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Design of Masonry Structures for Accidental Damage -Types of Accidental Loads

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Accidental damages in masonry structures occur due to unexpected loads. Design of masonry structures for accidental damages due to various types of accidental loads are discussed.

Design of Masonry Structures for Accidental Damage

Generally, masonry structures are designed for normal loadings, but it is possible that masonry building subjected to abnormal loading conditions and commonly masonry structure is not designed to accommodate such unusual loadings. In this article the design of masonry for accidental damage will be discussed.

Types of Accidental Loads on Masonry Structures

Accidental loading can be defined as loads which the building is usually not designed for and the main types are as follows:

Explosive Loads on Masonry Structures

Explosive loads on masonry buildings can occur inside and outside of a structure. There are various types of explosive loads, for example, bomb detonation, gas ignition, or transporting explosive chemical materials or gas. Pressure versus time curve for each of explosive sources is different. Despite the fact loads caused by explosive is of dynamic nature but it is considered as a static load and design checks are conducted depend on this assumption.

Impact Loads on Masonry Structures

Sources of this type of accidental load can be vehicles, such as collision of a car with wall or column of a multi storey structure, and construction equipments for instance accidental impact of crane load against a wall.

Other Loads

There are other loads which may be considered as accidental load. For example, settlement of foundation, making changes to structure considerations of safety measure etc.

Risk of Occurrence of Accidental Loads on Masonry Structures

The risk of occurrence of accidental loads is considerably important because the occurrence of specific risks. For instance, lightening might be acceptable while others might not be. The total cost of the structure is increased because of designing for accidental damage. Therefore, it is crucial to consider the increase of cost versus degree of danger for suggested design methods to become acceptable. Dangers that society can take can be compared numerically by considering the likelihood of death per person per annum for various accident type. It is clear that not only does this evaluation changes according to geographical location but also according to time as well. An example of accidental death statistics is provided in Table-1. Table-1: Accidental Death Statistics for United States
Cause Risk per person per annum
Motor vehicle 2.7 x 10-4
Falling 1.0 x 10-4
Fire 4.0 x 10-4
Drowning 2.8 x 10-4
Firearms 1.3 x 10-4
Poisoning 1.1 x 10-4
Earthquake 8.0 x 10-4
Lightening 5.5 x 10-4
It is demonstrated that the risk of accidental damage and fire are similar, and in the case of fire design measure against fire is established. Similar justification must be applied to take necessary measures against accidental loading. Estimation of accidental damage to masonry structures depends on the occurrence of abnormal loading. Table-2 Provides lower limit to number of abnormal loadings per annum. Table-2 Number of Low Limit of Abnormal Loadings in United States, 1966
Accidental loading type Number per annum
Explosive bombing 204
Gas explosions 131
Explosion of hazardous materials 177
Highway vehicle impact 190
Total/ annum 702

Likelihood of Progressive Collapse of Masonry Structures

It is important to consider the possibility of progressive collapse of masonry structures in the event of an accidental loadings. Generally, there are three kind of load bearing masonry constructions that need investigation for accidental damages which are: Type A: When there is an outside wall without returns or only one internal return. If the panel is removed, the remained section will suspend on the slab above. Type A construction is illustrated in Figure-1 & Figure-2.

Fig.1: Plan View, Outside Wall with Small Return

Fig.2: Plan View, Outside Wall Without Return

Type B: When there is an internal wall without return. The floor slab will support the wall above damaged wall, Figure-3.

Fig.3: Plan View, Internal Wall Without Return

Type C: Where the returning wall or a wall is subjected to large local bearing stress because of the removal of a section of a wall and the remaining wall is supported by return wall.

Fig.4: High Local Stress Imposed on Return Wall Due to Section Wall Removal

It is demonstrated by studying various high-rise and low-rise structures for possible occurrence of one of the types of load bearing masonry structure collapse because of removing a panel. Masonry structures that meet partial collapse requirements can be designed without facing considerable difficulty. Based on results of tests performed on a section of five storey brickwork cross wall structure in which main section of cross wall of the ground floor is removed to show the stability of the structure in a damaged situation, it was found that, masonry structure can easily be designed to offer alternate load transfer path when accidental damage is occurred. Additionally, it is pointed out that, the possibility of occurrence of progressive collapse is 0.045%. Finally, it can be said that, the danger of progressive collapse in structures of load bearing masonry wall is considerably small. Nonetheless, given the large social implication of progressive collapse occurrence and low cost of measures required to deal with this failure, providing additional design precautions is required to prevent such collapse. An example of progressive collapse is Ronan Point in which the progressive collapse at a corner of 23rd storey building occurred as result of accidental explosion of gas.

Methods of Masonry Structures Design for Accidental Damage

There are two design methods which can be used to deal with accidental damages include:

Design of Masonry Structures Against the Occurrence of Accidental Damage

This technique is clearly costly if it is applied in general case. But in specific situations, this might be useful to decrease the likelihood of local failure in masonry structures. For instance, the danger of explosion can be deduced through limiting gas utilization in a building. The decrease of likelihood does not guarantee the elimination the possibility of progressive collapse occurrence.

Permit the occurrence of accidental damage then design against progressive collapse

This method is used to avoid progressive collapse of masonry structures when local failure has happened. It is clear that all kinds of failure cannot be dealt with, so the level of permissible local failure, which is considered in the design, should be determined. Permissible local failure level of external wall may be larger than that of internal wall and might be related to number of storeys. With respect to the extent of allowable local failure, various rules are used by various countries. When it is decided that local failure is likely to occur, then the structure need to be analyzed to specify whether to experience progressive collapse or not and there are three methods which can be used for that purpose. · A three-dimensional analysis of the structure · Two-dimensional analysis of sections taken through the building · A storey by storey approach The first two methods are not suitable for design applications. However, the third method assumes the removal of a load bearing element in specific storey and residual stability evaluation is conducted from within that storey.

Design Recommendations for Masonry Structures Provided by BS 5628

Theoretical analysis methods provided in the above sections with experimental investigations led to design recommendations provided by BS 5628:

Use of Ties in Masonry Structures

BS 5628 recommends the use of ties as a way of restricting accidental damage of masonry structures. Specifications regarding accidental damage design provided by the code are divided to four storeys or lesser and five storeys or higher. There is no any specification for the former option whereas three options are provided for the latter option which can be found in BS 5628 table-12. Various types of ties provided in masonry structures are:

Vertical Ties

These are continuous wall or column ties except of lapping or anchoring from foundation to roof and they need to be anchored properly at each end and floor. It is advised that vertical ties should be independent in each storey and need to be staggered instead of continuous because the failure of the ties must be restricted to the storey where the incident occurred. As per BS 5628 the value of tie force is the greater of the following:



Where: A: Horizontal cross sectional area excluding non-load bearing leaf of cavity construction but including pier h: Clear height of column or wall between restraining surfaces t: Thickness of the wall or column It is recommended by the code that, the thickness of solid wall or one load bearing of a cavity wall is at least 150 mm and characteristic compressive strength of masonry is at least 5 KN/m2. Moreover, ties should be installed at maximum distance of 5 m centers along the wall and maximum of 2.5 m from an unrestrained end of any wall. Finally, the ratio of (h/t) is 25 for narrow masonry wall and 20 for other wall types.

Horizontal Ties

There are four different types of horizontal ties namely: For each type, different design rule is applied. The smaller of the following two values are horizontal tie force:


Where: Ns: is the number of storeys however the actual number employed is varied with the type of the tie Peripheral ties are installed within 1.2 m of the edge of floor or roof, or wall perimeter. The force of the tie can be computed using equation-2 and the ties need to be anchored at re-entrant corners or changes of constructions. Internal ties are arranged to span both ways and need to be anchored to the perimeter ties or continue as column or wall tie. The magnitude of tie force (Ft') is computed by using the following formula: Where: Ft: is computed as per equation-2 Gk + Qk: Sum of the average characteristic dead and imposed load in KN/m2 La: is the smaller of largest distance in the tie direction between column centers or other vertical load bearing members or five times clear storey height as defined in Figure-5.

Fig.5: Height of Storey

External Wall or Column Tie - In both external walls and columns, the tie force is smallest of (2Ft) or (h ? 2.5Ft), h is in meter and the tie force is in KN for columns whereas it is in KN/m for length of load bearing walls. Ties should be applied in both direction for external columns and the ties are likely to be provided partly or entirely by the same reinforcement as perimeter and internal ties. Masonry wall ties are required to be distributed uniformly or concentrated at centers with maximum spacing of 5m and maximum distance of 2.5 from the end of the wall. The wall ties might be provided partly or wholly by the same reinforcement as perimeter and internal ties. The tie force may be based on shear strength or friction rather than steel ties. Read More: Causes and Types of Cracks in Masonry Buildings and their Repair Methods
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