Faculty of Safety Engineering - vsb.cz and... · 2019-03-07 · • Strengthening by concrete topping: - Roughing up the surface of original concrete in combination with concrete

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Faculty of Safety Engineering

Explosion Protection of Buildings

Author: Miroslav Mynarz

VŠB – Technical University of Ostrava

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Explosion Protection of Buildings

Methods of Repair and Strengthening of

Structures


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- Strengthening has to be always well-founded by static analysis,

drawing documentation and it must take into account overall

condition of the strengthened structure.

- It can be related either to the whole structure or to its part.

- Strengthening can be done by:

Structural solution of repairs



- enlargement of a cross-section;

- prestress;

- change of a load-bearing system.

• Strengthening in general:

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- For particular methods of strengthening, it is possible to use

different technical solutions; their design and execution has to

be in agreement with relevant standards and regulations.

- Enlargement of a cross-section can be achieved by:




- shotcrete (with or without steel reinforcement);

- concrete jacketing (with or without steel reinforcement);

- composite reinforcement glued to the surface or placed in the

channel.


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- Provision of interaction between new material and original

concrete is important assumption of functioning of enlargement

cross-section.

- Calculation should bear in mind involves that original part of

elements is under the influence of load in the state of stress

while new part of concrete is only hardening and it is subjected

to volume changes (shrinkage, hydration processes).





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- For concrete jacketing, it is advantageous to use self-compacting

concrete. With this material, compact concrete with quality

surface can be achieved without compact devices even in zones

with rich reinforcement.

- With the help of prestress, favourable state of stress is evoked in

concrete element – prestressing unit is acting as an active

additional reinforcement.





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- Despite of static advantages of bonded prestressing, for

strengthening the external unbonded tendons are usually

preferred for structural reasons. Change of load-bearing system

is usually executed by modification of element's support

conditions (external change of load-bearing system).





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• Strengthening of reinforced-concrete slab can be executed by

several ways; choice is affected by following facts:

Strengthening of slabs



- economic indicators;

- possibilities and experience of a provider;

- enginery;

- time factor;

- space potentials etc.

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• Reinforced-concrete slabs are strengthened by:



- concreting;

- addition of reinforcement;

- reduction of a span;

- combination of listed methods.


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• For concrete topping, strength class of concrete is designed at

least the same as original concrete slab, but better one level

higher. For many reasons, thickness of topping should be at

least 30 mm, better 50 mm. Compressed zone in a slab usually

does not exceed this value.



• Strengthening by concrete topping:


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• Strengthening of a slab by concrete topping can be executed by:




- acting of new and original concrete

(interaction of a slab);

- without interaction (relieving slab).


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• When interaction between new and original concrete is ensured,

thickness of a slab is considered for calculation as a sum of

original and new slab;

• Tension force in reinforcement passes to compressed concrete

through horizontal shear forces. Joint in connection between

new and old concrete is a critical zone of strengthening of a slab.





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• Interaction of old and new concrete can be improved by:




- Roughing up the surface of original concrete in combination

with concrete bonding adhesive. Mechanic ways of roughing

up are preferred to chemical ones. Roughing up should not

be too deep otherwise the structure of original concrete can

be damaged by microcracks or large local stresses can occur.


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• Interaction of old and new concrete can be improved by:




- Placing steel mandrels or bolts to pre-drilled holes in original

concrete. Mandrels or bolts are embedded in the holes and

grouted with epoxy. They are positioned throughout the slab

area in distances necessary to carry shear forces. Clamping

of bolts improves the efficiency of the connection.


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new

concrete mandrel bolt

new

concrete

original

concrete original concrete

epoxy

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• Topping layer is usually reinforced by wire fabrics. At support,

reinforcement is complemented by splices to carry the support

moment. In the case of reinforcement of concrete topping,

minimum thickness of the layer is 50 mm.

• Concrete topping of isolated acting slab (relieving slab) belongs

to common method of slabs strengthening. New concrete is not

connected with old one, it means that each slab reacts isolated,

but their deflection is the same. With simplification, ratio of load

transmission by slabs is directly proportional to ratio of their

flexural rigidities.





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• Actually, ratio of resistance of both slabs is much more

complicated.

• Following items are considered: effect of shrinkage, change of

elastic modulus of new concrete, creep of both concretes and

other factors needed to be considered in that specific case.





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• Although the thickness of a slab and consumption of

reinforcement is higher at relieving slab, this type of

strengthening is used mainly for the following reasons:




- problem of connection between old and new concrete does not

occur. Cleaning of concrete surface can quite often be

problematic, for example at strong oil pollution;

- there is no need to remove cement screed and to settle the

mandrels or bolts;

- acceleration of works.


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• At simple slabs, reinforcement is added to the bottom surface

(according to the course of bending moments, it is not necessary

to lead the reinforcement to supports).

• At continuous slabs, reinforcement is placed also to the upper

surface above the supports. Added reinforcement has to interact

with concrete therefore it must have necessary bond with

concrete.



• Strengthening by additional reinforcement:


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• Bond is achieved by:




- location of reinforcing bars to a chase made by milling cutter

and filling of the chase with material ensuring bond between

concrete and reinforcement. Due to minimizing of interference

with the structure, added reinforcement is placed tightly below

the concrete surface. To avoid its corrosion, stainless bars or

fiber-reinforced polymer (FRP) bars are used. High strength of

materials and finish of reinforcement enables to use bars with

small diameter or short anchorage length.


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• Bond is achieved by:




- gluing of carbon or steel reinforcement (straps) to concrete by

two-component glue;

- embedded of steel bar in a chase made by milling cutter.


chase

reinforcing bar

polymer cement mortar

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• For strengthening of structures exposed to atmospheric

condition enabling the corrosion, different reinforcing material

should be used. To that effect, straps with fiber reinforced

polymer (FRP) are used.

• In recent years, straps with Carbon Fiber Reinforced Polymer

(CRP) became widely used. Their elastic modulus is high and

their behaviour is linear elastic until failure. Long delivery

lengths – up to 250 meters, delivered as the coils of wire – and

small thickness enable minimizing of joints number, or more

precisely smooth crossing of straps.





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• Stress-strain diagrams of carbon straps are in a shape of

straight line.

• Two-component epoxy glue is used for straps sticking.

Properties of the glue are getting worse at temperatures between

+50 and +70 °C. Structures threatened by fire demand fire

protection of bonded reinforcement.

• Main disadvantages are manipulation with rather heavy, less

flexible steel straps and corrosion hazard. For carbon straps,

disadvantage is their price and transfer of forces in one

direction. Other criterions are in favour of carbon straps.





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• With the help of a span reduction, decrease of internal forces in

a load-bearing structure can be achieved. At slabs, reduction is

reached by embedding of reinforced-concrete or steel beam in

the middle of a span of a slab.



• Strengthening by reduction of a span:


concreting of embedded beam

embedded

composite

I beam

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• Concreting of embedded reinforced-concrete beam is executed

through holes in the slab to placed shuttering with

reinforcement. Steel mandrels are put to the holes for better

interaction between slab and new beam. Mandrels are also used

for placing of a slab to steel beams (tough rolled steel sections,

usually leg angle).

• A cross-section is not able to transfer the negative bending

moment and crack appears. Embedded beam forms pin support.

Even so, midspan and support moments decrease.





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• Beams (girders) are horizontal 1D structural members taking

load from less stiff horizontal elements (e. g. slabs) supported by

those beams. They are subjected to flexure and shear,

eventually torsion and normal force.

• Strengthening of beams can be achieved by:

Strengthening of beams



- enlargement of a cross-section by adding concrete layer

with longitudinal and transverse reinforcement;

- embedding subsidiary supports;

- adding tough rolled steel sections;

- adding bonded reinforcement;

- prestressing using external prestressed reinforcement.

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• The most common ways of enlargement of a beam (or girder) cross-

section by adding of concrete layer with reinforcement are:



• Strengthening by enlargement of a cross-section:

a) strengthening at midspan moment;

b) strengthening at midspan moment and shear;

c) strengthening at support moment;

d) strengthening at support moment and shear.


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• Strengthening by enlargement of a cross-section :


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• Beam resistance is increased by reduction of a span, namely by

additional embedding of one or more supports.

• From static point of view, embedding of supports causes the

change of static system of the structure. Support moments and

transverse forces developed above embedded supports are not

caught by reinforcement, only insufficient structural

reinforcement is placed there. Cracks that arise cause pin

supports.

• New supports for beams and girders can be rigid (columns,

walls) or flexible (transverse beams, ties).





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Embedded support

Embedded

supports

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• Tough rolled steel sections interacting with reinforced-concrete

beam can markedly increase its resistance by their own flexural

rigidity. This can be functioning only when interaction from the

moment of load is reliable – it is provided by bolts.

• For better bonding, concrete surface is roughened: cement

screed layer is applied to the interface and rolled steel section is

tighten to reinforced-concrete beam with the help of bolts.

Proper supports are created for added rolled sections; these

supports can be the components of columns strengthening.



• Strengthening by adding tough reinforcement:


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• Strengthening by adding tough reinforcement:


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• Beams strengthening by bonded strap reinforcement is used for

increase of their moment and shear resistance. At beams, gluing

to the side faces of the beam is also possible.

• When beams are strengthened at bending moment using bonded

straps, their shear resistance is often needed to be increase.

• Besides straps, carbon fabric can also be used for this purpose.

It is laminated by resin directly to prepared concrete surface of

the beam. Carbon fabric is anchored to the compressed part of

the beam.



• Strengthening by adding of bonded reinforcement:


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• Additional prestress by external prestressing tendons is effective

instrument for strengthening of concrete and masonry

structures. External prestressing tendons consist of strands,

cables or bars installed right to the concrete cross-section.

• Detailing of reinforcement depends on particular conditions of

strengthened structure: static system, arrangement of

longitudinal section, shape of a cross section, etc.

• Reinforcement can be straight or curved.



• Strengthening by external prestress:


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• Advantage of straight reinforcement is that it requires only minimum

interference to original load-bearing structure. Curved tendons are

organized in the shape of polygon. Compared to straight tendons,

curved reinforcement is more efficient because its trajectory can

conform to a course of internal forces in a load-bearing structure.

However, it is technically more demanding due to the so-called

deviators that have to be created in each turning point of trajectory.

• For design of reinforcement it is assumed that external forces of

prestress are transferred into the structure. It means that the whole

structure is loaded by external forces in numerical model.





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• For strengthening of structure, following tendons are used as

prestressing reinforcement:




- unbonded strands, trademarked as Monostrand. It

concerns special-treated seven-wired tendons with low

relaxation and diameter of 12.5 or 15.5 mm. Corrosion

protection of reinforcement is ensured by polyethylene

envelope filled with grout which minimizes losses due to

friction as well. Strands treated like this can be even used

for forming larger prestressing units;


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- external cables represent bigger units. They are usually

prepared from prestressing strands with diameter of 15.5 mm.

Strands are usually embedded to steel ducts and typically

injected by grout, or polyethylene ducts are used and their

corrosion protection is ensured by special grouting vaseline;

- bars made of high-quality steel 13 180.9 with yield strength fy

= 835 MPa; bars can be fully threaded, or plain with special

cold-rolled threads at the ends. Basic bars are provided up to 6

m or 12 m. Couplers can be used for continuation of bars to

any length.


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• Big advantage of external prestress is that its parts can be

controlled, repaired and changed easily.





Detail of

deviator

A - A cross-section

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• Requirements for strengthening of horizontal load-bearing

elements is very often connected to the need of strengthening of

columns. These are subjected to combination of normal force

and bending moment. In the case of slender columns, effect of

buckling should be also considered.

• The most frequent ways of columns strengthening are:

Strengthening of columns



- adding of longitudinal and transverse reinforcement and

concrete;

- adding of tough rolled steel sections;

- confinement by carbon fabric;

- embedding of reinforcing bars in channel.

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• Necessary amount of added reinforcement and concrete results

from static analysis. For design and assessment of column,

shrinkage and creep of old and new concrete should be

considered.

• For circular and polygonal cross-sections, minimum of 6 pieces

of longitudinal reinforcement is added and transverse

reinforcement is formed into spiral shape. Columns of square

and rectangular cross-sections are strengthened by longitudinal

reinforcement and stirrups.

• Vertical reinforcement must be anchored into floor, eventually

foundational structure using pre-drilled holes and epoxy.




• Strengthening by adding reinforcing steel and concrete:

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• New concrete layer of

thickness between 40 and

60 mm is executed using

shotcrete; for higher

thicknesses, fresh concrete

is placed in shuttering.




• Strengthening by adding reinforcing steel and concrete:

A - A cross-section

B - B cross-section

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• Columns strengthening with the help of tough steel sections is

usually provided by adding rolled angles and transversal straps.

Angles in the corners of columns are embedded to cement

screed. After its hardening, heated straps of reinforcement are

welded to angles.

• Steel bandage constrains deformation of concrete in transversal

direction by which means it increases concrete compressive

strength. Effectiveness of bandage decreases with increasing

eccentricity of normal force.




• Strengthening by adding tough rolled steel sections :

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• From aesthetic point of view and for corrosion protection and

fire prevention, after loading of the structure it is recommended

to wrap strengthened column by wire fabric and to spray it by

concrete layer of thickness between 30 to 50 mm.




• Strengthening by adding tough rolled steel sections :

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• Interacting of added concrete, reinforcing steel or rolled steel

with load transfer depends on their efficient activating that is

reached by:




• Strengthening by adding tough rolled steel sections:

- maximum unloading of the part of the structure that loads

the column during strengthening;

- elimination of variable action, eventually even of a part of

permanent action;

- transient support of the structure by subsidiary supports;

- keeping tough reinforcement or steel sleeve at horizontal

load-bearing elements in soffits with the help of the presses

or steel gussets.

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• Modern concepts of columns strengthening are based on using

carbon fabric and reinforcing bars embedded in channel made

by milling cutter.

• Essence of increasing columns resistance by fabric confinement

rests in preventing the concrete deformation in transversal

direction. Then multi-axial stress of concrete occurs which

increases its strength.

• For increasing the bending resistance of columns, reinforcing

bars (steel or FRP) embedded in channel can be used in the

direction of column axis. For their anchorage, holes are drilled

into foundations or beams.




• Strengthening by confinement and bars in channel:

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• Line representing original rectangular cross-section of column

subjected to combination of normal force and bending moment:





- between A and B, failure is caused by crushing of concrete;

- between B and C, failure is caused by exceeding yield

strength of steel;

- after reinforcing bars are embedded, a resistance line

becomes markedly wider between B and C, proportionaly to

the increased reinforcement ratio for longitudinal

reinforcement;

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- on the other hand, using confinement fabrics causes

increasing of resistance between A and B;

- combination of reinforcing bars and confinement fabrics

leads to increasing of resistance in both zones. Moreover,

fabric contributes to a better stability of placed bars;

- for strengthening by confinement, fabrics reinforced by

carbon, glass or aramid fibres are used most often;

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Bars in channel

Bars in channel

Tensile failure

zone

Compression failure zone

Compression and tension failure

Confined fabric

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• Similar to strap, fibres in fabric are also unidirectional and

straight. Therefore, they are suitable for transfer of tension force

in direction of fibres already at small deformations. Carbon

fibres are used most frequently for this purpose.

• In matting, fibres are lead in two directions (usually

orthogonally) and they are waved depending on the kind of

weaving. Here, glass fibres proved to be good. Transfer of

tension force in matting is activated at bigger deformations.

• Mattings are suitable for increase of ductility of load-bearing

elements, e. g. for increasing their seismic or explosion

resistance.





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• Spraying of fibre reinforced polymer is ranked among rather new

methods of strengthening concrete structures.

• Epoxy or other (e. g. vinyl ester) resin reinforced by short glass

or carbon fibres is sprayed to prepared concrete surface of

strengthened structure. After spraying, applied layer is

smoothed with roller.

• This way is possible for continuous strengthening of all elements

of load-bearing structure (slab, beam, wall and column).

• Increased ductility results from the fact that failure of applied

material occurs by combination of tearing and pulling of fibres.

Non-linear course and plastic deformation enable detection of

coming failure and they increase the structure resistance.

Continuous strengthening of the structure



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• After the collapse of the structure, inspections are recommended

to assess the degree of the buildings damage.

• The volume of the accident documentation should be

commensurate with the importance of the accident from the

point of view of its extent, casualties, meaning, disaster recovery

assumptions, decision about repair works (yes, no) and

maintaining database record used in the event of any future

accidents.

Assessment of blast effects



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• At more significant structures or after strong damages, detailed

inspection can be carried out to specify the degree of damage

and to determine other necessary requirements for following

works and their terms (e. g. for samples withdrawals, mapping

of cracks etc.).




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• For decision-making about remaining resistance of damaged

structures, whether they should be repaired or demolished,

static or dynamic load tests at damaged structure together with

complete structure analysis are recommended.

• At higher degree of damage or damage to personnel or material,

it is necessary to take samples from the damaged structure for

their further analysis or to do on-side inspections using

approximate non-destructive methods, load test etc.




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• Specimen collecting of load-bearing parts of the structure

follows the standards requirements for materials testing.

• Size and amount of collected samples should be consulted with

the testing room carrying out the analysis.




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• For accident damage of degree 3 (see the table) or higher,

character and damage conditions of the structure material are

determined (masonry, concrete, timber, steel or frame

structures, structures made of plastics or other types of

materials, ground structures, ...).

• For damaged masonry or concrete debris, it is necessary to

determine whether it was crushed or it only fell apart to single

elements, like the whole or half bricks, blocks, wall block etc.).

Failures caused by explosion



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• For undamaged masonry closely situated to the damaged

structure, it is necessary to assess weathering of masonry,

moisture, release of mortar in joints, probable quality of mortars

etc.

• Disturbances in material strength of undamaged structure

closely to the damage epicentre should be assessed or samples

for further analyses should be taken.

• For timber and frame buildings, wood structure is determined.




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• For steel and cast iron structures, corrosion rate of the

structure and its curing against corrosion is investigated.

• For ground structures, compaction rate, consistence, humidity

etc. should be found.

• Damage should be photographically documented; location of

crack formation, their length, opening, displacements of

structural elements etc. should be described in detail.




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• The following damages are the main indicators for determining

the extent of an explosion:




- tearing out of whole structural elements;

- bursting of glass window panels, doors, shop windows;

- opening of pressure safety valve;

- collapse of partition or brick walls, infills of frameworks

etc.;

- displacements of structural elements along possible joints;

- displacements of masonry blocks in joints;

- shear of chimneys, columns, girders, tie beams of

frameworks etc.;

- shearing off the chimneys;

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permanent displacements along expansion joints and

cracks formatted before accident;

permanent buckling of walls, slabs, shells;

exposed reinforcing of reinforced concrete elements;

fractures of floor panels, beams, columns, slabs;

torn off anchor screws of machines, hinges of pipes etc.,

torn off machines parts;

overturn or extrusion of railway vehicles and free-wheeled

vehicles from roadway;

craters or cracks in the ground or geological environment,

in mine pits at their hard and unconsolidated surfaces,

roadways, yards, galleries etc.

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• Documentation about collapsed structures should involve

objective information about accident and its consequences that

can be used for detailed analysis of the accident or any possible

damage to the structure in the future.

• Information about the structure before the accident should be

sufficient enough for considering load history and development

of structure failure before accident.

• Documentation about accident should be brief; its range should

enable reliable analysis of situation both before and after the

accident.

Damage classification



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• To assess the extent of the damage to structure, it has to be

classified.

• There are five damage degrees as presented in the following

table:

Damage classification



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Damage

grade

Specification

of damage Description of damage

0 No damage No visible damages occur; waterproof of tanks or gasproof of

rooms remain unaffected.

1 Slight damage

Part of glass window panels and doors are broken. Cracks of

width of 1 mm occur in interfaces of structural elements

(between load-bearing structure and partition walls, in ceiling

cavettos, closely to corners of walls).

2 Moderate

damage

All glass panels are broken or cracked. Cracks of width under 5

mm in plaster of walls and floors are usually continuous.

Unopened cracks occur in corners of walls as a result of

settlement. Unopened cracks occur also in sill masonry; roofing

and sheath metal panelling is released.

3 Heavy damage

Opened cracks wider than 5 mm occur in partition and load-

bearing walls, they do not impair stability of the structure.

Chimneys in buildings and parts of roofing fall. Torn off supply

line and parts of pipes. Release of machines in industry

operation. Opened cracks are in roadways and ground

structures.

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Damage

grade

Specification

of damage Description of damage

4 Very heavy

damage

Cracks in load-bearing walls, lintels or damages in framework

load-bearing structure which endanger their static function.

Collapse of parts of partition walls, infills and chimneys.

Serious cracks in plain concrete elements. Damage of structural

stability. Torn and deformed pipes and service lines. Torn off

machines anchorage. Derailed overhead cranes. Overturned or

seriously damaged external cranes. Loss of tightness of silos

etc.

5 Destruction

Collapse of masonry buildings or their parts with load-bearing

elements. Roof frames or ceilings cave in . Continuous cracks

on dangerous cross-sections of reinforced concrete structures.

Serious deformations of power poles. Destruction of parts of

external pipes in chemical industry. Rupture of silos and their

deformation etc.

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Thank you for your attention.



Documents

Faculty of Safety Engineering - vsb.cz and... · 2019-03-07 · • Strengthening by concrete topping: - Roughing up the surface of original concrete in combination with concrete