Essay: Progressive collapse of buildings

Literature Review
2.1. Introduction
This chapter provides background and a literature review regarding the progressive collapse of buildings. First, the definition and famous examples of progressive collapse are presented. Next, design approaches and analysis procedures for progressive collapse of buildings are described. Next, the current guidelines for the prevention of progressive collapse are reviewed. Especially, overviews of the General Services Administration (GSA, 2003) and the Department of Defense (DOD, 2005) guidelines are described. Lastly, review from other researches.
2.2. Definition of Progressive Collapse
Progressive collapse has been used to describe the spread of a local failure in a manner analogous to a chain reaction that leads to partial or total collapse of a building. The underlying characteristic of progressive collapse is that the final state of failure is disproportionately greater than the failure that initiated the collapse. This progressive collapse may come from gas explosions, bomb explosions, vehicular collisions, aircraft collisions, tornados, and the like. Thus, when buildings are subjected to such abnormal loads, they may sustain extensive damage
(ASCE) Standard 7-05 defines progressive collapse as ���the spread of an
initial local failure from element to element resulting, eventually, in the collapse of an entire structure or a disproportionately large part of it��� (ASCE 2005). The disproportionality refers to the situation in which failure of one member causes a major collapse, with a magnitude disproportionate to the initial event. Thus, ���progressive collapse��� is an incremental type of failure wherein the total damage is out of proportion to the initial cause. In some countries, the term ���disproportionate collapse��� is used to describe this type of failure.
A similar definition of progressive collapse is provided in GSA 2003 guidelines, ���a situation where local failure of a primary structural component leads to the collapse of adjoining members, and hence, the total damage is disproportionate to the original cause��� (GSA, 2003).
2.3. Examples of Progressive Collapse
This section has some of the most famous examples will be described below
Ronan Point Apartment Tower in 1968 , Alfred P. Murrah Federal Building in 1995 and Jackson Landing Skating Rink in 1996.
2.3.1. Ronan Point
2.3.1.1. Introduction
Ronan Point was a development of apartment buildings in London. It was built between 1966 and 1968. On the morning of May 16, 1968, a gas leak caused an explosion in an apartment of the 18th floor of one of the buildings. The explosion blew out an exterior wall panel. The loss of an exterior wall triggered the collapse of the upper floors followed by the collapse of the floors below due to the impact of the falling upper floors.
2.3.1.2. Description of the structure
The Ronan Point buildings were 64 m tall, 22 story apartment buildings. With five apartments per floor, the footprint of the building with the collapse measured
24.4 m by 18.3 m. The structural system, including the walls, floors, and staircases was precast concrete. Each floor was supported directly by the walls in the lower stories. The wall and floor system fitted together through slots and were bolted. The connections were filled with dry packed mortar to secure the connection. The system used in Ronan Point was selected because of ease of construction. The structure was assembled by lifting the precast concrete panels with a crane and then bolting them together. In essence, the structure was like a ���house of cards��� with no redundancy for load redistribution in the event of a local failure.
2.3.1.3. The event
On the morning of May 16, 1968 a gas explosion blew out an outer panel of the 18th floor of one the buildings in Ronan Point. The loss of a bearing wall in the 18th floor caused the progressive collapsed of floors nineteen through twenty-two. Then, a second phase of progressive collapse occurred. The dynamic loading imparted by the falling debris triggered the progressive collapse of floors seventeen and below. The southeast corner of the building collapsed to the ground level. The collapse destroyed the living room portions of the apartments, leaving intact the bedrooms, except for floors seventeen through twenty-two.
2.3.1.4. Progressive collapse issues
The British government formed a team to investigate the causes of the collapse of the Ronan Point Tower. The investigating team concluded that the explosion was small and estimated that a pressure less than 10 psi was originated from the outburst. The primary evidence of a small explosion was that the hearing of the person who lit the match was not damaged. Tests were performed to estimate the structural capacity of the as built Ronan Point tower.
The results showed that the kitchen and living room walls would fail at a pressure of about approximately 11.7 kPa (1.7 psi), while the exterior wall would fail at a pressure of approximately 20.7 kPa (3 psi).
The collapse of the Ronan Point building was attributed to its lack of structural integrity. There was no alternate load path for redistribution of forces at the onset of the loss of a bearing wall. Therefore, as the exterior wall of the 18th floor apartment was blown out, the exterior walls of the upper floors immediately collapsed. The impact loading of the falling debris on floor seventeen was sufficient to exceed its capacity and to trigger the sequential failure of the lower 16 floors as shown in Figure (2.1).
The investigation of the Ronan Point collapse showed other interesting aspects of the design. It was determined that strong winds and/or the effects of a fire could also have caused progressive collapse. In addition, it was found that the building had been built with very poor workmanship. Only half of the specified mortar had been placed in the connections and bolts and nuts were not tightened as specified. However, it was concluded that this issue had little impact on the event of May 16, 1968. Although the building was rehabilitated and strengthened, continuing concerns about structural safety led to its demolition in 1986.
Figure 2.1. A partial collapse of Ronan Point apartment tower in 1968 (Wikipedia).
2.3.2. Alfred P. Murrah Federal Building
2.3.2.1. Introduction
The Alfred P. Murrah Building ( Murrah Building), located in Oklahoma City, Oklahoma, was an office facility for the U.S. government. On the morning of April 19, 1995 the Murrah Building was the target of a terrorist attack in which a truck bomb was detonated in front of its north side. The explosion caused extensive structural damage to the building.
2.3.2.2. Description of the structure
The Murrah Building was designed and constructed between 1970 and 1976. The structural configuration of the Murrah Building was composed of a nine-story reinforced concrete ordinary moment frame. The overall plan dimensions were approximately 67 m in the east-west direction and 30.5 m in the north-south direction. The architectural plan of the building consisted of ten 6.1 m bays in the east-west direction and two 10.7 m bays in the north-south direction with shear walls and other localized columns in the middle of the south side of the building. The typical floor height was 4 m from the third to the eighth floor and 4.3 m for the ninth floor. A key feature of the building was a transfer girder at the third floor level in the north side of the building. The transfer girder had a span of 12.2 m and supported columns spaced at 6.1 m in the upper floors. The curtain wall located in the north side was set back about 1 m in the first two levels, providing an open space below the third level. Vertical circular ventilation shafts serve as support to spandrel beams in the east and west ends. The infill walls between spandrels were 75-mm granite panels backed by vertical steel studs and drywall.
2.3.2.3. The event
On the morning of April 19, 1995 a truck bomb was parked on the street right in front of the north side of the Murrah Building. The bomb was detonated, causing extensive damage to the Murrah Building and various degrees of damage to other buildings in the vicinity of the explosion. Most of the north half of the rectangular footprint was destroyed. The damage spread the entire 21.3 m width of the building. Three columns that supported the transfer girder in the third floor were immediately destroyed by the blast, triggering progressive collapse of the upper stories. It was estimated that roughly half of the usable space in the building collapsed. The east end wall suffered significant damage. The blast removed some of the granite panels of the infill walls from the third through the sixth floor. Some the granite panels in the seventh floor were fractured. Other panels randomly failed in flexure due to inward or outward pressure. All granite panels in the west end wall remained in place but some failed in flexure due to outward or inward pressure from the blast. Damage to the south side of the building was mild and limited to failure of glazing and door frames.
2.3.2.4. Progressive collapse issues
The structure was designed as a reinforced concrete ordinary frame in accordance with ACI 318-71. A building performance evaluation team assembled by the Federal Management Emergency Agency (FEMA) determined that the design was adequately performed following the existing codes at the time and that the building was very well detailed. In agreement with the governing codes, the building was not designed to resist blast, earthquakes, or any other type of extreme loading condition. It was estimated that the explosion had a yield equivalent to approximately 1.8 Mg of TNT and that the bomb was located approximately 4.9 m from one of the columns in the north side of the building. The blast immediately removed this column, which supported the transfer girder of the third floor. The investigating team determined that an ordinary moment frame could not allow for the redistribution of load that resulted from the removal of a first floor column. The assessment team found during the investigation that the removal of a column in the first floor would redirect additional loads to the laterally adjacent columns, exceeding their yield moment and shear capacity. The loss of these columns would leave the transfer girder only partly supported as shown in Figure (2.2).
The investigating team suggested that if more recently developed detailing, such as those present in special moment frames used in seismic regions had been in place, the collapsed area would have been reduced at least by 50 % and at most by 80 %.
Figure 2.2.The Alfred P. Murrah Building after the bombing and just shortly before the May 23, 1995 demolition of the building (Social Security).
2.3.3. Jackson Landing Skating Rink
2.3.3.1. Introduction
Jackson Landing Skating Rink was an unheated, covered skating rink in Durham, New Hampshire .After a heavy snow storm in 1996, the entire roof covering the ice collapsed completely.
2.3.3.2 Description of the structure
The roof structure was a pre-engineered rigid frame structure (Figure 2.3), approximately 210 ft (64 m) long by 30.5 m (100 ft) wide. There were 9 bents spaced at 6.4 m (21 ft), and column-and-beam end walls. The roof structure was metal deck over steel Z-shaped and C-shaped purlins that were bolted to the top flange of the bents. lateral loads were carried by cable cross bracing in 3 bays.
The bents rested on cast-in-place circular concrete piers that extended typically approximately 1.5 m (5 ft) below grade. The thrust at the base of each bent was supported by two steel tie rods that extended under the structure, embedded in concrete-filled trenches below the ice, to connect the concrete piers on opposite sides of the bents. Anchorage for the steel rods was to be through steel plates that were to be cast into the piers, positioned outboard of the anchor bolts for the columns.
Figure 2.3.The Jackson Landing Skating Rink framing.
2.3.3.3 The event
Over the days preceding the collapse, snow accumulated to about 750 mm (30 in) depth. The unheated, relatively low-slope rink roof structure accumulated all the new snow, loading it to approximately 1.3 kPa (27 psf) at the time of the failure (the design load was approximately 1.9 kPa [40 psf]).
The failure began at one end of the rink when anchorage for the thrust tie rods for one of the bends failed suddenly. The collapse of one bent caused the anchorage to fail at two adjacent bents. The next three bents failed by overload as the collapsing purlins successively pulled down on the roof structure. The last three bents at the far end of the structure from the failure origin point toppled over from lateral load induced by the progressive collapse.
2.3.3.4 Progressive collapse issues
An investigation of the failure traced the initial failure to misplaced anchor plates. At the first failure location, the anchor plates had been placed inboard of the anchor bolts for the columns. The progressive collapse began when one of the piers with a misplaced anchor plate split vertically due to column thrust, allowing the column base to kick outward. Subsequent failures were from either overload as the downward pull on purlins added to stresses caused by roof snow or by lateral instability. Lateral instability was a contributor to this failure in part because the open structure required design for relatively low wind loads. As such, bracing in the direction perpendicular to the bents was relatively light, and insufficient to carry the forces from the collapsing structure.
This failure is particularly interesting because it was a horizontal progression of failure, rather than the more common vertical progression. It calls attention to the need, when evaluating progressive collapse potential, to consider the ability of horizontal bracing to support extraordinary overloads, or for providing means to limit horizontal progression of failure in otherwise vulnerable structures by detailing connections to avoid transfer of failure loads beyond pre-defined failure-limiting locations in such structures.
2.4. Design Approaches for Progressive Collapse
This section will describe the design approaches used to prevent the progressive collapse, according to ASCE -07-2005 there is two approaches first the direct design and second the indirect design.
2.4.1. Indirect Design Approach
In the indirect method, must use a prescriptive approach to increase the overall robustness of the structure. the indirect method is likely to be the primary method used to enhance the robustness of the type of buildings because the risk of progressive collapse is low for most buildings.
Provisions for general structural integrity may be in the form of prescriptive requirements for minimum joint resistance, continuity and inter-member ties that will provide a robust, stable and economical design. The indirect design approach has the distinct advantage of being the easiest to apply and provides a uniformity of compliance on all projects. Although this event independent approach is not based on detailed calculations of the structural response to abnormal loading, it results in continuous tied reinforcement for concrete frame structures and stronger connections for steel frame structures to allow the structural elements to develop more of their capacity (either in flexure or membrane action) when subjected to abnormal loading conditions. Although vertical loads are not resisted efficiently by
horizontal ties, loads that were supported initially by the damaged portions of the structure will be redistributed to undamaged elements.
2.4.1.1.Tie Requirements
If all members are structurally connected by joints capable of transferring the specified capacity in tension, shear, or compression without reliance on friction due to gravity loads or when additional tie members are provided as specified below.
Structures are designed to withstand at a minimum level of horizontal load, typically defined as a percentage of each floor���s weight applied simultaneously on all floors, which is checked separately from the effects of seismic or wind load. The magnitude of the horizontal load must be determined; however, this may be as low as 0.2 %, as recommended by the Structural Stability Research Council (SSRC 1998) for overall stability checks. To resist progressive collapse, however, key elements of a structure must be tied together so that redistribution of forces could occur due to a local failure. The specified capacities of such ties shouldn’t be less than the capacities determined by the normal design loads.
2.4.2. Direct Design Approach
In the direct design methods, resistance against progressive collapse is provided by enhancing the strength of key structural elements to resist failure under postulated abnormal loads or designing the structure so that it can bridge across the local failure zone.
2.4.2.1. Specific local resistance method
Explicitly designs critical vertical load bearing building components to resist the design level threat, such as blast pressures.
The specific local resistance is often the only rational approach when retrofitting an existing building. The cost of bringing the building into compliance using other methods may be so great as to make the solution impractical.
2.4.2.2. Alternate load path method
Localizes response by designing the structure to carry loads by means of an alternate path in the event of the loss of a primary load bearing component.
An advantage of this approach is that it promotes structural systems
with ductility, continuity, and energy absorbing properties that are desirable in preventing progressive collapse. This approach would certainly discourage the use of a large transfer girder that prevents a significant number of the columns from extending to the ground floor. This method is also consistent with the seismic design approach used in many building codes throughout the world. The seismic codes promote regular structures that are well tied together. They also require ductile details so that plastic rotations can take place.The alternate load path method has been selected by (GSA 2003) & In the DOD UFC 4-023-04 (DOD 2005).
2.5. Analysis Procedure for Progressive Collapse
There is four different analytical procedures may be used to investigate the structures behavior;
Linear Static (LS),
Nonlinear Static (NLS),
Linear Dynamic (LD), and
Nonlinear Dynamic (NLD).
Many previous researchers investigated the advantage and disadvantage of each analysis procedures for progressive collapse analysis will be viewed in section (Review from old researches).
A complex analysis is desired to obtain better and more realistic results representing the actual nonlinear and dynamic response of the structure during the progressive collapse. However, both GSA and DOD guidelines prefer the simplest method, linear static, for the progressive collapse analysis since this method is cost-effective and easy to perform. Therefore, one of the objectives in this research is to compare the performance of the simplest and most complicated analysis procedures (i.e., Linear Static and Nonlinear Dynamic procedures, respectively) for evaluation of the progressive collapse.
2.5.1. Linear Static Procedure
The primary method of analysis presented in the GSA guidelines is the linear
static (LS) approach. In general, the (LS) procedure is the most simplified of the four procedures, and thus the analysis can be completed quickly and easy to evaluate the results. However, it is difficult to predict accurate behavior in a structure, due to the lack of the dynamic effect and material nonlinearity by sudden loss of one or more members. The analysis is run under the assumptions that the structure only undergoes small deformations and that the materials respond in a linear elastic fashion. The (LS) procedure were developed to provide minimum requirements for evaluating the potential for progressive collapse particularly for buildings 10-stories or less as the guide lines of (GSA 2003).
2.5.2. Nonlinear Static Procedure
The nonlinear static (NLS) procedure, geometric and material nonlinear behaviors
are considered during the analysis. The (NLS) procedure is widely performed for a lateral load called pushover analysis and for a vertical load called vertical pushover analysis. For progressive collapse vertical pushover analysis, a stepwise increase of vertical loads is applied until the maximum loads are reached or until the structure collapses. This procedure is a step above the linear static procedure because structural members are allowed to undergo nonlinear behavior during the (NLS) analysis. However, vertical push over analysis for the progressive collapse potential might lead to overly conservative results. Also, the (NLS) procedure still does not account for the dynamic effects, therefore it is ineffective to use for progressive collapse analysis.
2.5.3. Linear Dynamic Procedure
Dynamic analysis accounts for dynamic amplification factors, inertia, and damping forces, which are calculated during analysis. Considering these dynamic parameters, dynamic analysis is much more complex and time-consuming than static analysis, whether it is linear or nonlinear. However, the linear dynamic (LD) procedure provides more accurate results, compared with static analysis. The (LD) procedure still needs to consider nonlinear behaviors for better results. For the structure with large plastic deformations, it should be careful to use this analysis because of incorrectly calculated dynamic parameters.
2.5.4. Nonlinear Dynamic Procedure
The nonlinear dynamic (NLD) procedure is the most detailed and thorough method of progressive collapse analysis. This method includes both dynamic effect and nonlinear behavior of the progressive collapse. More accurate and realistic results can be obtained from the (NLD) method while it is very time consuming to evaluate and validate analysis results. (NLD) analysis is performed by instantaneously removing a load bearing member from the already loaded structure and analyzing time history of the structure response caused by the loss of that member.
2.5.4.1. Dynamic Effect
Dynamic effects may come from many sources during the collapse. When a structural member is failed, the structure transfers the load of that member and comes to rest in a new equilibrium position. During this dynamic load redistribution, internal dynamic forces affected by inertia and damping are produced and vibrations of building elements are involved. A sudden release in forces from any failed member can be another source of dynamic effects. Progressive collapse is generally initiated by dynamic event such as explosion, impact, and instantaneous failure of a structural member such as a connection. Therefore, dynamic effects for frame structures should be taken into consideration in progressive collapse analysis.
2.5.4.2 Nonlinear Effect
Geometrical and Material Nonlinearity
The performance of any structure under abnormal loadings depends not only on
its geometrical properties, but also on the properties of the materials used to construct the structure. Member stiffness ratio is derived to account for geometrical nonlinearity and member shear deformation. The effect of shear deformation is generally insignificant for the conventional framed structure, but it can be considerably important for heavy transverse loading. Geometric nonlinearity is commonly described in terms of ���P-Delta Effect��� in the model. Member axial compressive forces act through the displacement of one end of a member relative to the other amplify the lateral bending response of a beam column. Therefore, the P-Delta effect influences the transverse bending stiffness of an element.
Most failure or collapse causing in typical structures are mainly due to the advent
of nonlinear material behavior, referred to as post-elastic or plastic behavior. Therefore, material properties such as yield strength, ultimate strength, and ductility are important parameters to design buildings with safety.
Catenary Action
Failure of a column creates a double span condition in the adjoining beams above the failed column. If the beams have large moment capacity and the connections have sufficient ductility and substantial inelastic rotational capacity, excessive deformation occurs in the double span, resulting in the sagging floor. The beams act as cables between columns, developing significant tensile forces that the connection must be able to withstand. The double span across the failed column can be supported by catenary action. Alternately, the vertical loads start to be transferred upward through tension in columns above the failed column and the remaining structure transfers the loads to adjacent and unfailed spans.
Catenary action has a significant effect on progressive collapse mitigation. About
20 story buildings can be supported by catenary action after the removal of a column at the first floor. Very conservative results are obtained if the progressive collapse analysis ignores the effect of catenary action. Catenary action can be applied to the finite element models, as ���P-Delta with large displacement��� in SAP2000.
2.6. Guidelines for Prevention of Progressive Collapse
Design guidelines divided into two ways to prevent progressive collapse
Firstly Practical Design ways which is not related to any code and secondly the Design Guide line from The DOD UFC (DOD 2005) and (GSA 2003).
2.6.1. Practical Design Guidelines
Structural features
* Closely spaced beams framing into a girder may improve load redistribution.
* Closely spaced columns may allow for improved load redistribution.
* Allow framing to cantilever from first bay in to the perimeter; this recesses the exterior columns relative to the fa��ade and reduces their exposure to exterior hazards.
* Consider resistance to collapse in both directions; do not assume ���plane frame��� behavior.
* Avoid discontinuities that will cause load concentrations.
* A regular, symmetric building plan will allow for load sharing and redundancy.
* Multi-span beams/girders will provide greater continuity, resulting in less deflection and increased load redistribution in the event of column loss.
* Eccentricities may create large moment demand under additional load.
Steel structures features
Steel is a ductile material and provides good resistance to progressive collapse. It has a relatively high strength to weight ratio that allows for reduced weight of construction, which is advantageous for progressive collapse resistance. This lightness may become a liability, however, when assessing blast resistance. In buildings not specifically detailed for blast resistance, it is likely that the cladding and floor system will fail, leaving the steel frame intact. Steel structural systems with enhanced resistance to progressive collapse include perimeter moment frames and systems with special features such as strong floors or belt trusses.
(i) Beam design
* Lateral support provided for full length of beam will prevent lateral-torsional-buckling. Loss of floor slab adjacent to a beam or change in support conditions can change the un braced length and weaken the beam.
* Adding stiffener plates to specific beams will reduce local buckling.
* Using seismically compact sections.
* Beam should be laterally braced to reach plastic moment capacity in both positive and negative moments assuming that the slab is ineffective in lateral bracing.
* Consider high-strength bolted connections to prevent brittle failure from concentrated stresses at weld locations.
If welds are used, use notch-tough weld metal recommended.
* Design connections for two limit states:
1) developing beam plastic moment.
2) developing beam axial tension capacity.
* Connections should be such that they permit large plastic deformations without brittle failures.
* Size bolted connections to prevent block shear and other brittle-type rupture failures.
* For nonlinear analyses, consider maximum plastic rotation after formation of plastic hinges.
* If possible make all beam-column connections fully restrained.
* For a composite floor system, design beams to be un shored rather than shored to provide extra strength in the beam.
* When using plastic analysis, ensure that local buckling or shear failure will not occur prior to developing full plastic moment capacity
(iii) Slab design
* A concrete slab on metal deck can be used to provide full lateral support to beams.
* Provide additional reinforcing steel, bars in both directions as opposed to welded wire fabric to allow slab to develop adequate membrane capacity.
* Lap reinforcement for continuity; do not use mechanical splices unless well staggered.
* Lightweight concrete floor slabs will reduce load but the blast resistance performance can be enhanced by use of normal weight concrete.
* Reinforce slab to carry self-weight in case of column or beam loss.
(ii) Column design
* Check column stability for greater un braced length due to loss of adjacent beams, increased axial load due to loss of adjacent columns, and for axial-moment interaction from beams delivering their plastic moment to the columns.
* If possible, use concrete-filled tube columns or concrete-encased wide flange shapes designed .
* For built-up columns, use notch-tough weld metal.
* Columns should be designed stronger than beams to ensure plastic hinging of beams.
* Provide column stiffener plates (continuity plates) to prevent prying of column flanges when beams develop catenary tension. Stiffeners must be capable of transferring catenary tension from beam to beam across the column web.
* Add doubler plates to column web.
* For narrow columns, provide lateral flange bracing to reduce un braced length.
* Size column splices to develop axial tension capacity of columns and permit large plastic deformations.
2.6.2. Design Codes Guidelines
The progressive collapse by disasters and terrorist attacks in recent years has further created an urgent need for all code-writing bodies and governmental agencies to provide design guidelines and criteria to prevent or minimize progressive collapse. below (Figure 2.4), shows the timeline of major catastrophic events followed by major building codes changes for progressive collapse mitigation.
Figure 2.4 Timeline of major catastrophic events followed by major building
Codes changes for progressive collapse mitigation.
2.6.2.1. The DOD UFC (DOD 2005)
The U.S. Department of Defense published a document, ���Design of buildings to
resist progressive collapse���, in the frame work of the Unified Facilities Criteria (UFC) (DOD, 2005).
This document was prepared for the new DOD construction such as military buildings and major renovations. Especially, all DOD buildings with three or more stories are required to consider progressive collapse. The DOD guideline can be
applied to reinforced concrete, steel structures masonry, wood and cold-formed steel
structural components.
The DOD guideline describes how to analyze and design the building structures to
resist progressive collapse. A combination of direct and indirect design approaches was used, which depends on the required level of protection for the facility: indirect design for very low and low levels of protection, and both indirect and direct design (Alternate Path) for medium and high levels of protection.
2.6.2.2. The GSA 2003
The U.S. General Services Administration (GSA) guideline, entitled ���Progressive
collapse analysis and design guidelines for new federal office buildings and major
modernization projects���, was specifically prepared to ensure that the potential for
progressive collapse is addressed in the design, planning, and construction of new federal office buildings and major modernization projects (GSA, 2003). The intent of the guidelines is to prevent widespread collapse after a local failure has occurred.
Based on the GSA guidelines, progressive collapse analysis is accomplished by
the implementation of the alternate path method of design. The primary method of
analysis in this design guideline is the linear elastic and static approach.
Linear procedures are used for low- to medium- rise structures, with ten or less stories and typical structural configurations.
The GSA guideline recommends that the use of nonlinear procedures should be considered for the buildings with more than ten stories.
The GSA guidelines are useful guidance for minimizing the potential for
progressive collapse in the design of new and upgraded buildings, as well as for
evaluating the potential for progressive collapse in existing buildings.
In the Review from old researches Comparison between GSA and DOD will be done for their approaches and evaluating structure,
As well in this study GSA progressive collapse guidelines are used to assess the progressive collapse potential of two existing steel buildings. The detailed GSA recommendations and loading conditions for a computer model and column removal used in this study are described.
2.7. Review From Other Researches
In this section, a number of technical papers and reports concerned with the progressive collapse of steel building with different design guidelines from GSA and DOD and different analysis procedures.
The aim of this section is to explore what is the currently procedure used for analyzing current building and how to prevent progressive collapse and improve building situation.
R. Shankar Nair [ 6 ]
Provide a definition for progressive collapse and different approaches with codes,
– Progressive collapse is the collapse of all or a large part of a structure precipitated by damage or failure of a relatively small part of it. Prevention of progressive collapse is one of the unchallenged imperatives in structural engineering. But the building���s susceptibility to progressive collapse should be of particular concern only if the collapse is also disproportionate. the engineering imperative should be not the prevention of progressive collapse but the prevention of disproportionate collapse (be it progressive or not).
– There are three approaches to designing structures to reduce their susceptibility to disproportionate collapse:
*Redundancy or alternate load paths, where the structure is designed such that if any one component fails, alternate paths are available for the load in that component and a general collapse does not occur.
*Local resistance, where susceptibility to progressive/disproportionate collapse is reduced by providing critical components that might be subject to attack with additional resistance to such attacks.
*Interconnection or continuity, which is not a third approach separate from redundancy and local resistance, but a means of improving redundancy or local resistance or both.
– Approaches Used in Codes and Standards
The following Figure shows which of these approaches to preventing disproportionate collapse are used in each of the five codes and standards.
Figure 2.5 Different approaches of design used in codes
A. Saad, A. Said & Y. Tian [ 7 ]
Provide a comparison between GSA 2003 and DOD 2005 and a case study using non linear static method with this two codes,
1- General Services Administration (GSA 2003)
Different analysis techniques are considered in the GSA guidelines including: linear elastic static and dynamic analysis and nonlinear static and dynamic analysis techniques. For each of these techniques the GSA guidelines mandates loading values and acceptance criteria for evaluation.
For static analysis procedures the loading value is taken as:
Load = 2(DL + 0.25 LL) (1)
while for the dynamic analysis procedures the loading value is:
Load = DL + 0.25 LL (2)
where DL and LL are the dead and live loads of the structure, respectively. An amplification factor of 2 is used in the static analysis loading equation to account for the dynamic effect.
Evaluation in the linear elastic analysis procedures is based on the demand capacity ratio (DCR), while in the nonlinear analysis procedures it is based on the plastic hinge rotation and displacement ductility ratios.
2- Department of Defense (DOD 2005)
Following the same scenario of the GSA .Three analysis techniques are presented as the linear static, nonlinear static and the nonlinear dynamic analysis procedures.
For linear and nonlinear static analysis procedures, the following amplified load combination is applied to the bays adjacent to the removed element:
Load = 2[( 0.9 or 1.2)DL + (0.5LL or 0.2S)] + 0.2W (3)
While for the nonlinear dynamic analysis procedure, the following load combination is used:
Load = (0.9 or 1.2)DL + (0.5LL or 0.2S) + 0.2W (4)
where S and W are the snow and wind loads, respectively. For linear static analysis procedures, iterations are requited since elements are removed from the model if their ultimate capacities are exceeded. For the nonlinear analyses, the evaluation is performed based on the stresses and forces in the elements and connections as well as deflection and plastic hinge rotation values which may require additional analysis iterations with new initial.
A study was performed to compare the two main standards for evaluating structures��� vulnerability to progressive collapse with non linear static procedure, namely the GSA and DOD standards. It was concluded that ,
��� The percentage of load at which the potential collapse occurs (for nonlinear static analyses) in the DOD procedures are less than the percentage of load at which the collapse occurs in the GSA procedures. This is due to the higher applied load intensity in the DOD than that of the GSA.
��� Structure may have a huge difference in its design or evaluation results if the two standards are used.
��� One of the main reasons resulting in such a difference is the application of the wind load in the DOD, even though the wind speed used in the analyses of the study is a basic wind speed with a moderate exposure.
��� The value of deflection at the collapse point in case of the DOD in slightly higher than those in case of the GSA.