ASSESSMENT OF BLAST LOADING

ASSESSMENT OF BLAST LOADING (due to vibration from bomb explosion) ON BRIDGES

Aim of the Study

Ø  Analyze the structures under the effects of blast loading.

Ø  Including; the threat definition, blast wave parameters, dynamic properties of materials and damage assessment.

Definition of Blast Loading

Ø  Blast loading may result from the detonation of high Explosives, chemical ammunitions.

Ø  Type of extraordinary dynamic load and it has to be described by two parameters; peak overpressure and duration.

Ø   Blast loading is a function of: Distance of the structure from the explosion, and the charge weight or a weapon size.

Bomb explosion create a loading effect on the bridges, bridges are made up of element such as columns or piers, slab and beams, each of this element undergo stresses that need to be assessed to prevent a future failure of structures.

Columns can be under a pure compression, if all load are transferred axially along it centred, but loads affecting a column can be along x-x axis or y-y or even both, columns can be bi-axially loaded, uni-axially loaded and axially loaded, which ever form of loading, there are instance of the unexpected loading that occur during the structural life span, at preliminary design stage, bending reinforcement is provided and checks are done, such as deflection. shear and crack are checked for to ensure that the building material meet the design permissible limit. area of steel bars required are calculated with respect to the expected loading but live and dead load, but impact load (loading due to bombs) are usually not inclusive in preliminary design stage, so it is imperative that impact assessment is carried out on columns to prevent axial failure.

Basic checks in column include, the following the designer needs to check present degree of plumb deviation from the initial design specification before the explosion, and the displacement due to differential or tilt settlement must be checked if it is still within a permissible limit. If there is need for replacement of columns, racking or flying shore can be used to make alterations or replacement, for the foundation jack piles can be used to replace pile foundation if the piers used as foundation have failed axially.

Slabs are under the influence of bending stresses this stress can cause hogging or sagging, the excessive stresses caused by the bomb can cause any of the above defect mentioned above, assessing the bridge will require checks that include levels and degree of tilt from level, need to ask what is the required slope base on design prescription, if there is any case of sag, beams can be introduced at the point where bending is at the peak, the point of inflection should be checked also to ensure that bending is kept zero, the depth of slab can also be increase to with stand the instantaneous effect of the bomb blast, as far as there is no total failure of structure the tensile strength of steel is still within the permissible limit.

Beams transfer loads from slab to the columns, they are  constantly under the influence of shear and bending stresses, the beam must be checked for if there is any form of deflection and this is done by making sure that the depth of beams can still withstand the present stresses, check for cracks to ensure that the spacing of reinforcement can still prevent cracks due to the present blast loading(explosive),

Threat Definition

·         Charge Weight or weapon size (W): W is expressed in weight or mass of TNT; the equivalent W of any other explosive material is based on experimentally determined factors or the ratio of its heat of detonation to that of TNT.

·         Stand-off Distances (R): R measures how close to the building a bomb could explode and is therefore a function of the physical characteristics of the surrounding site.