The chapter also discusses how explosions that occur in process plants are characterized in order to determine the blast loads for structural design. The flexural resistance at this point is essentially ail Resistance provided beyond this region is due to membrane response which is characterized by stretching of the panel rather than flexure.
 
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of the Energy Division of ASCE published the manual Design of Blast Resistant. Buildings in Petrochemical Facilities in (American Society of Civil Engi. “Design of Structures to Resist Nuclear Weapons Effects,” ASCE –. Manuals and Reports on Engineering Practice – No. 42,

 
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Our policy towards the use of cookies Techstreet uses cookies to improve your online experience. They were placed on your computer when you launched this website. You can change your cookie settings through your browser. Request Free Trial. Language: English. Although such incidents may be relatively rare, when they do occur the consequences can be extremely severe involving personnel casualty and financial loss and potentially impacting public safety.

In some instances the consequences have involved plant buildings. The property loss was reported to have exceeded 50 million dollars. Similarly, in the US, recent petrochemical plant explosions have resulted in a significant number of fatalities from the severe damage or collapse of buildings. The concentration of such fatalities in buildings points to the need to design plant buildings to withstand explosion effects in order to protect the people inside so that, at least, the building does not pose an added hazard to the occupants.

In addition to personnel safety. However, in addition to the air blast effects, such incidents can result in fires, projectiles and ground transmitted shocks that also can be damaging to buildings and their contents, Historically, blast resistant design technology in the petrochemical industry has evolved from equivalent static loads and conventional static design methods Bradford and Culbertson , to simplified dynamic design methods that take into account dynamic characteristics and ductility of structural components, and based on TNT equivalent blast loading Forbes , and finally to more complex and rational methods involving vapor cloud explosion models to characterize the blast loading and nonlinear multi-degree of freedom dynamic models to analyze the building structure.

Current practices within the industry appear to cover all these approaches. This report is intended to provide guidelines on the various methods available for the structural design of blast resistant buildings in petroleum and chemical process plants. Chapter 2 discusses the basic philosophy and general considerations involved ia establishing design requirements for blast resistance in buildings to resist che effects of accidental explosions in petrochemical processing plants, Chapter 3 describes the types of explosions that may occur and the general characteristics of the resulting blast load, but does not prescribe magnitudes for design.

The chapter provides a brief review of the approaches used in the industry to quantify blast loads for design purposes and gives typical examples of such loads. In Chapter 4 the types of building construction appropriate for various levels of blast resistance are discussed. The dynamic ultimate strength design criteria, including the dynamic material properties and deformation limits applicable to blast resistant design are covered in Chapter 5 The methods and procedures for blast resistant design can vary considerably in complexity, accuracy, cost and efficiency from simple conventional static design approach to complex transient nonlinear, multi-degree of freedom dynamic design methods.

Chapter 7 outlines recommended procedures and provides aids for the design of the various components Of reinforced concrete, reinforced masonry and structural steel buildings. Chapter 8 provides some typical structural details for doors and frames, wall penetrations, and connections for steel and reinforced concrete components. Blast protection considerations for non-structural items such as interior details, windows, openings, and HVAC ducts are covered in Chapter 9.

Chapter 10 gives guidance on strategies for evaluating the blast resistance of existing buildings and provides practical measures for upgrading masonry and metal buildings, the most common types of building construction for plants in the petrochemical industry.

Design examples are provided in Chapters 11 to 13 to illustrate the use of these procedures and tools in the design of typical buildings for blast resistance 1. However, the design practices used by some operating companies and contractors are based on a number of existing documents dealing with this subject including.

Structures to Resist the Effects of Accidental Explosions, TM , Department of the Army, Navy, and Air Force The SG and CIA documents are similar and cover the siting, design and construction of control buildings in petrochemical plants for a specified set of TNT equivalent blast loads and the simplified dynamic elasto-plastic, single degree of freedom design approach.

However, the fundamentals and design principles covered in these documents are applicable to designs for other types of explosions In addition to the publications cited above, the American Institute of Chemical Engineers, Center for Chemical Process Safety CCPS committee and the American Petroleum Institute API recently have addressed various aspects of blast protection technology relevant to this report.

This flowchart shows fifteen basic steps in the overall blast assessment and design process, as follows: a. Define Scope: Steps 1 and 2 are to define the owner’s requirements and needs for the building b. Analyze Explosion Hazards: Steps 3 and 4 are to identify the explosion scenarios to be used to quantify the design biast overpressures see Chapter 3 , c.

Determine Performance Criteria: Step 5 is to determine how the building should perform during the explosion scenario see Chapter 3 d. Determine Blast Loads: Step 7 is to determine the blast loadings for the various components of the building see Chapter 3.

The design engineer’s responsibilities fall in d to g steps 6 to 15 of the process. Such explosions have demolished plant buildings, in some cases cesulting in substantial personnel casualties and business losses.

Such events have heightened the concems of the industry, plant management, and regulatory agencies about the issues of blast protection in plants having the potential for explosions, Generally, these issues relate to plant safety and risk management to prevent or minimize the occurrence of such incidents and to siting, design, and construction practices for plant buildings to mitigate the effects on plant workers and operations.

This chapter covers the general considerations pertaining to the design of plant buildings to resist the effects of accidental explosions in petrochemical plants. First the relevant regulatory requirements are briefly discussed. Next is a discussion of current industry practice and the objectives for providing blast resistance in plant buildings. In Section 2.

Siting plays a key role blast protection of buildings in a plant. Often the need for blast protection has to be weighed against functional or operational needs. These siting considerations are discussed in Section 2. Blast resistant design should provide a level of safety for persons in the building that is no less than that for persons outside the building in the event of an explosion.

Evidence from past incidents have shown that many of the fatalities and serious injuries were due to collapse of buildings onto the persons inside the building. This Objective is to reduce the probability that the building itself becomes 2 hazard in an explosion. Preventing cascading events due to loss of control of process units not involved in the event is another objective of biast resistant design. An incident in one unit should not affect the continued safe operation or orderly shutdown of other units.

Preventing or minimizing financial losses is another objective of blast resistant design. For example, a critical building sited far enough from a potential blast source may not need increased blast resistance. One should keep in mind that every building has some level of blast resistance and che cerm is not synonymous with a bunker design.

A blast resistant design is then recommended if either of the following apply: The building meets the owner’s occupancy criteria API RP Even where evacuation is used as a mitigation strategy, blast resistance should be considered for accupied buildings because complete evacuation is unlikely in the short response time due to the aumber of occupants or size and layout of the building.

The building or installation is expected to perform critical services. One critical service is where procedures require that personnel remain inside during an accident to regain, or maintain control, or to safely shut down operating units, Another critical service is where a building controls multiple units or controls a particularly high risk unit.

Risk relates to the volume of stored flammables, the proximity to a blast source, and the consequences of 2 major accident. Hazards, exposures, furure expansions, and spacing establishes the selected site Siting a plant building should consider the hazards in the adjacent and nearby processing operations and the possible results of an incident involving these hazards. Blast protection can be provided by adequate spacing from a potential hazard or by strengthening the building.

Spacing should be the primary choice in providing blast protection. Generally, buildings designed for conventional Iga can be sited in areas where the peak side-on overpressure is less than. DoD Buildings should be oriented such that the short side faces the most probable explosion source. Buildings housing personnel not required for actual operation of the unit should be sited as far away as possible.

Buildings should be sited away from areas of congestion and confinement as these contribute to the severity of the explosion. Buildings should not be sited downhill from potential release sources of heavier than air materials. Buildings should not be sited in prevailing downwind direction from potential release sources. Structural strengthening, or design to resist the effects of accidental explosions, was identified as one of the options available to achieve the appropriate level of blast protection, Blast resistant design requires that the loads from such events be quantified and that the structural performance requirements be established for buildings subjected to these loads.

This chapter provides general information. The chapter also discusses how explosions that occur in process plants are characterized in order to determine the blast loads for structural design. First, Section 3. Section 3. Some of the methods currently in use in the industry and some blast overpressure values for accidental explosions used for design are covered in Section 3.

Finally, Section 3. This dispersion is called a vapor cloud. Baie, Second, ignition must be delayed long enough for a vapor cloud of sufficient size to form. Maximum flammable cloud size is usually reached in 30 to 60 seconds, so the ignition delay is not long. If ignition occurs nearly instantly, a fire or fireball, but not a VCE, would occur.

Third, the fuel-air ratio of a sufficient amount of the vapor cloud must be in the flammable range. The more uniform the fuel-air mixture, near the stoichiometric fuel- air ratio, the stronger the explosion.

Finally, there must be a flame acceleration mechanism, such as congested areas, within the flammable portion of the vapor cloud. The overpressures produced by a vapor cloud explosion are determined by.

Objects in the flame pathway such as congested areas of piping, process equipment, etc. This turbulence results in a much faster flame speed which, in tum, can produce significant overpressures Confinement that limits flame expansion, such as solid decks in multi-level process structures, also increases flame speed.

Without flame acceleration, a large fireball or flash fire can result, but not an explosion. Thus, the center of a VCE is not necessarily where the flammable material is released, the point of ignition, or the center of the vapor cloud. However, acetylene alone, with no oxygen, can also detlagrate or detonate ». Runaway Exothermic Chemical Reactions: Many industrial chemical reactions are exothermic, ie. Certain reactions can go into acceleraced runaway conditions if the released energy is not removed fast enough, If a containment vessel has insufficient venting capabilities, considerable pressure can build up.

If this pressure exceeds the pressure capabilities of the vessel, it will explode. Rupture of pressure vessels due to overpressure may occur if human error or ancillary equipment failures allow too high an internal pressure to accumulate.

Physical Vapor Explosions: Physical vapor explosions occur when two streams of widely differing temperatures mix suddenly, such that the cooler liquid dashes rapidly to vapor and generates a pressure beyond the pressure capability of the container. The container thus explodes. Foundries may experience such explosions if molten metal is accidentally poured into a moist moid, or water into hot oil.

In this case, the vessel is aot pressured above its rated pressure, but is weakened by the heat. Much of the liquid flash vaporizes, and much of the remainder is broken up into aerosol droplets The vapor aerosol mixture is typically ignited as the material is suddenly vented to the atmosphere, The combustion rate is limited to the rate at which air can mix into the fuel.

In terms relative to the speed of flames, the rate of mixing with air is relatively slow. A huge, billowing, highly radiant fireball results, and a pressure wave may also occur 3. Some materials found in petrochemical plants have properties that cause them to explode under upset process conditions. It is significant that, in a dust suspension in air, small concentrations of flammable gas, even well below the lower flammable limit of the gas, can contribute to a more severe explosion than that of the dust alone.

Such mixtures are called hybrid mixtures 3. The blast wave propagates outward in all directions from the source at supersonic or sonic speed. The magnitude and shape of the blast wave depends on the nature of the energy release and on the distance from the explosion epicenter The characteristic shapes of blast waves are shown in Figure 3.

Shock Wave: This has a sudden, almost instantaneous rise in pressure above ambient atmospheric conditions to a peak free field side-on or incident overpressure, The peak side-on overpressure gradually returns to ambient with some highly damped pressure oscillations. Pressure Wave This has a gradual pressure rise to.

Shock waves in the near and far fields usually result from condensed phase detonations, or from an extremely energetic vapor cloud explosion. For situations where the negative phase blast loading may be important, the reader is referred to TM for the characterization and treatment of this loading In Figure 3. These sources do not provide data on the negative phase of the blast wave from a vapor cloud explosion.

Because negative phase pressures are relatively small, and oppose the primary lateral force, it is usually conservative to ignore them for design. The values of blast overpressure and duration appropriate for petrochemical design are discussed in Section 3. The net dynamic pressure on a structure is the product of the dynamic pressure and a drag coefficient, Cy.

The drag coefficient depends on the shane aad orientation of the obstructing surface. The dynamic pressure also influences, but to a lesser extent, the net blast loads on the walls and roof of an enclosed building as discussed in Section 3. There are no similar plots available for pressure wave propagation. However, for design purposes it can be conservatively assumed that a pressure wave travels at the same velocity as a shock wave.

In the low pressure range, and for normal atmospheric conditions. In the low pressure range, the length of the blast wave can be approximated by: G8 3. This simplification is shown in Figure 3. The blast loads on the various parts of 2 building based on these simplified blast wave parameters are discussed in Section 3. Because there are no codes or industry standards for determining whet blast overpressures should be used, the design blast loads are usually supplied by the facility owner.

A further refinement is to specify overpressures and durations based on the distance between the structure and a potential source. The distances may be given in stepped blocks or a continuous function. A site specific study is the most comprehensive approach. Site specific studies to identify and quantify explosion hazards are usually conducted by the owner’s process safety specialist or by specialty consultants.

There are several steps which need to be taken, each of which may be done in a variety of ways. The steps are outlined below with some of the available methods. Define the release. This step may be based on a worst possible case based on the maximum amount of material within a process loop, or a worst probable credible case selected from a hazards review Formation of an explosive cloud: This step is often done using two computer models.

The first is a Source emissions model which calculates what happens at the interface between the contained material and the atmosphere into which it is being released. The second is a dispersion model which calculates how the celeased material disperses and mixes with the air. Amount of energy contributing to the explosion: This may be based on a fraction of the total amount of material available or by determining the mass of the cloud that is within the fammable limits, [t may be further refined by looking at the level of confinement within the area of the cloud.

Calculation of blast overpressure parameters: There are three major methods in use today, One is the TNT Equivalency Method which gives inaccurate results for vapor cloud explosions. Both provide a family of curves based on flame speed or explosion strength These curves are used to select dimensionless parameters which are then unscaled to determine the actual overpressures.

If the structure is large, the average overpressure on the surface or the overpressure at the centroid of the surface may be used. Normally a building should be designed considering the potential blast wave fom any horizontal direction, but not all directions simultaneously Commonly used criteria includes SG, and Cl , Both documents specify at least nwo blast overpressures for buildings spaced feet 30 meters from a vapor cloud explosion hazard as follows: 4.

High pressure, short duration, triangular shock loading: Side-on overpressure of 10 psi 69 kPa with a duration of 20 milliseconds b. Low pressure, long duration, triangular loading: Side-on overpressure of 3 psi 21 kPa with a duration of milliseconds. These blast loadings have been widely used in the past for blast resistant design throughout the industry.

However, many owners have developed specific blast loading criteria more in line with their specific circumstances. Overall, the specified blast loads used for design have side-on overpressures ranging from 15 to 15 psi 10 to “kPa with positive phase duration ranging from 20 to ms. These loads are for buildings spaced from to feet 30 to 60 meters from an explosion source, Generally, the greater the spacing, the lesser the overpressure and impulse, but the longer the duration of the blast loading, Historical data from industrial explosions are hard to accurately quantify as these can only be approximated by back calculating from observed deformations of structures.

Blast overpressures from vapor cloud explosions are especially difficult to quantify because they tend to be directional, come from multiple sources, and vary with site conditions. Additionally, there is less information available than for high explosives. In one company’s review of five recent vapor cloud explosion incidents, as measured at a range of to 1, feet 60 to meters , peak reflected pressures in the range from 2 psi 14 kPa with a 35 ms duration to 12 psi 83 kPa with a 33 ms duration have occurred.

These pressures correspond to side-on overpressures ranging from 1 psi 7 kPa to 5. An extensive list of this type of explosion data is included in Lenoir To establish these loads, the design engineer should understand the interaction of the propagating blast wave with the building When a blast wave strikes a building, the building is loaded by the overpressure and drag forces of the blast wave, The interaction between the blast wave and a structure is quite complex as shown schematically in Figure 3.

For the purpose of design, the resulting blast loading can be simplified, as illustrated in Figure 3. The blast wave in Figure 3. Based on the owner specified side-on overpressure and duration, the design engineer can determine the blast loads for the various components of the building, as illustrated below, for a closed rectangular box-shaped building. Panels can be quite strong since this is a very efficient structural action; however, end anchorage is extremely important to achieving significant capacity Resistance to blast loads of more than psi kP2 will normally require tensile membrane response Where fragment hazards are a concer, cold formed panels may not be suitable because they have 2 very low resistance to fragment penetration.

Joists Conventional reinforced masonry structures as well as steel frame buildings often utilize open web steel joists to provide support for roof decks, Principal concems for these members are crushing of the web at the ends due to high shear forces and instability in the bottom chord during rebound of the section.

Older steel joists have performed surprisingly well in many explosion accidents provided they are adequately attached at the supports. This typically requires additional welding of the chord members to the embedded plate.

Bracing for the bottom chord throughout the length of the member is not normally provided for conventional designs but is crucial to achieving acceptable response. Quality of joist welds is also critical to achieving a ductile response. Welding is Performed to Steel Joist Institute standards and the lack of specific criteria may prevent development of a predictable ultimate capacity. This usually requires substantial base plates as well as high capacity anchor bolts Achieving full anchorage of these bolts is of primary importance and will usually require headed bolts or plates at the embedded end of the bolts co prevent pullout When anchor bolts are securely anchored into concrete, the failure mechanism is 2 ductile, tensile failure of the bolt steel.

Insufficient edge distance or insufficient spacing between boits results in a lower anchorage capacity and a brittle failure mode Post-installed bolts will be required at times for attachment of equipment which may be subjected to large accelerations during a blast. Expansion anchors should be avoided for most blast design applications unless the load levels are low. Typically “wedge” type anchors are qualified for dynamic loads although most of these ratings are for vibratory loads and are based on cyclic tests at low stress levels.

These should only be used where ultimate loads are less than the rated capacity with a margin of safety. Epoxy anchors have shown excellent dynamic capacity and. Often anchor bolts are designed for the maximum axial and shear reactions at the base of the columns as a static load. This method requires a large qumber of boits even using dynamic material properties. In reality, the bolts will yield under tensile loads and to some degree, shear loads.

That is why it is important to use ductile materials for bolts to guard against sudden failure under peak stress. It is possible to model the tensile response dynamically and take advantage of the strain energy capacity of the bolts. This allows the bolts to respond to the load-time history rather than just a peak load. A dynamic analysis is warranted only for special situations, such as where the reuise of existing bolts is important.

For typical designs, a dynamic analysis is not performed because there may aot be a cost benefit over a static bolt design. Because shear deformations are more difficult to model and generally don’t control bolt sizing, bolts are designed for the. Soil properties should be obtained from a subsurface investigation. This factor of safety can be used to convert service load capacities to ultimate strength values. A geotechnical engineer should be retained co provide soil properties for blast loads.

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Provisions must be made in the design to resist uplift loads in columns foundations and other areas where soil is placed in tension. Typically, foundations are designed to resist the peak blast load or the maximum dynamic reactions of the supported member applied as a static load. It is possible to model dynamic response but the engineer must be careful not to overestimate allowable response. TR ref provides a detailed discussion of soil behavior and recommendations for analysis and design.

Static properties are available from a number of references and are not repeated in this chapter, except to indicate minimum acceptable values. Dynamic response of these materials has been studied extensively, however, their dynamic properties are not as widely published.

Procedures for obtaining these properties will be covered here in sufficient detail to permit an accurate determination for design and analysis of petrochemical structures Stress, ksi 0 0. A typical stress-strain diagram for concrete is shown in Figure 5. As the fibers of a material are deformed, stress in the material is changed in accordance with its stress-strain diagram, In the elastic region, stress increases linearly with increasing strain for most steels.

This relation is quantified by the modulus of elasticity of the material Concrete does not have well defined elastic and plastic regions due to its brittle nature. A maximum compressive stress value is reached at relatively low strains and is maintained for small deformations until crushing occurs. The stress-strain telationship for concrete is a nonlinear curve. Thus, the elastic modulus varies continuously with strain, The secant modulus at service load is normally used to define a single value for the modulus of elasticity.

This procedure is given in most concrete texts, Masonry has a stress-strain diagram similar to concrete but is typically of lower compressive strength and modulus of elasticity For steel materials, the shape of the curve is much different than for concrete as can be seen in Figure 5. Steel is celatively ductile and is able to achieve large strains prior to rupture. Low carbon structural grade steels e.

A36, A exhibit a well defined yield point followed by a flat yield plateau. High strength steels do not have a sharp break at the elastic limit and the yield region is very nonlinear. Low carbon steel materials are particularly suited to blast resistant design because they are able to deform well beyond the elastic limit without rupturing.

This groduces a long resistance-deflection curve to absorb the blast energy while avoiding brittle fracture problems. High strength steels should be avoided for general construction due t their low ductility. Special applications, such as blast doors and shields, may require high strength materials to achieve the desired resistance.

Selection of static properties for high strength materials should be made conservatively Stress-strain relationships for soil are difficult to model due to their complexity In normal practice, response of soil consists of analyzing compression and shear stresses produced by the structure, applied as static loads.

Change in soil strength with deformation is usually disregarded. Clay soils will exhibit some elastic response and are capable of absorbing blast energy; however, there may be insufficient test data to define this response quantitatively.

Soil has a very low tensile capacity thus the stress-strain relationship is radically different in the tension region than in compression, Strength Increase Factor SIF Static properties are readily available from a variety of sources and are well defined by national codes and standards organizations.

Specifications referenced in the codes define minimum mechanical properties for various grades of material. A strength increase factor is used to account for this condition and is unrelated to strain rate properties of the material TM suggests using a 1. Application of the recommended 1. Concrete strength is specified as minimum compressive strength at 28 days, This value is used for design and is not typically increased to account for an increase in strength with age.

For evaluation of an existing structure, it may be worthwhile to determine the in-situ strength of the concrete to use in the analysis. This will not make a great difference in flexural capacity but it could be very important when examining shear resistance 5. At a fast strain rate, a greater load is required to produce the same deformation than at a lower cate This increase in the yield stress is quite significant for lower strength materials and decreases as the static yield strength increases.

For steel, the modulus of elasticity is the same in the elastic region and yield plateau for static and dynamic response. In the strain hardening region the slope of the stress-strain curve is different for static and dynamic response, although this difference is not important for most structural design applications.

A strength increase is also produced at ultimate strength F. A typical stress-strain curve describing dynamic and static response of steel is shown in Figure 5.

Lindholm surveyed available test data on dynamic properties for a number of materials. This is an extremely useful resource for information on less commonly used materials, Ultimate strength for concrete is greater under dynamic loads. Though the modulus of elasticity is also greater, this difference is small and is usually ignored. Figure 5. The faster 2 material is strained, the higher the increase in dynamic yield and ultimate strength.

In these situations, it is necessary to use the static Properties. Dynamic soil properties which are reported may be based on low strain amplitude tests which may or may not be applicable to the situation of interest.

These can be used along with ultimate bearing capacities to Perform a dynamic response calculation of the foundation for the applied blast load 5. Dynamic Increase Factors To incorporate the effect of material strength increase with strain rate, a dynamic increase factor DIF is applied to static strength values. DIFs are simply ratios of dynamic material strength to static strength and are a function of material type as well as strain rate as described above, DIFs are also dependent on the type of stress ie flexural, direct shear because peak values for these stresses occur at different times Flexural stresses occur very quickly while peak shears may occur relatively late in time resulting in a lower strain rate for shear.

A rate must be assumed and a DIF selected. The dynamic strength is determined by multiplying the static strength increased by the strength increase factor by the DIF. The time required to reach maximum response can be used to determine a revised strain rate and a revised DIF This process is repeated until the computed strain rate matches the assumed value.

There are uncertainties in many of the variables used to calculate this response and determination of strain rates with great accuracy is not warranted, TM and other references suggest selecting DIF values based on pressure range or scaled distance to the explosion source, This method groups blast loads of less than a few hundred psi into the low pressure category with a single DIF value for each stress type.

For petrochemical facilities, the vast majority of structures will fall in this low pressure category. DIF values vary for different stress types in both concrete and steel for several reasons. Flexural response is ductile and DIF values are permitted which reflect actual strain ates, Shear stresses in concrete produce brittle failures and thus require a degree of conservatism to be applied to the selection of a DIF.

Much of the data that has been published is based-on high strain rate tests and many of the recommended values are arbitrarily chosen, Table 5.

A2 contains values for structural steel, cold-formed steel and aluminum. During dynamic response, the stress level at critical sections in a member vary with strain of the section. In the elastic region, the strain across the section varies with location from the neutral axis of the member.

Beyond this region, the member experiences plastic response in which the fber stress of the entire section exceeds the elastic limit, At this point, the stress is constant over the cross section but is still changing with total member strain. Steel members experience an increase in stress in the strain hardening region until the ultimate dynamic material stress is reached. After this point, the fiber stress decreases with increasing strain until rupture occurs.

Concrete exhibits an increasing stress until the maximum compressive stress is reached after which the stress level decreases with additional deformation. Because of its brittle nature, strain hardening does not occur in concrete; however, reinforcing steel will exhibit this effect, To predict true dynamic response, it would be necessary to continuously vary the material stress with deformation.

This variation is difficult to model using SDOF analysis methods because it requires tracking a complex resistance-deflection curve at each time step. It is desirable to represent the design material stress as a bilinear stress-strain curve in which stress increases linearly with strain to yield and a constant value after yield refer to Section 7.

This produces a simple, bilinear resistance- deflection curve as shown in Figure 5. To achieve this simplification, while accurately modeling the dynamic response, it is necessary to select a design Stress equal to the average stress occurring in the actual response. This can be done by estimating a maximum response range and using recommendations in Tables 5. At higher response ranges, the design stress is increased to account for strain hardening, In the initial portion of the response, this increased design scress will result in an overprediction of resistance.

While these methods are widely available, they are quite complex and, in many cases, their use is not warranted due to uncertainties in blast load prediction, The dynamic material properties presented in this section can be used in FEM calculations; however, the simplified response limits in the next section may not be suitable.

Most FEM codes contain complex failure models which are better indicators of acceptable response. See Chapter 6, Dynamic Analysis Methods, for additional information. These limits are based on the type of structure or component, construction materials used, location of the structure and desired protection level The primary method for determining adequacy of a structure for conventional design is evaluation of the stress level achieved compared with the maximum stress permitted.

Deflections are also checked for certain members although this is typically done for serviceability or architectural reasons cather than structural requirements. Blast loaded members however, reach or exceed yield stresses to achieve an economic design. In general, the more deformation the structure or member is able to undergo without failure, the more blast energy that can be absorbed, As member stresses exceed the yield limit, stress level is not appropriate for judging, member response as is done for static elastic analysis.

In dynamic design, the adequacy of the structure is judged on maximum deformations. Limits on displacements are based on test data or other empirical evidence. Load bearing walls should normally be allowed less deformation than non-load bearing elements also because of the consequences associated with failure – The structure’s performance goal becomes an important factor in selection of maximum response values. IF it is desired to provide a high degree of protection to personnel or equipment, a low response limit is chosen, This situation may be typical of control room in which personnel are required to remain at their workstation during an emergency or for critical equipment which must be protected to implement a safe shutdown.

On the other hand, if a building is frequently unoccupied or contains low value equipment, significant damage may be permitted, up to the point of failure. Structures which are required to be reusable following a blast are typically designed to remain elastic under the predicted loads. The capacity of a member to deform significantly and absorb energy is dependent on the ability of the connections to maintain strength throughout the response.

If connections become unstable at large responses, catastrophic failure can occur. The resistance will drop thereby increasing deflections. Connections often control blast capacity for structures which have been designed for conventional loads only. Risk assessments which evaluate accident probability and potential consequences can be helpful in making the appropriate selection, The deformation limits chosen relate to a specified degree of response which can be characterized as low, medium or high.

At the highest response limits, catastrophic failure of the structure should not occur. Points of highest stress in the members will be near incipient collapse and local failures may occur but the overall structure should remain intact. It is important to remember that predicted responses may not always account for local instabilities and the actual response can be significantly greater.

The engineer must take these factors into consideration when designing or analyzing the structure to ensure the proper degree of protection is provided. The performance goal for the structure then becomes incipient failure in which portions of the structure are damaged severely but do not tear loose and become missiles. Structural collapse is not permitted and suspended equipment must be adequately anchored within the structure.

Chapter 2, General Considerations, contains additional discussion of protection philosophies. All-bolted single-angle connection beam-to-girder web. Above this angle of skew, it becomes impractical to bend rolled angles. Bent plates are not subject to the deformation problem described for bent angles, but the radius and direction of the bend must be considered to avoid cracking during the cold-bending operation.

Bent plates exhibit better ductility when bent perpendicular to the rolling direction and are, therefore, less likely to crack. Whenever possible, bent connection plates should be billed with the width dimension parallel to the bend line. The length of the plate is measured on its mid-thickness, without regard to the radius of the bend.

While this will provide a plate that is slightly longer than necessary, this will be corrected when the bend is laid out to the proper radius prior to fabrication. Skewed beam connections with bent double angles. Skewed beam connections with double bent plates. Use 8 in. Select tension flange plate dimensions To provide for an 8-in. Design the compression flange plate and connection The compression flange plate should have approximately the same area as the tension flange plate 4.

The plate width, then, is 7. Comment: The column section should be checked for stiffening requirements. Blodgett, O. Carter, C. Deierlein, G. Hsieh, and Y. Disque, R. Design the compression flange plate and connection.

Comment: The column must be checked for stiffening requirements. Welded flange-plated FR moment connection beam-to-column flange. Design the single-plate web connection. Calculate the flange force Puf. Try a 1 in. Solution: Check beam design flexural strength. From Example Design the bolts a minimum of four bolts is required at the tension flange; a minimum of two bolts is required at the compression flange. Calculate the flange force Pu f. Determine number of 1-in. Try six 1-in. Check bolt shear: From Table for six 1-in.

Check shear yielding of the end plate. Deter- mine size required to develop web flexural strength near tension bolts: 0. Determine size required for the factored shear Ru. Ru is resisted by weld between the mid-depth of the beam and the inside face of the compres- sion flange or between the inner row of tension bolts plus two bolt di- ameters, whichever is smaller.

By inspection the former governs for this example. Force Transfer in Diagonal Bracing Connections There has been some controversy as to which of several available analysis methods pro- vides the best means for the safe and economical design and analysis of diagonal bracing connections. To resolve this situation, starting in , AISC sponsored extensive computer studies of this connection by Richard In , this task group recommended three methods for further study; refer to Appendix A of Thornton Using the results of the aforementioned full scale tests, Thornton showed that these three methods yield safe designs, and that of the three methods, the Uniform Force Method see Model 3 of Thornton, best predicts both the design strength and critical limit state of the connection.

For the above reasons, and also because it is the most versatile method, the Uniform Force Method has been adopted for use in this book. The Uniform Force Method The essence of the Uniform Force Method is to select the geometry of the connection so that moments do not exist on the three connection interfaces; i.

Design by this method may be uneconomical. It is very punishing to the gusset and beam because of the moment Mub induced on the gusset-to-beam connection. This moment will require a larger connection and a thicker gusset.

Additionally, the limit state of local web yielding may limit the strength of the beam. This special case interrupts the natural flow of forces assumed in the Uniform Force Method and thus is best used when the beam-to- column interface is already highly loaded, independently of the brace, by a high shear Ru in the beam-to-column connection.

Since there is to be no gusset-to-column connection, Vuc and Huc also equal zero. Note that, since the connection is to a column web, ec is zero and hence Hc is also zero. For a connection to a column flange, if the gusset-to-column-flange connection is eliminated, the beam-to-column connection must be a moment connection designed for the moment Vu ec in addition to the shear Vu.

Thus, uniform forces on all interfaces are no longer possible. When this happens, uniform interface forces will not satisfy equilibrium and moments will exist on one or both gusset edges or at the beam-to-column interface. If the connection at one edge of the gusset is more rigid than the other, it is logical to assume that the more rigid edge takes all of the moment necessary for equilibrium. For instance, the gusset of Figure is shown welded to the beam and bolted with double angles to the column.

Force transfer, UF method special case 3. Check tension rupture of the angles. Check column flange. By inspection, the 4. Note that, for this example, l2 is negative since part of the Whitmore section is in the beam web.

The effective length factor K has been established as 0. It assumes that the gusset is supported on both edges as is the case in Figure In cases where the gusset is supported on one edge only, such as that illustrated in Figure d and possibly Figure a , the brace can more readily move out-of-plane and a sidesway mode of buckling can occur in the gusset. For this case, K should be taken as 1. Bracing connection design. Given: Refer to Figure Check web crippling N 10 in.

Table cont. If requested, the fabricator shall provide an affidavit stat- ing that the structural steel furnished meets the requirements of the grade specified. The impact test shall meet a minimum average value of 20 ft-lbs. For plates exceeding two-in. For structures [g] For members with unequal flanges, use hp instead Rev. Assumes an inelastic ductility ratio ratio of strain at fracture to strain at yield of 3.

When the seismic response modification factor R is taken greater than 3, a greater rotation capacity may be required. Fillet Welds 2a. If the load factors and combinations stipulated in Section A4 are used to design concrete structural elements, the provi- sions of ACI shall be used with appropriate f factors as given in ACI , Appendix C. Unstiffened Compression Elements The design strength of unstiffened compression elements whose width-thickness ratio exceeds the applicable limit lr as stipulated in Section B5.

The design strength of axially loaded compression members shall be modified by the appropriate reduction factor Q, as provided in Appendix B5. Stiffened Compression Elements When the width-thickness ratio of uniformly compressed stiffened elements except perforated cover plates exceeds the limit lr stipulated in Section B5.

The shear coefficient Cv is determined as follows: kv E h kE a For 1. Stiffeners may be required in certain portions of a plate girder to develop the required shear or to satisfy the limitations given in Appendix G1. Transverse stiffeners shall satisfy the requirements of Appendix F2.

AM bolts 2 – 6. The values for Fv in Table A-J3. When specified by the designer, the nominal slip resis- tance for connections having special faying surface conditions is permitted to be adjusted to the applicable values in the RCSC Load and Resistance Factor Design Specification.

When the loading combination includes wind loads in addition to dead and live loads, the total shear on the bolt due to combined load effects, at service load, may be multiplied by 0. Slip-Critical Connections Designed at Service Loads When a slip-critical connection is subjected to an applied tension T that reduces the net clamping force, the slip resistance per bolt, fFvAb, according to Appendix J3.

Appendix K3 per- tains to the design of members and connections subject to high cyclic loading fatigue. The combined stiffness of the primary and secondary framing is sufficient to prevent ponding if the flexibility constant read from this latter scale is more than the value of Cp computed for the given primary member; if not, a stiffer primary or secondary beam, or combination of both, is required.

A load factor of 1. If the center of gravity of the connecting welds lies outside this zone, the total stresses, including those due to joint eccentricity, shall be included in the calculation of stress range. Design Stress Range The range of stress at service loads shall not exceed the stress range computed as follows.

Rather, the decision as to which welding process and which filler metal is to be utilized is usually left with the fabricator or erector. To ensure that the proper filler metals are used, codes restrict the usage of certain filler materials, or impose qualification testing to prove the suitability of the specific electrode.

The load factors and load combinations recognize that when several loads act in combination with the dead load e. The mean value of arbitrary point-in-time live load La is on the order of 0.

The arbitrary point-in-time wind load Wa, acting in conjunction with the maximum lifetime live load, is the maximum daily wind. The reader is referred to the commentaries to these documents for an expanded dis- cussion on seismic loads, load factors, and seismic design of steel buildings.

Ic is the moment of inertia and Lc the un- supported length of a column section, and Ig is the moment of inertia and Lg the unsupported length of a girder or other restraining member. Ic and Ig are taken about axes perpendicular to the plane of buckling being considered. If the column end is rigidly attached to a properly designed footing, G may be taken as 1. Smaller values may be used if justified by analysis. Alignment chart for effective length of columns in continuous frames —Sidesway Inhibited.

It is not intended that these provisions be applicable to limit nonlinear secondary flex- ure that might be encountered in large amplitude earthquake stability design ATC,