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discussionand

updatesonstructuralmaterials

BUILDING

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Resistance of Historic UnreinforcedMasonry Walls to Air-Blast LoadsBy Jessica Godinho, M.S., Elizabeth Agnew, M.S., and Shalva Marjanishvilli, Ph.D, S.E.

Blast resistant analysis and de-sign of historic structures can bechallenging. Original designs of

these structures utilize historic construc-tion methods and materials, and were

often not intended for abnormal loadingscenarios such as air-blasts.When an external explosion imposesan air-blast load on a heavy, external

wall system, the air-blast pressure mustovercome the inertia forces of the wallitself before putting the system intomotion and causing damage. In the caseof thick, unreinforced masonry walls,these forces are very large and thereforethe resulting damage is less than one

would initially imagine.

BackgroundPrior to the 20thcentury, unreinforced

masonry construction was the primarybuilding material for both residentialand commercial construction. As a re-sult, unreinforced masonry structuresconstitute a large portion of existingbuildings, many of which hold historicaland architectural importance.Beginning in the early 1980s with

terrorist attacks in Lebanon and Kuwait,high-profile and high-risk civilian federalbuildings have been designed to resistthe effects of explosive attacks. Witheach major attack on the United Statesinterests, more attention has been paidto this low probability/high consequencethreat scenario. As a result, recent workhas been dedicated to the design andanalysis of masonry structures to resistair-blast loads.

Air-Blast EffectsBefore explicitly addressing air-blast

effects on a particular building system, it

is important to understand the effects ofexplosions themselves.An explosion is a rapid release of energyin the form of light, heat, sound, and ashock wave. The shock wave consists ofhighly compressed air traveling radiallyoutward from the source at supersonicvelocities. Pressures reduce rapidly withdistance and can be amplify by a factor ofup to thirteen when reflected off a build-ing surface. These pressures decay expo-nentially with time and their durationis typically measured in milliseconds.

Diffraction effects at reentrant cornersof the building may confine the air-blastand prolong its duration. Late in theexplosive event, the shock wave becomesnegative, creating a high-intensity drag

pressure. This wind picks up and carriesflying debris from the building.

Air-Blast Effects on HeavyUnreinforced Masonry

BuildingsWhen a wall is first subjected to an air-blast load it experiences out-of-planeflexure, producing tensile strains on theinterior face of the wall and compres-sive strains on the exterior. Once themaximum positive deflection associated

with the out-of-plane flexure has beenattained, the wall will begin to vibratedue to rebound forces created by nega-tive blast pressures. The wall undergoesnegative deflection and curvature, caus-ing tensile strains to develop on theexterior face of the wall and increasingshear stresses at wall supports.

WindowsWhen designing exterior masonry wallsystems to resist air-blast, it is importantto begin the analysis with the windowglass. A primary goal of building enve-lope design for blast is to ensure that thesupporting walls are stronger than theglass itself. This provides a balanced de-sign of the envelope so as to control the

spread of potential damage if a largerthan-designed for weapon threat is applied to the wall. This is based on thassumption that the failure of the glas

will release some of the blast pressurethereby reducing the blast load applieto the remaining structure.The glass should be designed by balanc

ing manufacturing limitations againsair-blast response requirements. The keis to design the glass layup such that imeets the air-blast response requirements while limiting over-strengthThe maximum capacity of the glass ithen analyzed to determine the pressurat which the glass will release from thframing systems, and this is the desigblast pressure that imparts load to thsupporting walls.

Anchorage

The next step in designing masonrinfill walls is to ensure that the windows can be adequately anchored tthe supporting walls. In considerinhistoric structures, there are many conditions that can make this a challengindesign requirement.The ideal placement of a window in

supporting wall is at the center of thwall depth. This allows the maximumedge distance for the anchors to resisboth inward and outward forces. In nonhistoric retrofits, it is often acceptable tchange the location of the windows relative to the wall depth. However, this irarely an acceptable modification in historic buildings. If adequate edge distancis not available, alternative anchorage details or wall retrofits will be required.

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While the masonry piers between windowsare generally constructed of solid masonry, itis common to find voids in the walls below

window sills. If voids occur at the windows,anchoring the window into the sill is notpossible without drilling diagonally into themasonry behind. This is a costly endeavor,and one which contractors are often reluctantto undertake.Depending on the size and placement of the

windows relative to the floor systems, the heador sill may be only inches away from spandrelbeams or, in the case of steel frame structures,the concrete encasement of beams. For con-crete members, anchorage of the windows

will be relatively straightforward. In the caseof concrete-encased steel members, construc-tability considerations may require findingalternate solutions for window anchorage.

Other Constraints

As with all structural retrofits on historicstructures, there are other constraints toconsider when developing design solutions.These may include: Interior Finishes: interior finishes,

especially below the windows, areoften designated as remaining andmay be fragile, as in the case ofHVAC equipment.

Floor system: several of the retrofitsolutions require that retrofit elementsbe attached to the existing floor system.Older floor construction systems maynot be able to support these additionaldead loads.

Wall AnalysisIt is best to begin calculations assuming that

the window can be anchored to the walls atall four sides, distributing the forces from the

windows to the supporting walls at the head,sill, and jambs.A common configuration for windows inhistoric unreinforced masonry buildings, es-pecially those constructed in the late 1800sand the early 1900s, is to have two or three

the wall as a slender member subjecteda uniformly-applied dynamic load. Itassumed that there is no contribution of acompressive forces at the top and bottomthe wall, and the wall is analyzed as a simpsupported beam. The flexural resistance of

wall is dependent on the compressive strefrom its own self weight, and the tenstrength as defined by the mortars moduof rupture. Because unreinforced masonrylittle tensile resistance, the flexural capaof the wall is minimal, resulting in extenscracking and brittle failure at the interior fof the wall.

windows per structural bay. Thiscreates masonry piers betweenindividual window openings.These piers tend to be criticalsupporting wall elements.Two computational models for

analyzing wall response are de-scribed below.

Simply-Supported

A conservative approach tomodeling the resistance of anunreinforced masonry wall foran air-blast load is to represent

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Rigid Supports

Another model, based on the military tri-services technical manual TM5-1300 (therecognized air-blast resistance design manualthroughout the United States) assumes thatthe top and bottom supports are completelyrigid and provide restraint against elongation.Strength comes from compressive blocksthat form at the top, bottom and mid-heighthinge locations of the wall, behavior knownas a compressive membrane phenomenon.This model also assumes that the mortarjoints will be cracked, disregarding whatlittle flexural tensile strength remains in themasonry. The deflected wall wedges itselfbetween floor slabs and also forms a plastichinge at mid-height. This model takes intoaccount that masonry infill walls will oftenhave a gap at the top, either from a mortarjoint or by design that accounts for flexure ofthe floor systems above.

RetrofitsOnce the basic calculations on the windows

and walls have been performed and theconstraints have been identified, design ofretrofit solutions can begin. In a perfectworld, the windows would be anchored at allfour sides and the surrounding wall elementswould be able to support the design loads.This is rarely the case; therefore multipleoptions for retrofitting these window wallsystems are considered.

Steel Supporting Frames

The most straightforward retrofit is to installsteel frames, interior to the existing walls,

which support the window systems. Thissolution is appropriate where the wall piersare not able to support the additional air-blastload imposed by the windows. This retrofitis often unacceptable to architects as it eithercreates an uneven wall surface or requiresreducing the overall room size by furring outthe walls.

Shotcrete Walls

In some cases, other loading condition re-quirements may be combined with air-blastrequirements to develop multi-purpose solu-tions. Where progressive collapse or seismicdesign requirements would benefit fromshotcrete walls, this may be used as an air-blast retrofit of unreinforced masonry wallsas well. Window systems could be mountedin the new concrete wall system. This wouldbe an especially useful technique for relativelylarger blast loads.

Fiber Reinforced Polymer (FRP)

Another retrofit method is to apply vertical

FRP composite strips along the inside face ofthe masonry wall. FRP composites have sig-nificant stiffness and tensile strength in thedirection of the fibers, and can be highly ef-fective in increasing the walls out-of-planeflexural capacity. Additional advantages ofthis method are that it is relatively unobtru-sive to the architectural detailing of the walland existing structure, and its applicationprocess is not labor intensive, requiring littledisruption to building occupants.The flexural capacity of a FRP-retrofitted

unreinforced masonry wall can be assessed

using basic moment-equilibrium relationssimilar to that of a steel-reinforced masonr

wall. The tensile strength of the masonris disregarded and all tensile resistance iassumed to be provided by the FRP. Thflexural resistance can then be computed aa function of the compressive strength of thmasonry, the tensile strength of the FRP, ththickness of the masonry panel, and the valuof the applied axial force.Blast tests have demonstrated it is unlikel

that the walls predominant failure mode wibe failure of the FRP. Also, tests of unreinforced masonry units to out-of-plane air-blasloading have demonstrated that shear failure at the supports is a predominant failurmechanism. The FRP reinforcing provides nadditional shear resistance to the wall sectionand if connections at the walls supports dnot allow transfer of excessive shear forces tother structural elements, shear failure at th

wall supports can occur. Also, connections awall supports are instrumental in preventin

the out-of-plane flexural collapse towards thfront side of the wall due to rebound forces.

ConclusionWhile not considered an ideal buildinmaterial, unreinforced masonry infill wallconstitute a large stock of structural systemthroughout the worlds historic building

Analysis techniques, supported by air-blastesting data, can estimate the capacity othese wall systems for extreme out-of-planloading. Existing walls, including window

and supporting structure, are a complex system that should be analyzed from a capacity-based approach when designing a viablretrofit solution.

Jessica Godinho, M.S. is a Project Engineerat Hinman Consulting Engineers, Inc. inSan Francisco, California. She specializesin design and research of blast resistantstructures and can be reached atjgodinho@hce.com.

Elizabeth Agnew, M.S. is also a ProjectEngineer at Hinman in San Francisco.She specializes in blast resistant design and

progressive collapse research, and can bereached at eagnew@hce.com.

Shalva Marjanishvilli, Ph.D., S.E. is theTechnical Director at Hinman in SanFrancisco. He is an expert in the dynamicnon-linear response of structures fromseismic, impact, and explosive loadings.He can be reached via email atShalva@hce.com.

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