masonry design for blast loading - group · pdf file15th international brick and block masonry...

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

MASONRY DESIGN FOR BLAST LOADING

ElSayed, M.1, El-Dakhakhni, W.W.2 1Ph.D. Candidate,Department of Civil Engineering, McMaster University, Canada, [email protected]

2Martini Mascarin and George Chair in Masonry Design, Department of Civil Engineering, McMaster University, Canada, [email protected]

In order to protect the building occupants against accidental or deliberate blast loads, special expertise and knowledge are necessary to ensure adequate performance of the structural system. Although the structure may require an extensive repair following a blast event, the main goal of the protective design is to avoid structural progressive collapse and minimize fragments. Blast loading is very different from other forms of dynamic loading generally analyzed by structural engineers. Peak pressures are several orders of magnitude higher than those associated with other typical dynamic loads, and blast load durations are usually much shorten than the fundamental period of the structure. This paper will focus on the blast loading phenomena as a part of the design requirement implemented in the two recently developed North America’s codes, ASCE SEI59-11 (2011) and CAN CSA S850-12 (2012). A description of different level of protections and various masonry loads is presented in this paper, in addition to the general considerations in standards and design guidelines for constructing masonry structures to resist explosive loads.

Keywords: Blast Load, cube root scaling, Masonry, North American Blast Standards. INTRODUCTION In recent years, especially following the explosion in Oklahoma City (1995), the numbers of stakeholder seeking blast resistance design to their structures have increased. Due to the threat caused from such extreme loading conditions, efforts have been made during the past three decades to develop methods of structural analysis and design to resist blast loads. These efforts have led to the publication of some guidelines for the analysis and design of blast resistance structures, such as the U.S. manual TM 5-1300 and other relevant materials in the UFC. The most significant technical factor limiting growth of load-bearing and masonry infill wall construction is the lack of authoritative information and guidelines to assess and improve the performance of masonry structures under blast. Although the lack of information has existed for some time, the introduction of the new standards in Canada will have a significant impact on the masonry industry as a result of the lack of basic understanding of masonry wall performance under blast loads. On the other hand, the development of the Canadian Standards: CAN CSA S850 “Design and Assessment of Buildings Against Blast” is currently being finalized with an estimated publication date sometime in the spring of 2012.



CHARACTERISTICS OF BLAST WAVE Blast is a pressure disturbance caused by the sudden release of energy. It is a common misconception that blast is usually associated with the detonation of a high explosive charge. However, there are many other blast sources that have the potential to cause damage, such as propane gas explosion. An idealized pressure-time history diagram for an open air explosion is shown in Figure 1. The characteristics of the pressure wave propagated on the surface of a structure due to an explosion depend on the properties of the source explosion. Such properties are the peak incident pressure ‟Pso” and the duration of explosion, which varies from one explosion to another based on charge weight, shape and its type. Basically, Figure 1 describes the typical time history for the overpressure where it decays exponentially within a short period (known by positive phase) followed by a negative phase where the pressure is less than the ambient pressure Po. It is also important to mention that there is a non-linear variation between the two positive and negative impulse phases.

Figure 1:Pressure-time variation

Figure 2:Blast environment from an air-blast (TM5-1300)

A reflected pressure wave will be developed due to the reflection of the source wave from ground surface, as shown in Figure 2. The reflected wave depends on many factors such as:



the transfer medium density; charge shape; charge weight; distance between source and target; humidity degree; medium temperature; inclination angle between source and target and other factors that will also affect the characteristics of this reflected pressure wave. In this way, the pressure wave time history that will be applied on a structural element, taking into consideration the effect of reflected wave, will be plotted against time. One of the most common presentation for blast wave is similar to the one shown in Figure 3, where a pressure-time relationship is plotted using actual pressure-time history diagram, or idealizing it into other shapes such as: triangular, rectangular or exponentially decaying function. The main factors that affect the peak structural responses are the peak pressure and duration of the blast load. The Impulse ‟I” of this blast load can be defined as the area enclosed by the load-time curve, and therefore blast load can be defined under a certain shape by both peak pressure ‟Po” and Impulse “I”. Therefore, maximum responses of structure under the action of such blast load can be determined by performing non-linear dynamic analysis considering the material nonlinearity.

Figure 3:Blast environment from an air-blast (TM5-1300) BLAST LOADING SCALING Arrival times, blast pressure, impulse, load duration, shock wave velocity, and other blast parameters are frequently presented in scaled form. Research has shown that scaling laws can be applied to explosions, allowing data from one explosion test to be applied to a geometrically similar (larger or smaller) case. As a result, scaling has incredible utility in blast prediction, allowing compilations of explosion test and numerical modelling data to be used to predict loads for any combination of explosion energy and standoff distance within the range of data. The most common form of scaling is called “cube root scaling” owing to the fact that blast parameters are scaled by the cube root of the explosion energy. See Eq. 1 below.



3 WRz = (1)

Where term ! is defined as the scale distance, and R is the distance from a reference explosion of a charge having a weight of W. As mentioned earlier, employing the cube root scaling method can compare different explosions. Such comparisons for the blast effect of spherical TNT charges are presented in Figure 4.

Figure 4:Shock wave parameters for spherical TNT explosions in free air (TM5-1300) The basic principle of cube root scaling is that the energy released from a point explosion will propagate with an expanding sphere of the shock wave. In other words, the various blast effects will be proportional to the energy per unit volume (specific energy). Since the volume of the sphere will be proportional to R3, the scaling will contain the cube root. Generally, explosive threat can be grouped into three general categories based on the explosive weight and standoff distance. Such categories are: far field, near field, and contact / near contact. Far field threats are generally exist at scaled distances greater than 2.0 and blast pressure loading can be considered uniform across the structure. Scale distances ranging from 0.5 to 2.0 fall into the near field range. If charge shape is not an issue and the structure surface is flat and regular, simplified blast load functions are applicable. Contact and near contact place the explosive threat in close proximity to the structure, generally at scale distance less than 0.5. At this standoff distance, the explosive effects are highly localized, and the blast pressures have extremely high pressure and short duration pulses. MODELING OF MASONRY WALLS After estimating the peak pressure and corresponding impulse that would take place during an explosion events, it will be required to evaluate the damage level for the various elements in the

Scaled Distance Z = R/W^(1/3)

Figure 2-7. Positive phase shock wave parameters for aspherical TNT explosion in free air at sea level

0.1 1.0 10 100.001

.01

0.1

1.0

10

100

1000

10000

1.E+5Pr, psiPso, psiIr, psi-ms/lb^(1/3)Is, psi-ms/lb^(1/3)ta, ms/lb^(1/3)to, ms/lb^(1/3)U, ft/msLw, ft/lb^(1/3)



structures. In general, a dynamic analysis should be considered for critical structural elements during the design process and for the entire system after completion of the design. Different computational approaches can be used to address a specific analysis needed. Such approaches are: finite element (FE) approach, continuous system approach, multi-degree of freedom (MDOF) approach, and single degree of freedom (SDOF) approach. For regular masonry elements subjected to uniform blast pressure, nonlinear SDOF approach is considered sufficient accurate, but very rapid analysis procedure compared to the other approaches. The masonry walls will be modelled as an equivalent nonlinear SDOF system subjected to a blast load as illustrated in Figure 5.

Figure 5:Equivalent Single Degree of Freedom System

The perpendicular load to the wall surface is usually referred to as the overpressure. It is more common to idealize the positive phase into a triangular load, which has the same peak pressure and shorter load duration TL. As shown in Figure 6, the duration of the idealized load depends on the time to reach to the maximum response. If the maximum response occurs within the positive phase, the idealized duration will be determined by equating the area under the triangular load and actual load curves. Vice versa, if the peak response occurs in the negative phase, the triangular load duration will be determined by equating the slopes of the triangular load and the tangent line of the actual load at peak load position.



Figure 6: Idealized air blast pressure-time diagram

Non-Linear Time History Analysis (NLTHA) is considered the most accurate analytical tool that governs the performance of structures under dynamic loads. Such methods usually require a large amount of input data and consume a long duration during solution. Unlike time history analysis, pressure-impulse (P-I) diagrams gives the ultimate deflections (e.g. like response spectrum analysis) and not time histories, which is more important to designers who needs to know the maximum responses. It is now evident why such diagrams are gaining much attention from engineers and decision makers. P-I diagram is a curve which is generated to show the response over a wide range of blast loadings, and generally takes the following form as shown in Figure 7. This curve consists of three main regions: impulsive, dynamic and quasi-static regimes, where the first and third regimes are almost linear.

Figure 7: Normalized Pressure-Impulse diagram

BLAST LOAD LEVEL OF PROTECTIONS (LOP) The primary purpose of blast resistant design is to reduce, to a certain extent, the risk to occupants of injury and death and to contents of damage and destruction in the event of an explosion of a specified magnitude and location within or near the building. Modern North



American codes, such as CAN CSA-S580-12 (2012) and ASCE SEI 59-11 (2011), established four different level of protections (LOP) based on the performance and degree of damage to structural component under the action of an explosion. Table 1 correlates qualitative damage description with the performance levels: collapse prevention, life safety, immediate occupancy, and operational and their corresponding levels of protections from very low through high. The structural components are members whose loss would affect a number of other supporting components and whose loss could potentially affect the overall structural stability of a building area. Examples of structural components include columns, girders, and any load bearing structural components such as walls. Secondary structural components in general are carried by the main structural components, and can fail without creating extensive structural damage. Such components include non-load bearing infill masonry walls, steel purlins, studs and joints. Non-structural elements are generally not designed against blast load. Non-structural components interior infill walls, and architectural items attached to building structural components. Table 1: Building LOP, and component and envelop levels of damages

LOP Building Performance

Building Component Damage Levels Components

Glazing Structural Secondary Structural

Non-Structural

Very Low (VL)

Collapse Prevention: • Able to evacuate • Re-entry unlikely • Contents not intact

Heavy: • Unlikely to fail • Significant

permanent deflections likely

• Repair unlikely

Hazardous: • Likely to fail and

produce debris

Low hazard rating: • Glazing will facture

and come out of frame

• Glazing likely to propel into building

• Injuries likely

Low (L)

Life Safety: • Able to evacuate • Temporary re-entry • Repair not economically

viable

Moderate: • Unlikely to fail • Some permanent

deflections • Repair or

replacement likely

Heavy: • Unlikely to fail • Significant permanent

deflections likely • Repair unlikely

Very low hazard rating: • Glazing will facture

and potentially come out of frame

• No significant injury hazard

Medium (M)

Immediate Occupancy: • Able to evacuate • Operational after cleanup

or repair • Partially functional after

repairs • Repair economically

viable

Superficial: • Unlikely to

exhibit permanent visible damage

Moderate: • Unlikely to fail • Some permanent

deflections • Repair or replacement

likely

Minimal hazard rating: • Glazing will facture

but remain in frame • Minimal injury

hazard

High (H)

Operational: • Uninterrupted occupancy • Contents fully functional • Minimal local damage

only

Superficial: • Unlikely to exhibit permanent visible

damage

No break: • Glazing observed not

to facture • No visible damage to

glazing system



In typical static design, the stress level of component response is limited to prevent collapse. However, this approach cannot be used for blast design, since controlled, ductile yielding is part of the design intent. The first step in air blast design of masonry wall is to establish the desirable performance level in terms of acceptable damage levels, as shown in table 1. Then, structural components are designed by conducing dynamic analysis to calculate damage indicators and maximum component response, as described under each column for corresponding damage level. Next step is to check that response against quantitative limits that are consistent with the overall building damage level. If it is determined that the damage indicators are exceeded for a desired performance level, the wall must be redesigned and re-analyzed. Thus, air blast design is an iterative process. The amount of conservatism in the design depends on the allowable component deflection / end support rotation limit. GENERAL STANDARDS CONSIDERATIONS Modern North American Codes recommend using masonry blocks that their maximum strength normal to the bed joint, f’m, shall not exceed 40 MPa, and the minimum strength, f’m, shall not be less than 10 MPa for all LOP in new constructions. In case of existing masonry structural elements, the design engineer should determine the material properties to ensure proper design to achieve the LOP required for the existing masonry structure. It worth mentioning that ASCE-SEI 59 (2011) does not permit using unreinforced masonry completely and all concrete masonry units shall be fully grouted for heavy, and medium LOP. On the other hand, CSA-S850 (2012) permit using unreinforced masonry only in case of very low LOP depending on its arching action and for existing buildings only, and allow using partially grouted reinforced walls for the different LOP. In addition, a bond beam course shall be placed all around the top of the wall at each story. It is desirable to use reinforcement with high ductility demand capability. Use of welded splices shall be limited heave LOP and to zones remains elastic during blast response, unless the mechanical splices can be shown to develop the expected tensile strength of the bar under conditions generated by blast loads. Both ASCE SEI 59 (2011) and CSA-S580 (2012) indicate that the maximum vertical reinforcement spacing shall not exceed 400 mm and the minimum reinforcement ratio shall not be less than 0.0025. In addition, at least one vertical bar shall be placed at the corners, each side of openings, and either side of movement joints. The design shear force for the direct shear shall be based on the flexural capacity of the masonry element. Hence, in-plane shear reinforcement of reinforced masonry walls shall have 180° hooks around vertical reinforcing bars at the end of the wall. Horizontal reinforcement shall have a maximum spacing of half the wall depth (dv) or 1200 mm, whichever is less. Running bond is suggested by both Canadian and American standards as a construction procedure for blast resistance masonry elements. CONSIDERED FAILURE MODES One of the preferred failure mechanisms and occur in properly designed and detailed masonry is the flexural failure mode. Such mode provides some ductility to the attacked masonry element. This mechanism can be achieved by designing masonry under balanced conditions whereby reinforcement will yield before masonry concrete blocks start crushing. Typical deflected shape for such failure mode can be described as illustrated in Figure 8-a.



In case of relatively low air blast pressure, it is applicable to design a reinforced masonry wall using only one reinforcement curtain located in the middle. In this case, the wall should be designed to that reinforcement will yield before the compressive block of masonry begins crushing. On the other hand when air blast pressures are relatively high (in excess of 70 kPa), it may be needed to use two equal reinforcement curtains at each side of the wall. Furthermore, it may be required to increase wall thickness to have room for the two reinforcement curtains and hence, the flexural capacity of the masonry wall will be increased. The second possible failure mode is the diagonal tension shear. This failure mode occurs when a masonry wall response reaches the diagonal tension resistance before its flexural one. Typical deformed shape corresponding to such failure modes is illustrated in Figure 8-b. Unlike flexural failure mode, diagonal tension shear failure mode provides limited ductility and therefore by nature, it is a brittle failure mode. For this reason, this failure mode should be avoided in blast resistance masonry wall. One technique used to achieve this is to decrease the flexural capacity of the wall in such a way that the wall undergoes flexural yielding before shear capacity is exhausted. However, the flexural resistance of the wall should not be reduced when doing so will compromise the desired level of protection. The other alternative is to increase the diagonal tension shear capacity of the wall. To achieve that, there are many ways, such as increasing the thickness of the masonry; grouting more cells; or placing shear reinforcement. Another possible failure mode can be created in masonry wall, which is widely referred to by direct shear. Such failure mode can arise when explosion happens very close to the masonry wall and mortar joints are too weak to transfer the shear stress flow through concrete blocks. Figure 8-c describes the typical deformed shape corresponding to this failure mechanism. Direct shear may be of concern when scaled distance “Z” is less than 4. At the corresponding standoff, it is expected that the magnitude of blast pressure is very high while load duration is extremely short. For such case, the will be no time for the masonry wall to respond in flexural mode, and direct shear response will dominate. Same as diagonal tension shear failure, direct shear failure mode is very brittle and therefore it should be avoided. One way to avoid direct shear failure is to increase the standoff between the masonry wall and the expected explosion source. The last failure mechanism is breaching. Breaching occurs when an explosive charge is stuck or placed directly against a masonry wall structure. Detonation at this close proximity cause shattering of the masonry units. Breach effects can be reduced by proper confinement, masonry wall thickness, and application of anti-spall laminates such as fiber-reinforced polymer (FRP). In case of full breach does not happen, the backside of the masonry wall will be exposed to high tensile stresses that may exceed the tensile strength of the masonry element. Hence, spall typically occurs on the backside of the wall. Both breach and spall will cause fragmentation on the back face of the wall. Spall can be mitigated by application of laminate material, such as FRP, which is permanently adhered to the wall surface.



(a) (b) (c)

Figure 8:Different failure mechanisms for masonry structures: (a) flexural failure mode, (b) diagonal tension shear response, and (c) direct shear response

ACKNOWLEDGEMENTS The financial support of the ongoing research on the performance of masonry structures under blast load is funded by the McMaster University Centre for Effective Design of Structures (CEDS) funded through the Ontario Research and Development Challenge Fund (ORDCF) as well as the Natural Sciences and Engineering Research Council (NSERC) of Canada. The continuing provision of mason time by Ontario Masonry Contractors Association (OMCA) and Canada Masonry Design Centre is always appreciated. The one-third blocks to be used in this research are made available by the Canadian Concrete Masonry Producers Association (CCMPA) which is gratefully acknowledged. REFERENCES ASCE Standards: ASCE 59-11 (2011) “Blast Protection of Buildings”. Virginia, USA:American Society of Civil Engineers, , 108pp. Baker, W.E., Cox, P.A., Westine, P.S., Kulesz, J.J., and Strehlow, R.A. (1983) “Explosion Hazards and Evaluation”. New York: Elsevier Scientific Inc., 807pp. Biggs, J.M. 1(964) “Introduction to Structural Dynamics”. New York: McGraw-Hill Inc., , 341pp. Chopra, A.K. Dynamics f Structures. New-Jersey, USA: Pearson Edition Inc., Third Edition, 2007, 729pp. CSA-S850 (2012) “Design and Assessment of Buildings Subjected to Blast Loads”. Canada: Canadian Standards Association. Drysdale, R.G.,Hamid, A.A. (2005) “Masonry Structures: Behaviour and Design”, First Canadian Edition, 769pp.



Dusenberry, D.O. (2010) “Handbook for Blast Resistant Design of Buildings”. New-Jersey, USA:Wiley& Sons Inc., First Edition, , 484pp. Krauthammer, T. (2008) “Modern Protective Structures” New-York, USA:CRC Press, Taylor & Francis Group, 509pp. Priestley, M.J.N.,Calvi, G.M., and Kowalsky, M.J. (2007) “Displacement Based Seismic Design of Structures”. Pavia ,Italy:IUSS Press, First Edition, , 721pp. UFC 3-340-02. (2008) “Design of Structures to Resist The Effects of Accidental Explosions.” US Department of the Army Technical Manual.

masonry design for blast loading - group · pdf file15th international brick and block masonry...

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