHE MConsE . J . Morton BSe

PhD CEng MICE MINstM

THE

BRICKDEVELOPMENTASSOCIATION

as 5628: The Structural Use of Ma sonry : Part 1: Unreinforced Masonry

Accidental damagerobustness &stabiIity

Prepared by

J. Morton BSe PhD CEng MICE MlnstMBased on independent presentations given at BDA seminars byJ.OA Korff SSe CEng FIStructE MICER.J.M. Sutherland SA FICE FIStructE FIHEMConsEJ. Morton SSe PhD CEngMICEMlnstM

FOREWORD

'"~ This document supersedes all previous BDA publications on~ the subject of Accidental Damage and the 5th Amendment, in~ particular Technical Note No 1, Vol 3, July 1971 and Its! forerunners." This new publication results from three independent~ presentations which were originally given during a series of" seminars on BS 5628: Structural use of masonry: Part 1 in lateI 1978 and subsequently.~ The aim is both to cover the thinking behind accidental~ damage analysis and to explain the basis used in the Code to~ fulfil the requirements of the Building Regulations. Because of!I the nature of the subject, accidental damage cannot be

studied in isolation; robustness and stability also need to beconsidered. It is, therefore, a blend of masonry designphilosophy combined with the detailed provisions to meetParliament's requirements.

Being based on three different presentations by threeauthors, it is inevitable that within the document certain of theviews expressed represent the opinion of a particular authorrather than of all three. In this way a blend of views arerecorded which represent an up to the minute approach toaccidental damage, robustness and stability which, not beingan exact science. is based on reasonable assumptionscombined with engineering judgement.

CONTENTS

1. INTRODUCTION ........................................ . 2Scope ............................................................................................... . 2Background ................. . 3The Building Regulalions 3

2. CODE PREAMBLE 3Strudural slabilily & 10rm . 3Vehicu lar impad 4H ig h ris k silualions ..................... . 4Damage proportionate to cau se _ 4The effed of venting . 4Responsibilily for overall slabilily . 5Min imum st iffness requi rement __ __ ,5

3. THE DETAILED REQUIREMENTS 5The lhree options . 5Horizontal ly ing force . F, . 5Protected members . 6Option 1 6Notional removal of members 6Definit ion of members _6Containing the damage 6Case 1: Inlersecling & return walls 6Cese 2: Inlersecling substsntiet partitions 7Cese 3: Piered w.lls 7Recognisable situations_............................. . 7&lern.1 wstts. . 7tntemel w.lls 8Resultsn! high locst slresses 8Option 2. . 8Horizontal ties . 8

1 INTRODUCTION

Peripher.,ties 9Inlern.llr.nsverse & longiludin.lties 9W.II &column ties 9Option 3 10Vertical lies......... . 10Summary of options 11Design philosophy 11

4. BACKGROUND PRINCiPLES 11Horizonlalties 11Contain ing Ihe damage 12

5. ROBUSTNESS 12Strudural synthesis 13Connedorslslrapslties 13Lateral movement. 13Uplift . 14Wa U layout & lIoor syst ems 14Precasl floors 14Save or sacrifice? 15Di.crete blocks 15Vertical tie. ............................................ . 15Non-cellular plan forms ...... . 15Diaphragm walls . 16Fin walls .......... . 16

6. MAKING SENSE OF THE FACTS 16Frequency of damage ..16Permitted damage level• ......................................................... ........17

7. CONCLUSiONS 17

8. REFERENCES back cover

2

ScopeThis publication is concerned WIth the subject of robuslnessand the avoidance of excessive accidental damage in retat.ooto the sl ructure of masonry buildinqs, It deals Wllh theprovisions contained in Part 1 of BS 5628:The Structural Useof Masonry: and m particular with Section 5 of the Code whichgives detailed recommendations for controlling and limiting

accidental damage. The subject is dealt with in its widestsense to give the reader an understanding of what the Coderecommends and, more importantly, why the Coderecommends it.

BackgroundThe accidental damage provisions in BS 5628 stem from thepartial collapse of a block of flats in North London, RonanPoint. in 1968" ', As can be seen in Figure 1, the amount ofdamage appears fo be out of proportion to the cause - a gasexplosion in one of the flats. Questions were raised in theHouse of Commons. The relevant govemment department. atthat time, was the Ministry of Housing & Local Government.and they issued many circulars'" as a result of the collapse.The Insfitution of Structural Engineers naturally becameinvolved and issued its own circulars'", These. together withthe circulars from the Ministry, resulted in the BuildingRegulalions being arnended by the addition of the FifthAmendment. This Fifth Amendment was later incorporated inthe 1976 Building Regulalions as requirements D17 and D18which limit the spread of darnage permitted following anaccidental event:017 'Further requirements for the structure of certainbuildings'.018 Deemed-to-setisiy provisions for localisation of structuralfailure',

Structural stability & formIn the Code, the main section on accidental damage isSection 5. But there is an earlier section in the Code onstability (clause 20) where several important points are madewhich are relevant to accidental damage and stability.

These are:1, Robustness and containment of damage;2. Resistance to a prescnbed horizontal force;3. Prevention of vehicular impact:4, Special hazards related to service use.

The members of the Technical Comrnittee responsible for

The Building' RegulationsSection D of the Building Regulations, which deals with thestructural aspects of buildings, contains the followingrequirement: 'The structure of a building above thefoundations shall safely sustain and transmit to thefoundations the cornbined dead load, irnposed load and windload without such deflection or deforrnalion as Will impair thestability, or cause damage to, the whole or any part of thebuilding. I This provision. clause 08, is a legal requirement forall structures.

For buildings with 5 or more storeys, however, D17 and D18also apply. Broadly speaking, D17 and D18 ensure that thesort of collapse that occurred at Ronan Point does not happenagain.

However, because it is recognised that design for nilcollapse is often not possible, the Regulations D17 and D18specify the limits of permissible damage as follows:The darnaged area within each storey must not exceed 70rn2

or 15% of the plan area whichever is the lesser, Vertically,failure is acceptable if it occurs within the storey where theincident took place and may also involve the storey next aboveand next below.

This was scarcely the first occasion in history whenregulations were introduced to govern the design andperformance of buildings. Hammurabi, a great law maker,who was king of Babylon from 2067 to 2025 BC, introduced alaw preserved for posterity on a clay tablet. The translationruns as follows:

If a builder builds a house for a man and does not make itsconstruction firm and the house collapses and causes thedeath of the owner of the house - that builder shall be put todeath. If it causes the death of a son of the owner - they shallput to death a son of that builder, If i t causes the death of aslave of the owner - he shall give to the owner a slave of equalvalue. If J/ destroys property - he shall restore whatever itdestroyed and because he did not make the house firm he shallrebUild the house which collapsed at his own expense. If abuilder builds a house and does not make its construction meetthe requirements and a wall falls in - that builder shallstrengthen the wall at his own expense.

The Babylonian approach makes the present accidentaldamage provisions in BS 5628 seem eminently reasonable.

CODE PREAMBLE 2

drafting the Code felt that the designer should be reminded ofthe way in which forces can best be carried in rnasonrystructures. Applied loads are best resisted by walls lyingparallel to the direction of the applied force. Wind force A cansuccessfully be carried by the cross walls lying parallel to theapplied force. These are the walls which point into the wind.

There are no spine walls along the centre of the buildingaligned parallel to force B. The structure could therefore'sway' in this direction - It would be 'floppy' in direction B,

So, the plan form at the top is not at all stable in thelongitudinal direction, Whereas the bottom two plan forms,with walls in both directions are stable in direction A as wellas in direction B. 3

This, of course, leads to a cellular type of bUi lding forwhich masonry offers an excellent structural solution.

between masonry walls and other parts of the structure,Thesepoints will be considered in greater depth later.

Damage proportionate to causeThe total collapse of the end bay of the Danish blockIllustrated resulted from damage to only part of the groundfloor gable wall. The damage that follovved was out of all

H igh risk situationsThe Code also suggests that special precautions should betaken where there are particular hazards such as chemicalplants or flour mills. This may result in accidental damageconcepts being a primary consideration at the initial designstage.

Vehicular impactWhere vehicular impact may prove a problem, the Codesuggests that attempts should be made to isolate thestructure from the vehicles. This could be achieved usingearth banks, boltarcs. etc, .

In Figure 8, the columns just visible on the right are beingprotected from a run-awayvehicle. This technrque can equallywell be used to safeguard masonry bUildings.

The high nse blocks of flats In Figure 9 were desiqned foraccioental damage and are structurally safe. Subsequent totheir completion. however, bonards and a crash barrier wereInstalled to protect them from run-away vehicles on the steeproad behind them.

,.~;;:"~proportion to the cause, and to avoid trus sort of situation

arising, the Code lays down the guideline that the damage [should not be disproportionate to the cause. -

~A gas explosion at Clarkston Toll, on the outskirts of •Glasgow, could arguably be placed in the same category as ~the Danish block. The picture, Figure 11 shows all that was ~left of a heavily reinforcedconcrete wall after the event. ~

The Code suggests that the layout is considered forcellulanty, Interaction between intersecting walls, and

4

The void in which the explosion took place did not have anysubstantial areas of weakness. Thus, it was effectively anexplosion within a uniformly strong pressure vessel wh ich.unfortunately, was not quite strong enough . Had an end wallor part of a side wall been designed to be specifically weakagainst lateral pressure, the terocity and effect of theexplosion could have been much reduced. This would haveresulted from the weak area failing at a relatively low pressureand allowing the hot gases to escape. This escape at highenergy gases is called venting. It is not mentioned in theCode, but It is of great importance.

or all of the desion and details are not made by the samedesigner.' The need for overall responsibility is clearlyextremely important.

Minimum stiffness requirement

Another point in clause 20 - referred to earlier - concerns the'unsbffened' structure. Stiff and stable in direction A, butlacking stiffness in direction B. This structure would requiresome spine walls to sbffen it agarnst force B. But. because ofits geometry, not much wind may act on the narrow end wall ,resulting in only a little spine wall being incorporated fromstructural necessity. This could still be thought of as lackingstiffness in direction B, even when a small amount of spinewall has been incorporated. To avoid this situation occurring,the Code recommends that the building should be designedfor either the wind force or ' ,5% of the total characteristicdead load above any level, whichever is the greater. This ' ,5%represents the force which must be resisted to achieve theminimum acceptable amount of stiffness in the building.

Responsibility for overall stabilityThe Code also includes a paragraph dealing with the Questionof who is responsible for overall stability. This is aimed atavoiding situations where the masonry is designed by oneconsultant and the timber roof by another. Each designer maythink that the other is dealing with the way the roof is fixed tothe top of the walls. Consecuently, the building may becompleted without this detail being constructed or evenconsidered in the design. To avoid this situation arising. theCode states that 'The designer responsible for the overallstability of the structure should ensure the cornpatitnhty of thedesign and details of parts and components. There should beno doubt of this responsibility for overall stability when some

THE DETAILED REQUIREMENTS 3

DesiSJIforrobusb ass

lesser of 60 kN or 20 kN + 4Ns where Ns is the number ofstoreys.

This means that the maximum value for F, is 60 kN. Theminimum value - for a 5 storey structure - is 40 kN. Ft.therefore, has a value which lies between 40 and 60 kN. F, is avalue that is constantly required. 5

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...•••••

..

14

The foregoing commentary, covers the general preamblecontained in clause 20. We now turn to the specificrequirements of Section 5 of the Code.

The three optionsThere are three routes through the accidental damage clauses- options 1. 2 and 3, as shown in Figure 14. However, theseonly apply to buildings of 5 or more storeys. If the building is 4storeys or less, there are no specific requirements. This

~ presupposes that low-rise buildings will be designed 'robustly'I and comply with clause 20 of the Code.-e The first route. option 1, requires fundamental thought ­! the thinking man's route. The third route, option 3, requires" litt le If any thought and is simply a matter of following specific~ rules. Option 2 is a blend of options ' and 3 in that although.. the rules need only be followed in part. fundamental thought~ is also required.~ Before considering any of the options in detail, two]; definitions must be established:j Horizontal tying force, F,, The first definition is the horizontal tying force. F,. This is the

6

Protected Mem bersIf a structural' elemen t is designed to carry. wi thout failure.both:(a) the reduced design load - derived from the reduced V,'sand Ym's for accidental damage analysis;and(b) 34 kNlm' . applied uniformly to Itself and othe r elementsattached to it, it is designated a protected member.

In other words. protected members remain intact after anaccidenta l event. They are regarded as being structu rallyunaffected by It. This is clearly important when such membersare indispensable to the stabilityof the structure.

When adjust ing VI values for acc idental damage. VI for O,becomes 1·05 whe re the imposed load is of a permanentnature, such as storage areas, computer rooms, etc.

The calculation for a protected mason ry member IS basedon a three pinned arch model. From tests. this has beenfound to correlate well with what actually happens when awall is laterally loaded. The formula giving the lateral fai lureload, Qlal' is:~ fQlat=- where ha* and t are the height and thickness 0h;

the wall respecltvely. and n is the precompression on the wa llper unit length. It must be remembered that for arching actionto be effective. adequate abutments in the form of concretesurfaces or other construction capable of providing sufficientresistance against rotation. lateral and axial movements arenecessary. In this Situation Ym becomes 1·05, whichrepresents the overall factor of safety. Normally. of cou rse.

With accidental damage. the materia ls safety tactors such asYmand Ymll are halved.Having estab lished these two defmitions - the hor izontal tyingforce. F,. and the protected member - It is now possible tolookat option 1 in more detail.

Option 1

VERTICAL & HORIZONTAL ELEMENTS. UNLESSPROTECTED. PROVED REMOVABLE. ONE AT A TIME.

WITHOUT CAUSING COLLAPSE

This clearly indicates that all st ructural members. un lessdesigned to be protected members. are at risk and mu st beassumed to be lost following an accidental event.

Notional removal of membersThe Code only requires one member at a time to be lost or'notionally removed', Following the notional removal of amember. the structure mu st be anafysed to pred ict the extentof collapse. If volume is lost and if it exceeds that perm itted bythe Build ing Regulaltons. the member in question must bestrengthened to become a protected member. or the structureof the building improved to eliminate or reduce the extent ofpredicted collapse. When this has been done . the member isnotionally replaced and the next member is notiona lly removedand the structure re-enalysed. This process continues until allthe non- protected horizontal and vertica l members have beenremoved one at a time and the consequences assessed.

Theoretically . this is a very good and basically soundapproach. It does. however. require some farruliarity With thetechnique. Once experience has been gained, dangeroussituations can readily be spotted. On the other hand. it maynot commend itself to those unfamiliar with the process.

Definition of membersIrrespective of the degree of familiarity, what constitutes amember for this type of analysis? Table 11 ot the Codeprovides the necessary guidance and is reproduced below

Type of Extentload bearingelement

Beam Clear spanbetvveen supportsor between asupport and the extremityof a member

Column Clear height between horizontal lateralsupports

Slab or other Clear span between supports and/or temporaryfloor and roof supportsor betweena support and the extremityconstruction ot a member

Wall incorporat- l ength between lateral supportsor length109 one or more between a lateral support and the endof the walllateral supports(note 2)

Wall without l ength not exceeding 2'25 h anywhere alongthelateral supports wall (for internal walls)

Full length (for external walls)(h is the clear height of a wall or columnbetween lateral supports)

NOTE t, TemfXJrary supports to slabs can be provided by substantIal orotheradequate partitions capable of carrying the required load.NOTE 2. Lateral supports to walls can be provided by mtersecting orreturn walls. piers. stJIfened sectJCN1S of wall, substantial non­Ioadbeaflng partdions in accordance with (a). (b) and (c) of clause 37-5.orpurpose-designed structuralelements.

Contain ing the damageThus. a member is defined as a beam, a column, a slab or awall. Beams. columns and slabs are fairty straightforward, but ~

walls are not. If a wall is not a protected member, what iamount of the wall shou ld be deemed to be removed follow ing ;::an Incident? The Code gives guidance on thi s by suggesting Ii}

three specific cases where the spread of damage is limited by jthe provision of vertical bracing in the form of intersecting or !b

return walls, substantial partitions or piers. ~

Case 1: Intersecting &: return walls ~

The first instance is where there is an intersecting or return :.~

* See the Code definitions for the specific difference between h ~

~~ ~

Case2: Inlersecting substantial partitions

portion of wall equal in length to 2-25 x height is deemed to beremoved from anywhere Within the wall length.

In the case of long external walls Without intersecting orreturn walls to provide vertical bracing, the total length of wallmust be assurned to be removed.

Case 3: Piered wallsThe Code recognises that a substantial pier will stiffen a walland contain the spread of damage, The guidance limits thelength of pier to 1 m, and the pier section should be adequateto resist. in bending or arching a force of l '5F/ m run ofheight. The choice of 1-5F, is quite logical. and is basedon thefact that the pier would carry the 34 kNlm' load over its fullarea and a triangular area either side - this area being ha +h '....!.. If the lateral pressure is 34 kNlm2, the load/unit2

height becomes 76.5 kNlm. This is roughly the mean of 90 kNand 60 kN. which is 1-5 times the 60 and 40 kN values whichare the maximum and minimum Fl values. It is on this basisthat the pier strength cnterion of ' ,5 F{ m height is suggested.

Both the wall (fig 22) and Its short return (fig 23) willdisappear following an internal explosion, The Coderecognises this by stipulabng that the length of the returnmust be 'Y.! - say 1·25 m for a standard domestic. clear storeyheight. If it is longer than this. It is deemed to remain even ifthe main wall is demolished.

Considering both situations, each floor must be capable of 7

To reiterate the requirements of option 1 - walls. or parts ofwalls. floors, beams or columns are removed one at a time(unless they are protected members) and the building isanalysed to predict the extent of damage. As noted earlier.this is the thinking engineer's approach. It is not particularlydifficult.

Recognisable situationsIn the aftermath of Ronan Point. although bnckwork was notinvolved in that collapse. a study was made to tty to establishthe susceptibility of loadbearing brickwork structures to partialcollapse follOWing the removal of a major wall or pie~51. Threeareas were identified which requiredspecial consideration.

External wallsThe first subject was an external wall Without returns. and asimilar external wall With short returns. as shown in Figures 22& 23.

wall which has a minimum length of ~, average weight not2

less than 340 kglm' , and a connection (bonded or tied) to thewall being braced which is capable of transmitting a tensileforce of F{ m height. This wall will limit the spread of damageand, If an explosion occurs, the length of the wall lost will bereduced to the distance between vertical braces or between avertical brace and the free end of a wall, To realise 340 kglm' ,a 215 mm wall or a 170 mm heavy calculon brick wall withplaster would be required,

The spread of damage can also be limited when the inter­sectinq wall is a substantial partition. This is a specialinstance of the previous case. In order to fulf il the Coderequirements, the partition must intersect with the wall at rightangles. It must have an average weight of not less than 150kglm' . A halt-brick wall would be adequate here. The junctionmust be capable of transmitt ing a tensile force of 0-5 F{mheight or greater, The Code suggests that the partition neednot be straight but It should in effect divide the bay intocompartments. It may be interrupted by door openings,However, the Code does not give any definitive guidance onthe details, The Greater London Council have decided that animportant factor is the portion of partition abutting the wall,and they believe that this should not be less than 1 m.

I"~

~~

f"~ Where intemal walls do not have any stiffening effect from, return walls or intersecting walls, one should assume that a

cantilevering back to the undamagec part of Ihe building if it IS10 remain stable. It must be capable of carrying all thereduced design loads plus the weight of the wall standing onIt. If each floor can carry Its own wall. there is no spread ofcollapse.

Inlernal walls

The second condition which may require further InvestigationIS simi lar to the first - namely. short Internal walls (5 . 6m)without vertical bracing from returns. piers or Intersectingwalls. If one of the internal walls shown in Figure 24 wereremoved, the floors above must span twice the length . at leastIn part. and must stili be capable of carrying Ihe dead weightof the wall immediately above - plus. of course. any imposedloads. This extra dead load will act at or near mid span.Because of considerations like this. care must be taken withlarge span buildings.

Resullanl high local stresses

The third situat ion that requires special consideration is wherethe removal of a wall may place high local stresses on otherwalls. In this case. Ihe removal of the ground floor wall willresult in all the walls above It being carried by the structure insuch a waythat overstressing mayoccur at the intersection ofthe remaining ground floor walls - at point A in Figure 25.

The checks necessary for stability in the first two caseswere on the strenglh of the floor slab. It has to be strongenough to carry back Into the core of the structure the dead

8 weight of the floor and the wall . A part structure. such as that

shown In Figure 26 can be thought of as a floor encastre alongall ItS Internal ecges - as shown in Figure 27.

Trus will not be far from the truth, and IS a convenientengineering approximation.

The strength of the floor can then be ascertained usingstandard yield line techniques.

So much for option 1. Fundamentally, It is based on theconcept of transfernng the loads of the unsupportec part ofthe structure. following an accident, back to the foundationsvia another route with minimal damage. For that reason, It isoften called the 'atternative path' approach.

Option 2The second option may be summarised as follows:

PROVIDE FULL HORIZONTAL TIES CHECK VERTICALMEMBERS PROTECTED ORREMOVABLE

It requires all horizontal ties to be present In the building. ?iwhich is then examined for the effects of the removal of f_vertical members fa predict the extent of damage .

1/'Horizontal ties ~.~Three types of horizontal lies are usee in option 2:1. Peripheral ~

2. lnternal lonqttudmal and transverse -3. Wall and column ties mVertical ties will be covered laterbut. forthe moment. they can :­be ignorec because under Option 2 they are assumec to be ~

~

absent. -:<

The internal ties can also be positioned in the lower andupper 0·5 m of walls - again with a 6 m maximum spacing. InFigure 34. the longitudinal ties are shown evenly spaced for aninsitu slab. As before. peripheral steel is required.

With precast floors. the steel already present at thejunctions of the slabs can be used for ties. This will bediscussed later. Again, peripheral steel is required.

So much for the internal ties. Now. the wall and columnties must be considered.

Wall ,( column liesWall and column ties hold the top and bottom of external 9

The internal ties can be concentrated into beams orsimilar, and need not always be evenly spread. However, thespacing for these concentrated tie zones must not exceed6m.

In addition to peripheral ties. internal longitudinal (in directionof span) and internal transverse (in direction perpendicular tospan) ties are required. The magnitude of the force for whichinternal longitudinal ties are to be designed is given by theformula shown in Figure 32. Normally. in domesticconstruction. La is 5 m or lessand Gk+ Ok is of the order of 7·5kN/m' . Therefore. F, usually suffices. Beyond the domesticscale, for example in a warehouse type of building with 10 mspans. the force required may well be 4F, or more. Themagnitudeof the force for which internal transverse ties are tobedesigned is F, in all cases.

The internal ties in Figure 32 are evenly spread. Of course,the peripheral steel is also required.

*tIDIII..--"""_ vcIb ..- ..--- "'- -p--.........~-c.=:,

A more straightforward case is shown in Figure 31.

Internal transverse &. longitudinalties

Honzontal ties are designed on the basis of thecharacteristic tensile strength of the material in question, andmay be provided by the elements of construction already fullystressed in their normal service function.

Peripheral tiesPeripheral ties must be capable of carrying a force of F,kN,and the steel must lie within a 1200 mm zone from theperipheryof the structure.

The ability to carry F,kN is also required at anchorages andre-entrant corners. This, of course, is provided mechanicallyor by bond length.

PROVIDE FULL HORIZONTAL & VERTICAL TIES NOREMOVAL CHECKS REOUIRED ~

~Vertical ties ::Vertical ties are eithervertical wall ties. Vw, or vertical column :ties. Vc. These are contained Within the walls or columns of ~the building, '<

Option 3Option 3 is the fully tied solution. and involves the fullhorizontal tying, just descnbec, plus vertical ties. The object is ,.to ensure that the vertical members remain after an accident. ~In other words. with the level of vertical tying prescribed. the >:vertical members are deemed to behave as protected "­members. j

generated either above or below the slab. It is assumed that itis resisted by the joints at the head of the wall below the slaband the base of the wal l abcve the slab, This assumption isquite reasonable since the upper and lower inner leaves areconnected by the same outer leaf.

For the friction case, F, must be equal to or less than IJ- (thecoefficient of friction between brickwork and concrete)multiplied by n (the least favourable vertical load per unitlength of wall) divided by Ym(the overall factor of safety equalto 1'05) mult iplied by 2 (the factor which takes account of thetwo fnction surfaces). The value of IJ- between bnckwork andconcrete is taken as 0 6.

Incidentally. when using the shear resistance calculanon.0·35 can be enhanced by the precompression on the wall. Thebasic shear equation is F = 0·35 + 0·6 QA for mortardesignations (i). (ii) and (iii) - the mortars which are normallyused for loadbearing bnckwork,

Thus. option 2 uses peripheral and internal ties plus walland column ties - but no vertical bes. The structural adequacyof the vertical members is then checked. They must either beprotected members (ie. able to carry 34 kNlm') or they mustbe capable of notional removal. one at a bme. Without unduedamage resulting.

This type of wall connection can be justified by either shearor friction (the factors of 2 are for double shear or fricbon)'"), Inthe case of shear. F, per m run must be less than 0·35 (theminimum value for shear) dwided by Y~. mult iplied by thearea of 1m run of wall multipl ied by 2 for double shear. Notethat Y~ becomes 1·25 for accidental damage analysis.

walls into the structure. The force which the horizontal wall orcolumn bes should be designed to carry is the lesser of 2F,or

~" For domestic constructon, h is normally approximately2525 m, and the tie force usually approximates to F,.

Wall and column ties can be provided by fricbon or shearbetween the masonry and the concrete slab. as illustrated inthe left hand wall In Figure 37.

By reinforcement: In certain instances. It may be possible touse steel and. in such an event, the force may be provided inconcentrated pockets as shown in the right hand wall. Themaximum soacnq IS 5 rn, and when this spacing is used theforce Will be 5F,. Ties should not be further than 2·5 m from afree or unrestrained edge. Columns should have a junctionstrength of F, and. of course. for corner columns this isrequired in both directions and the vector sum of the twoforces IS F, '</2.

By frict ion/shear: Conbnuing With the way that fncbon/shearforces can be mobilised - the concept is that when a gasexplosion occurs. a pressure of 34 kNlm2 is assumed to begenerated.

The pressure of 34 kNlm' acting on the wall Will produce a

force shown as F,'m , equal to 34x ~= 42·5 kNim width,2

assuming a normal storey height. (Remember that theaccidental pressure can onlyoccur in the upperor in the lower

10 compartment at any one bme.) Although the force is

•roelcn:e<n.£ t Nor 100kN wIIich_-""1A_a1__mrnI

IS150mmIgnoreouter_RIilIo I .20

UNb.SN mort.. 1 1 6

Design ph ilosophyThe deSign procedure relies on three principal tools (singly orin combination):1. Removal of memberanalysis to predict the consequences.2. Design of protected members.

3. Triorth ogonal tying (tying within the three major axes of thebuilding).

Remember that the use of these tools depends on certainassum ptions autho rised by Parliament. The three princ ipalonesare:1. The pressure wi ll not exceed 34 kNlm'.

2, Only one member w ill fail at a time.

3. A certain definable volume of a build ing is allowed tocollapse.

Remember also that the accidental damage analysis is notbased on any exact science. It is a rational prediction of theprobable consequences related to the assumptions made.

whi ch offers a more pract ical approach. When vertica l ties areconcentrated in pockets at 5 m centres, the use of horizontalwall ties sim ilarly concentrated becomes clearer. As With thehorizonta l case. vertica l wall ties, etc, should not be posit ionedfurther than 2·5 m from a free or unrestra ined edge.

The tension requirement, T. also applies to columns. But.since T is an axial tensile force, there is obviously no \/2requ irement for corner columns as in the case of horizontalcolumn ties.

That, in essence, is the th ird option where, by following therules. all will be well. There is a school of thought whichwould agree wi th this sentiment. Equally, a school of thoughtexists which sees this approach as having potentia l dangers ifit is applied blindly and without thought.

Summary of optionsThese requ irements of Section 5 of the Code can be satisfiedby followi ng one of three routes:Option 1 - the aiternative path approach .Opti on 2 - the partially tied solution com prising full horizontal

ties, followed by an alternative path analysislimi ted to vertica l members.

Option 3 - the fully tied solution which requires full horizontal,and vertical ties.

.......--"'"......__vc.............Ir-­- ...--p--w.- -- c.=

Such vertically t ied elements wi ll remain structurally intactfollow ing an accidental event. To achieve th is. the Code givesthe follow ing gu idance:A . Ties shou ld extend from roof to either:(i) foundation levelor(ii) a level below which the vertica l elements are protected.

B. The loadbeari ng leaf must not be less than 150 mm.

C. The mo rtar should be 1:1:6 or better.D. The masonry should have a minimum characteristiccom pressive strength of 5N1mm' .

E. The ratio of height to thiCkness( h; ) must not exceed

20.F. The tie force is given by:

34A (h)'T = aooo f N or 100 kN per m or per colu mn whichever

is the greater. In this formula, A is the horizontal cross­sectional area (mm') of the column or wall including piers. butexcluding the non-loadbearing leaf. if any. of an externa l wa llof cavity construct ion.

The vertical steel bars needed for the vert ical ties can bedistributed . in theory . th roughout the length of the wall. In·practice. it is hard to see how this could be achieved.Alternat ively. they can be concentrated at 5 m max centres.

BACKGROUND PRINCIPLES 4

11

~ To better appreciate some of the principles embodied in the~ Code, it is useful to consider a few examples of damaged"ffi buildinqs,"0

"~

iHorizontal tiesThe illustration shows a reinforced concrete beam, withupstand . which is acting as a catenary. It is well beyond its

c design condition. With an excessive deflection - yet it has not~ failed. This was a local authonty tenants' store, fu ll of small~ cubicles. The tenants decided to make it into a community~ centre, and began to knock down all the part ition walls. After! many of the supporting walls had been demol ished. the beam, suddenly lurched into the deflected shape shown.

The fact that the beam did not collapse has more to do withcatenary action than with the flexural strength of the beam.The concept of horizontal ties is similar to this. Even whenfloor slabs are taken beyond their safe strength limit. It ISlikelythat the floors will span any external gaps in a catenary mode.The floors may deflect excessively. but should not collapse.

Transverse & longitudinal ties

The picture above shows a timber floor cantilevenng in bothdirections. Main beams in one direction. with the deckingspanning in the other - similar to the principles of internaltransverse and longitudinal ties. Although it has deflected wellbeyond the service condition. the floor has not collapsed andis cantilevering from the two remaining walls.

Containing the damage

5 ROBUSTNESS

The concept of lateral supports limiting the spread of damageis illustrated in Figure 46. The end wall has been blown out ­but only up to the stlHened section provided by the chimneybreast.

The stiffened section restricted the damage to the outer wallin a similar manner to the pier discussed earlier. Itdemonstrates the way in which piers. substantial crosswallsand partitions can limit the spread of damage.

The same house again is seen in Figure 47, in more detail.Notice the damaged partition. At the instant of the explosion.It had equal pressure on both sides due to the door being leftopen. So. although cracked. the partition remained in positionrelatively undamaged.

This was not true of the external wall which completelydisappeared up to the chimney breast - as shown in theprevious illustration. Thus, external walls are vulnerablebecause the pressure cannot be equalised on both sides. It isthis reasoning which precludes unbraced external walls fromhaving a limit on the length of wall to be removed. Thebuilding also provides an example of the alternative pathapproach - but the Code does not recommend reliance onwardrobes to prop up roofsl

Buildings can. of course. fall down through not beingrobust enough to carry their own weight. The illustrationsshow the Campanile In St Mark's Square. Venice. whichcollapsed in 1902. In fairness to the mason who designed It.the Campanile was built rn 1329 - SO It lasted a long time. Itwas rebuilt in 191 2 (Figure 50).Another building that fell down was the gymnasium at RockFerry School (7), There was no gale, no explosion, no impact.Luckily. it wasemptyat thetime and no one was hurt. Figure 51.

12

[~~

~

I- - - - ---- ---- - ------- - - - - -----

capable of suppo rting the tops of the wall s, If this is done, theforce from wall C whi ch is transferred into the roof w ill then betransmitted into the end gable wa lls - walls A and B, Sincethese end gable walls are the stiff shear wal ls, when the windis blowing as shown, they w ill resist the applied wind load, Inorder to achieve th is effect, the roof may need to be stiffened,It also requires care in the way that the roof is connected tothe walls ,

ConnectorslstrapsltiesLateral movement

Details of some of these connections are provided in Append ixC of the Code, Figure 54 illustrates the principles required inany detail whose purpose is to hold a gable wall into a timberfloor,

Straps, bent Into the shape of an L, are fixed to the floorand extend into the cavity. These prevent the wa ll moving indirection A. By f ixing packing between the first jo ists and thewal l, movement in direcli on B can be restricted, But, all th ispresupposes that the floor is acting as a stiff plate and , toachieve this, blocking or herringbone strutting is introducedbetween the first two or th ree joists, as shown , This should beplaced at the strap posit ion,

These principles are shown over the page in a realbuilding at roof level. The L-shaped straps are positionedwhere the ceiling diaphrag m has been sli ffened - not. th istime , by blocking or strutt ing but by triangu lar type bracingtrusses runn ing longitudinally along the roof, There is no 13

--=I*:i<F4l=j,--- -- -- 8lrIp

=ii,.~.' -_I,' ,_ :, .:!'

- bIocIUng. 8lrIp----_ 8 "

54 ---------------...A _

Structural synthesis

.c In order to get a feel for robustness, it is necessary toi understand the way in wh ich masonry structures are made to-e work. Consider a simple box structure such as the small! flat -roofed domestic unit shown in Figure 52. There is a front~ door, a back door and two window openings. When the ~ind

15 blows on wall C (or D), half the lateral pressure IS transmitted~ down to the foundations and the other half up to the roof,~ Considering Section A, the flat roof has no moment:? capacity at Its junction with the wall. So, under the loading~ shown, the wall s, acting as cant ilevers, would simply rotate~ through the dotted position to failure.~ To avoid th is, the roof must be made to act as a stiff plate

It is, of course, essentially a comparison table. and is notintended to be used as a fine design tool. Nevertheless, thetable Illustrates how an irutral deSign might be improved byoptinq for a somewhat more robust combination of wall layoutand floor system.

Wall layout & floor systemsSutherland has suggested,9, that the robustness of a structure,and Its ability to sustain accidents with minimum damage. isdirectly related to the layout of the walls and the type of floorused. These can be combi ned to give the desired degree ofrobustness. Korff argues that It is also greatly dependent onthe fortuitous interactions of elements in an emergency(lO).This can be defined as a likely combined behaviou r ofcomponents whereby their aggregate strength greatly exceedstheir individual strengths.

Precast floors

i;;:

In themselves, precast units are acceptable, although care ~

should be taken with the jointing and tying. A mechanical ~locked loop type of joint should be used, as shown in Figure

00. ~Tests carned out at the Building Research Establishment -

f ~have shown this type a joint to be as strong. and even ::stronger, than the concrete units themselves. Note that the ~

panels have failed while the joint (centre) remains ~

undamaged . Figure 61. ~

Uplift

The principle of holding down roofs subject to net uplift isshown in Figure 56. Both the roof trusses and the wall platemust be held down onto the wall with a factor of safety of 1·4against uplift .

evidence of packing to stop the wall moving inwards.However. closer examination reveals that the longitudinal roofbracing actually abuts the wall and prevents It from movinginwards.

Above. the principle is demonstrated in practice. The rooftrusses are held down to a wall plate by proprietary f ixingclips . These can be seen on the two lefthand trusses abovethe window lintol. The wall plate IS held down onto the wall by10 mm diameter galvanised steel bars which have a pre-benthooked end. The hooked end is lowered down the cavity andIScaught on a solid vertical twist tie. The upper end of the baris then bent over the wall plate. The connection IS made tightby pushing the top of the bar sideways, as far as It wi ll go,and then holding It in position with staples. Finally, the end ofthe bar is tr immed (Figure 58). The details shown In Figures 57& 58 are qurte different from that shown In Figure 56, but theycontain all the principles of holding down a roof,

Appendix C of the Code gives details of some fixings,although those provided only apply to lateral restraint. Forholding-down details and a fuller treatment of strapping andtying, reference should be made to the 'Design guide forstrapping and tying of loadbearing brickwork in low-rise

14 construction'!",

------A suggestion, which has much to comme nd it, is to

stagger the vertical ties'· ) or anchor them independently in thetop and bottom of the floor slab" O). The building would still befully tied vertically, but the ties would be discontinuous.

Another idea would be to buttress both ends of the build,ng ­leaving the centre portion untied to collapse, in part, as It may- although, of course, this may not satisfy the BuildingRegulations as currently drafted.

Both Korft 's and Sutherland's papers express some concemabout the use of continuous vertical ties. It is argued that anexplosion could have the effeel of dragging out the elementsabove and below the member forcibly removed by theincident. Indeed, this is what happened during a test on amodel concrete panel high·rise block at the Building ResearchEstablishment' ".

Verlicalties

Pursuing this argumen t to its logical conclusion, stabilitycould be achieved by using buttressed and independentblocks to get the best of both worlds. Where there is abuttressed block at one end only, the other blocks would needto be tied back to it - thus becoming tied blocks.

Non-cellular plan formsThus tar, the discussion of accidental damage and robustnesshas been primarily concerned with cellular forms ofconstruction, for which loadbearing brickwork offers such aneffective and economicsolution.

However, tall single-storey open plan buildings must also beconsidered. Such structures work in the same way as the 15

damage. Perhaps this is suggested because of the dueluncertainty ot either not having enough tying or, on the otherhand, having too much .

Save or sacrifice?The same paper by Sutherland'· ) also drscusses the concept

of dividing the structure into component blocks, each stableon its own but with definitive breaks to limit the spread of

A fully t ied solution (Figure 63) might result in an explosioncausing damage which would then spread because of the tiednature of the building. It the building were split intoindependently stable parts, this spread of damage would notoccur.

Discrete blocks

simple box building illustrated earlier.Sbff plate acbon of the roof is important to transfer the top

half of the wall loads Into the shear walls. The design shouldbe based on sound engineering princtples(11.12). In suchbUildings. walls will doubtless span vertically due to thepresence of movement joints.

For many years now. two efficient and economical forms ofconstruction have been very successfully used In practice.One uses cellular or diaphragm wall construction (11). whilethe other exploits the structural lorm of piered walls known asfin .......alls(121.

Diaphragm wallsA diaphragm wall IS an extra-wide cavity wall - so wide thatnormal metal ties can no longer be used in a structural sense.The ties across the cavity are, therefore. short brick walls.These can be bonded into the inner and outer leaves. orbutted up to the external leaves and bed to them. There mustalways be a verncal shear resistance at the junction of thecrosswall and the flanges. If the crosswalls are not bonded tothe outer leaves. metal ties must be placed across thejunction, Within the bed joints. to give the junction the abilityto resist vertical shear.

Clearly visible in Figure 68 are the vertical movement jointswhich effectively place the walls into purely vertical bending.

Diaphragm wall construction has been widely used. andWith a vanety of elevanonal treatments. for many differenttypes of buildings including sports halls. SWimming pools(Figure 69). industrial buildings. garages. theatres. churches.etc.

Fin wallsFin wall constructk>n is structurallySimilar to diaphragmwallsrn as much as the heavily piered caVity walls have a large Ivalue in bending. Wide span buildings enclosing large openspaces can be safely built when the design is based on soundengineering principles. A great many fin wall structures havebeen budt for a wide vanety 01 purposes. over a considerablenumber of years. and all are performing well.

6 MAKING SENSE OF FACTS

4

Deaths of occupantsof dwellings dueto vehicle impact

1971 7Explosions Total

causing death deaths Road deaths per annum

1971 ·2 10 34 1960's aporox 6CXX)-8XXl1972·3 8 81973·4 8 9 As can be seen, there are approximately 10 explosions a1974·5 15 17 year. and each tends to claim at least one life, The tragic loss1975-& 12 13 of life at Clarkston Toll (22) makes the 1971 ·2 figures untypical.1976·7 10 10 Deaths 01 occupants of dwell ings resulting from vehicular

Frequency of damageReturning to the subject of accidental damage. It IS revealingto look at the frequency of explosions occurring in buildings.Consider the statrstics for the penod·lg71 .1977.

16

Certainly, it is true to say that the border line between successand failure is very thin. This can be appreciated by comparingthe collapse at Ronan Point with the permitted damage levelsallowed under 0 18 of the Building Regulations. It must, ofcourse, be pointed out that 0 18 refers to three storeys only ­the storey above and below the incident and, of course, thestorey in which the incident occurs,

Ronan Point involved failure of one corner of the blockthroughout the total height of the structure. Nevertheless, thenarrowness of the divide between success and failure can bedemonstrated by considering the volume of structure lost.According to 0 18, to succeed, the volume permitted to fail atRonan Point should have been restricted to 2%. Yet 6%, whichwas the volume of structure actually lost, was considered tobe a national disaster. However, it can be shown that in a 5storey building the permitted volume of lost structure may beas high as 9%.

Consideration of these points can help to put the topic ofaccidental damage into perspective. It must be dealt with, butthere is only a very small possibility of the measures taken tosafeguard the structure being put to the test in an accidentalincident.

3334

total 4027

overall total 67

Significant accident damag e(dwellingslp.a,)

Vehicle impacts

Explosions: pipedgascylinder gasother

If the statistics for explosions in buildings are broken downinto categories: (a> buildings of 4 storeys or less, and (b)buildings of 5 storeys or more, it can be seen that only 1·5incidents per annum (affecting 3 dwellings per annum) fallinto category (b). The probability of an accidental eventoccurring in a build ing or 5 or more storeys is, therefore. verysmall. Whilst the recuirements for accidental damage are setout in the Code, it is extremely unlikely that the requiredmeasures will ever be called upon to work.

impact are considerably less than one per annum. Compared Permitted damage levelsto road deaths, the effect of accidental damage is very smallindeed.

Furthermore, looking at the combined statistics forexplosions and vehicular impact. including those which do notresult in deaths, it can be seen that there are just over oneperweek.

Explosions in dwellings(per annum)

Incidents DwellingsaHected

4 storeys or less 38 575 storeysor more 1.5 3

- -total 39.5 00

CONCLUSIONS 7

That properly designed buildings can withstand gasexplosions was demonstrated in a seriesof tests(13

) carried outin the aftermath of Ronan Point. The tests were conducted ina building designed to simulate the top 31'1 storeys of amodern masonry structure.

The effect of venting, mentioned earlier, can clearly be seenin the illustration. When the air/gas mixture is ignited,pressure builds up very rapidly indeed. At a pressure of about7 to 14 kNlm2 (1·2 psi) the windows blow out. allowing muchof the unignited mixture to be expelled. This can be seen'exploding' harmlessly outside the building.

Whilst the walls were damaged in this series of tests, no. wall was blown out by the explosion and no partial collapseoccurred. When this evidence is viewed in the context ofcurrent legislation, the guidance available within the Code ofPractice, and the basic principles involved, the engineershould have every confidence in the inherent ability of welldesignedmasonrystructures to withstand unforeseen events.

17

8 REFERENCES

1. Repo rt of the Inqui ry Into the collapse of flats atRonan Point, Cann ing Town, London. Ministry ofHousing & Local Government. HMSO 1968.

2. Flats constructed with precast concrete panels:Appraisal and strengtheni ng of existing high blocks.Design of new blocks. Ministry of Housing & LocalGovernment Circular 62/68.

3. Notes for gu idance on the Interp retati on of AppendixI to Min istry of Hou sing & Local Govern ment Circular62/68. Institution of Structural En9ineers RP/68102. 1968.

Guidance on the design of domestic accommodation inloadbearing brickwork and blockwork to avo id collapsefollowing an Internal explosion. Institution of StructuralEngineers RP/68/03. 1968.

4. The Build ing Regulations 1976 No. 1676. HMSO.Reprinted 1978.

5. J . Morton. S. R. Davies & A. W. Hendry. The stabil ity ofloadbearing br ickwork structures following accidentaldamage to a major bearing wall or pier . Proceedings of2nd International Brick Masonry Conference, edited West &Speed. 1970.

6. J . O. A. Korff. Accidental loading - Part II, Application.Proceedings of symposium on the struclu ral use ofmasonry. The Institulion of Structural Engineers. 1974.

7. J . B. Menzies & G. D. Grainger. Report on the collapseof the sports hall at Rockferry Comprehens ive School,Blrkenhead. Building Research Establishment CurrentPaper 69/76. 1976.

8. Design Guide for strapping and tying of loadbearingbrickwork in low rise construction. British CeramicResearch Association SP93: 1977.

9. R. J . M. Sutherland. Principles for ensuring stability.Proceedings of symposium on stability of low rise buildingsof hybrid construction . Institution of Structural Engineers.1978.

10. J . O. A. Korff. The overall appraisal of brickworkbuildings. Paper read at the Conference for EngineeringTutors, Maidenhead 1978. Brick Development Association.

11. W. G. Curtin, G. Shaw, J . K. Beck & W. A. Bray. Designof brick diaphragm walls. Brick Development Association.Revised 1982 (Reprinted 1990).

12. W. G. Curtin, G. Shaw, J . K. Beck & W. A. Bray. Designof brick fi n walls in tall s ingle-storey buildings. BrickDevelopment Association . 1980.

13. N. F. Astbury et al. Gas explosions in loadbearingbrick structures. British Ceramic Research AssociationSP68: 1970.

Additional References (1996)

14. The Building Regulations 1991. Approved DocumentA · Structure A3 & A4 (1992 edition). HMSO.

15. BS 5628. Use of Masonry: Part I: 1992. Structuraluse of unre inforced masonry. SSI Standards.

16. BS 5628. Use of Masonry: Part 2: 1995. Structuraluse of re inforced and prestressed masonry. BSIStandards.

The Building Regulations have been amended since the issue of the 1976 edition referenced in lhis publicalion, although therequirements for accidental damage for five or more storey buildings have not altered significantly. The guidance given in BS 5628: Part1: 1992 has not altered from the provisions in the 1978 version of that Code in respect of design for accidental damage. BS 5628: Part2 was first published in 1985 and amended in 1995 and includes guidance for the selection of partial safety factors for material strengthfor the accidental damage design of reinforced and prestressed masonry structures.

ReadfHS 8re expressly 8dvisfld thst whilst1116 contents of this publication 8re believed to be BCCUrst9, correct and complete, no relisncfI should be placed upon its contents 8$ beingapplicsbltt to any particulafCirCumstances. Any advice, opinionor information containedis publishedonlyon the footing that the Brick Development Association. its servants or agents sndsl1contributors to this pUblicstion shsllbe under no liability whstsooll$f in respectof its contents.

Designed and produced to( the Brick Development Association, Woodside House, Winkfield, Windsor, Berkshire Sl 4 2DX. Tel. Winkfield Row (01344 885651)Printed in England by P.L Blake l td.. Perivale. Middlesex