Masonry Abaqus

8/10/2019 Masonry Abaqus

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Numerical Analysis of the Dynamic Response of Masonry

Structures Subjected to Impact and Explosive Loading

Wahid K. Arif,1 School of Civil Engineering, Faculty of Engineering

ABSTRACT

This paper presents a numerical analysis of the dynamic response of masonry

structures subjected to impact and explosive loading. Based on an investigative

project, two multi-storey brick masonry models have been modelled using

Abaqus/CAE 6.9.1 developed by Simulia. These models were subjected to various

types of impact and blast loadings. By using the analysis one may gain a clearer

understanding of the types of failure of masonry structures and of the stresses

transferred throughout the structures.

Keywords: Masonry; numerical analysis; Abaqus; dynamic response; impact/blast

loading, FEM

1email address for correspondence:[email protected]


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Numerical Analysis of the Dynamic Response of MasonryStructures Subjected to Impact and Explosive Loading

INTRODUCTIONStructural engineers are responsible for constructing and designing public buildingsproviding life safety in the face of explosions and heavy impact loading. Blastloadings are known to inflict the most damage in the vicinity of an explosion.Prevention of progressive collapse is therefore an important characteristic that mustbe included in terms of design and construction; it is the key to success for structuralengineers as considerable weaknesses in protecting structures against damage anddestruction are caused by impact and blast loadings. (Elliot et al., 1992). In thiscontext, computer program simulations could be extremely valuable in testing a widerange of building types and structural details over a broad range of hypotheticalevents. (National Academy Press, 1995).

When considering structures subjected to impact loading, the main issue which has

recently become of renewed interest is the design and analysis of masonrystructures. Available codes of practice contain recommendations for the control offlexure in terms of low strain rate loading, notably wind loading for masonry. Howeveronly basic suggestions are provided in the case of high strain rate loadings notablyimpact and blast loadings.

Failure in masonry structures due to terrorism highlights the fact that an effectivesystem to improve the physical and mechanical properties of masonry units isneeded. It is known that the degree of damage depends on the capacity of thedetonation, the location of the structure and its conditions. However, research hasshown that injuries from external explosions are not necessarily caused byfragments, heat or pressure of the detonation itself, but are caused from

disintegration and fragmentation of walls, panels as well as the collapsing of failedstructures. Therefore different methods of strengthening structures such aselastomeric coating and fibre glass composite units should be introduced in order tocontrol the scattering of debris at high velocities.

The type of impact and blast the building has to restrain when subjected to variousloadings is likely to affect the outcome or success of structural rigidity. The outcomeor success of structural rigidity will be dictated by the type of impact or blast thestructure will be subjected to, and the type of masonry used.

This paper presents two alternative criteria to quantify the state of unreinforcedmasonry structures subjected to impact and explosive loading. An eight story

structure with openings and a two storey house with internal wall partitions shall besubjected to different loading conditions. The reaction in terms of the impact/blast willenable one to thoroughly analyse the different types of dynamic loading and howextreme and severe they can be.

A detailed conclusion will reveal how accurately the analysis can simulate real lifesituations, and will put forward recommendations to solve the many problemsoccurring in this field of engineering.


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Figure 1: Gas Explosion Figure 2: Impact Loading

Figure 1 presents a simulation of a gas explosion in a room. In this case a rampingpressure is applied to the inner walls, ceiling and floor of the room. Figure 2 presentsthe impact loading of the structure. This type of impact will simulate the response ofthe building in terms of an object in motion colliding into the structure. A real lifeexample could be the 9/11 attack on the World Trade Centre in New York. All elasticmodels were constructed using Abaqus/CAE. An incremental step time betweeneach frame of the initial model had an increasing step time of 2.5E-04 seconds.Isometric views as well as different section cuts and plans are illustrated (below) tohelp examine the stresses of the structure when subjected to various loadings.


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EXPERIMENTAL PROGRAM AND TEST RESULTS FOR MODEL 1Maximum Principle Stress Results for the Gas Explosion


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Maximum Principle Stress Results for the Impact loading


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Key Points of Analysis for the Maximum Principle Stress of the Gas Explosion

The explosion of the room encompasses around the openings Stress waves from the room propagate in a circular fashion making their way to

the corners The walls of the structure give a bouncing back or recoil effect leading to a high

concentration of stress The largest stress value has been simulated to be +1.588E+10Pa Rebounding of the structure occurs causing high localised displacement.

Key Points of Analysis for the Maximum Principle Stress of the Impact loading

Stress contours spread outwards from the point of impact The greatest stresses contain a value of 1.398E+10Pa Creation of both shear and large stresses across the structure due to impact

which affects the shape of the structure resulting in deformation of the front faceleading to thrusting of the back of the building

It is noticeable that the stress values at the point of impact have rebounded afterthe fourth frame Over-damaged top part of the structure

MODEL 2, SIMULATION OF A TWO STOREY HOUSE SUBJECTED TOVARIOUS TYPES OF LOADINGSThe second structure modelled was an innovative two storey brick masonry house.This model created was inspired by Wu and Hao, and Zapata and Weggel. Thestructure was modelled as domestic structures are rarely taken into account whensubjected to blast or impact loading.

The width of the multi-storey structure is 38.0m. The height of both stories is 3.0m. The total height of the total structure is 6.0 m. The depth of the structure is25m. Wall thickness has been assumed to be 0.4 m with a floor thickness of 0.2m

The structure is assumed to be free standing, fixed at the bottom The density of the brick unit has been assumed as 1800 kg/m3 Youngs modulus E = 2.68 x 1010 Poissons ratio v = 0.2 A blast loading of 9E+007 kPa, pressurised within 0.005 seconds

Figure 3: Blast pressure target located at South face of the structure


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Figure 3 presents a situation in which a blast pressure hits the South face of thestructure. This type of impact loading on the structure was emphasised in the paperby Wu and Hao,the distance point in which the detonation of the explosion is locatedhas a rather large outcome in the shaking and drifting of the structure as theexplosion hits the house causing fracture, eventually leading to failure.

All elastic models were constructed using Abaqus/CAE. An incremental step timebetween each frame of the initial model had an increasing step time of 2.5E-04seconds. Isometric views as well as different section cuts and plans are illustrated(below) to help examine the stresses of the structure when subjected to variousloadings.


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EXPERIMENTAL PROGRAM AND TEST RESULTS FOR MODEL 2Maximum Principle Stress Results for Blast Loading


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TOP FLOOR SECTION P

Key Points of Analysis f The blast wave reache The highest stress valu The ground floor is rea Stress values show a

The increase is due to

Observations of the AnThe main observations of If an impact/blast loadi

bottom, at any given pimpact area reaches aFurthermore, stress, stthroughout the structurdeformation, cracking

Excessive stress, straiproperties of the struct

cause internal and ext

LAN B-B

or the Maximum Principle Stress of thed the south-facee was +3.348E+10Pa

cting more to the explosion than the top flooebound effectdue to high concentration of

the rebound effect

lysis of Modelsthe analysis of models can be summarisedg is applied on a brick masonry structure, fiint except the areas near the boundary con

plastic state forming a plastic hinge before frain and energy waves start to appear and die eventually reaching the corners and fixednd eventually collapsing of the structure.and energy waves can lead to failure, depe

re as well as the magnitude of loading. Suc

rnal cracking of the structure deteriorating o

last Loading

stresses

s below:ed at theition, theilure.

ispersends leading to

nding on thewaves may

er time


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leading to failure and permanent deformation of the structure. Cracking of brickmasonry generally occurs due to poor tensile resistance as previously discussed.

Wall openings increase the vulnerability of the structures against impact/blastloads, especially if the loads are closely applied to the openings. Having aminimum number of openings notably in the ground floor is recommended. It isvery important that appropriate sizes of openings are set. The magnitude of thereaction and the resistance of masonry structures against impact loading dependon the size and distance between the opening and the point of impact/blast.Moreover, the impact time duration also has major effects on the reaction of thestructure against loads.

The resistance of the wall against impact/blast loads are greater and moreeffective near the restrained/fixed edges of the wall. It is also noticeable that wallpartitions in a structure help in terms of resistance to impact/blast loading. Withoutwall partitions providing resistance to the structures, open spaces are prone tofailure as they offer less resistance.

CONCLUSION

A simple overview of the stresses has been critically discussed regarding thestandard factors affecting these structures under dynamic loading in which the stressis produced by tensile stress. In many load cases for both models the dynamicloading applied to the wall in the ZZ direction acts perpendicular towards each model.When actually hitting the structure the blast pressure affecting the models leads to anincrease of kinetic energy of the system producing larger velocity in each element.The higher velocity leads to movement of the elements resulting in largedisplacement of the structures.

Both structures were exposed to flexure and extreme lateral forces. The effects offorces on buildings are related to both the weight and height of the building. Thehigher the buildings weight is above its support base, the further the top of the

building will move under the same force.

With the models being fixed at the bottom, the stress properties from Model 2 werethe most vulnerable areas in which the transfer of waves and energy were at thecorners of the boundary condition, reaching a stress value of +3.348E+10 Pa. Thevalues listed in the contour legend may be rather surprising as the maximum stresshas been specified to be +3.348E+10 Pa, which should not be possible since thematerial was assumed to be perfectly plastic at this stress magnitude. Thismisleading result occurs because of the algorithm that Abaqus/Viewer uses to createcontour plots for element variables, such as maximum principle stresses. Abaqus/Viewer uses linear extrapolation to calculate the nodal values of element variables. Inorder to show a contour plot of the stress, Abaqus/Viewer extrapolates the stress

components from the integration points to the nodal locations within each elementand calculates the stress. If the differences in stress values fall within the specifiedaveraging threshold, nodal averaged stresses are calculated from each surroundingelement's invariant stress value. Invariant values exceeding the elastic limit can beproduced by the extrapolation process. (SIMULIA, 2008).

Abaqus CAE is quite capable of analysing the data represented and modelled inmany different shapes; however it is mostly used in determining the deformation andstress/strain levels of the model with pre-defined elastic properties. A computationalmodel analysis of a brick masonry structure will give an approximate reaction whensubjected to an impact/blast loading in relation to time, but it will not be an exact

replica of the actual response when subjected to the dynamic loading. The behaviourof masonry differs from a standard material model. Being a continuous and ahomogenous material consisting of small elements connected with mortar, its tensile


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strength is much lower than its compressive strength. Unfortunately it is a materialthat is not frequently used and is often ignored today, but if one were to compare theusefulness of the material back in the nineteenth century, the use of masonry wasconsidered the best; the invention of steel and concrete fabrication has changedthis situation however. The fact that there are no specific standards specifying codesfor the destruction of masonry, or the response of masonry projectiles due toblast/impact loading is a critical area that could be improved upon. A small amount oftensile resistance should be provided to avoid instability problems; perhaps theremoulding of masonry bricks may provide a more sturdy structure which could bemore blast efficient. Masonry behaves in an elastic brittle fashion with very lowcapacity to tolerate strain during heavy impacts. Therefore, the idea of reinforcing thestandard material can be an option to be taken into consideration.


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References

Elliot, C.L., Mays, G.C., Smith, P.D., (1992). "The protection of buildings against

terrorism and disorder".Proc Inst Civ Eng Structural - Structures and Buildings, Vol.

94, No. 3.

National Academy Press. (1995). Feasibility of applying blast mitigating technologies

and design methodologies from military facilities to civilian buildings. Washington:

National Academy Press.

SIMULIA. (2008). Getting Started . Abaqus tutorial - CAE manual.

X.Quan, N. (2003).Numerical simulation of structural deformation under shock and

impact loads using a coupled-multi solver approach. Hunan, China.

Wu C. Q. and Hao H. (2007). Safe Scaled Distance for Masonry Infilled RC FrameStructures Subjected to Airblast Loads. Journal of the Performance of ConstructedFacilities, Vol. 21, No. 6.

Zapata, B.J. and Weggel D.C. (2008). Collapse Study of an Unreinforced MasonryBearing Wall Building Subjected to Internal Blast Loading. J. Perf. Constr. Fac., Vol.22, No. 2.

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