B-4.7 Time Effect Factor for Wood.
As discussed in AFPA/AWC “LRFD Manual for Engineering Wood
Construction”, the time effect factors, λ, were derived based on reliability analysis that considered variability in strength properties, stochastic load process modeling and cumulative damage effects. The time effect factors are applied to the reference strengths used in the code, which are based on short-term loading test values. Time effect factors range in value from 1.25 for a load combination controlled by impact loading to 0.6 for a load combination controlled by permanent dead load. Common building applications will likely be designed for time effect factors of 0.80 for gravity load design and 1.0 for lateral load design. Further ANSI/ASCE 16-95 indicates time effect factors of 0.7 when the live load in the basic gravity load design combination is for storage, 0.8 when the live load is from occupancy, and 1.25 when the live load is from impact. It is desirable that the structure is stable following local damage to allow for rescue operations and the installation of temporary shoring, however stability in the damaged state is not a permanent condition. Therefore a time effect factor greater than that associated with permanent occupancy and less than that associated with impact is warranted. For this reason and to avoid overly conservative values for such an extreme loading, a time effect factor of 1.0, consistent with the time effect factors used for
Progressivecollapse is referred to a localized failure, due to an unexpected event such as an accidental blast, causes the failure of adjoining structural elements, which in turn spread further resulting in the collapse of the entire structure or a disproportionally large part of it. Since the collapse of World Trade Centre twin towers in 2001, structural design to resistprogressivecollapse has garnered tre- mendous attentions from civil engineering community. The DoD guideline  was the very first rigorous criteria on the design of buildings to resistprogressivecollapse. A recent Canadian standard  also reflected many new developments in this field. Both direct and indirect approaches to resistprogressivecollapse have been outlined in these guidelines with various levels of details and effec- tiveness.
The terminology of progressivecollapse is defined as‘the spread of an initial local failure from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it’ . After the event of 11 September 2001, more and more researchers started to refocus on the causes of progressivecollapse in building structures. There are design procedures to mitigate the potential for progressivecollapse in the design guidance of UK and US. In the United States the Department of Defense (DoD)  and the General Services Administration (GSA)  provide detailed information and guidelines regarding methodologies to resistprogressivecollapse of building structures. Both employ the alternate path method (APM). APM is a threat independent methodology, meaning that it does not consider the type of triggering event, but rather, considers building system response after the triggering event has destroyed critical structural members. If one component fails, alternate paths are available for the load and a general collapse does not occur. The methodology is generally applied in the context of a ‘missing column’ scenario to assess the potential for progressivecollapse and used to check if a building can successfully absorb loss of a critical member. In U.K., The UK Building Regulations  and BS5950  has led with requirements for the avoidance of disproportionate collapse. FEMA 2002  and NIST 2005  also provide some general design recommendations, which require Steel-framed structural systems have enough redundancy and resilience, such that alternative load paths and additional capacity are provided for redistributing gravity loads when structural damage occurs. Perimeter columns and floor framing in particular should have greater mass to enhance thermal and buckling resistance.
Moment Resisting Frame steel structures which have been designed based on seismic codes, are able to resistprogressivecollapse with damaged columns at different locations under seismic loading. The progressivecollapse potential has been assessed in connection with 4,7 and 15-story buildings with 4 bays by applying the alternate load path method recommended in GSA guidelines. Member removal in this manner is intended to represent a situation where an extreme event, such as vehicle impact or past earth quake shock or construction error, may cause a critical column, as a result of local or global buckling, to lose a part or whole of its load bearing capacity. In contrast with 3-D models, two- dimensional frames represent very high sensitivity to base shear reduction and element removal. In case of the middle column removal, the structure is more robust than in a corner column removal situation. The influence of story number, redundancy and location of critical eliminated elements has been discussed.
Progressivecollapse is mostly not in proportion to the cause of damage and the structure might be exposed to progressivecollapse due to a small event. In other words, during the progressivecollapse, the amount of damage is much more than the initial damage . Old buildings mostly with small- span frames had adequate strength and resistance against progressivecollapse. However, changes in the architectural styles associated with the evolution of computer- aided structural design and using high strength materials have led to advanced building systems of large spans, relatively light weight, and with more ductility. Accordingly the modern buildings have high risk under unforeseen loads .
model considers two main assumptions in the failure mechanism, which includes tensile cracking and compression crushing. Dynamic explicit analysis was used to analyze the models. This analysis is explicitly used to design large scale models with short dynamic response durations and for processes with high discontinuities. This type of analysis allows us to use large deformations theory. Tie constraint has been used to define the interaction between members. This allows the user to combine two surfaces that their meshing is different from each other. In order to determine the optimal meshing in modeling, examine the convergence response method was used. The proposed meshing is reliable enough to ensure that the applied forces were accurately calculated. The imposed loads on building include the weight of structural elements (beam, column, floor, and brace), dead and live loads on the floor of structure. Three different methods were used to investigate the progressivecollapse:
Astaneh (2002)  investigated the strength of a ty- pical steel building and floor system to resistprogressivecollapse in the event of removal of a column. They tested a specimen of size 60 ft by 20 ft one-story steel building with steel deck and concrete slab floor and wide flange beams and columns. The connections were either stan- dard shear tab or bolted seat angle under bottom flange and a bolted single angle on one side of the web. It was observed that after removing the middle perimeter column, the catenary action of the steel deck and girders was able to redistribute the load of removed column to other columns. The floor was able to resist the design dead load and live load without collapse. Damage to the system was primarily in the form of cracking of floor slab, tension yielding of the steel corrugated deck in the vicinity of collapsed column, bolt failure in the seat connections of the collapsed column and yielding of the web of the girders acting in a catenary configuration.
Progressivecollapse is defined by ASCE 7-05  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”. More generally, progressivecollapse is characterised by the loss of load-carrying capacity, of a relatively small portion of a structure. This initial damage triggers a cascade of failures, affecting a major proportion of the structure. It is worth noting that the definition of progressivecollapse is something that is under constant discussion within the engineering community, and that no single definition of the term exists at present. However, for the purpose of this paper the definition provided in ASCE 7-05 will be used. A collapse of this nature can be triggered by a many causes; including design and construction errors, as well as loading conditions with a low probability of
The gradual or instantaneous failure of a column due to explosion, accidental actions, or earthquakes leads to progressive failure. The General Services Administration of the United States (GSA  ,2003) defines this phenomenon as “a situation where local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse” and denoted as progressivecollapse. Also in ASCE  Standard 7-10: Minimum Design Loads for Buildings and Other Structures, progressivecollapse is defined 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”. The first research in progressive failure was motivated by the partial collapse of 22-story Ronan Point Tower in England in 1968, caused by a gas explosion that bumped the supporting reinforced concrete panels close to the corner of the building. The loss of supporting wall at the eighteenth floor caused the failure of above stories . The second case of progressivecollapse was the destructing of Alfred P. Murrah Federal Building in the Oklahoma City in 1995 by a bomb detonated outside of the Building .
A situation in which a localized failure in a structure, caused by an abnormal load (see paragraph 2.2), triggers a cascade of failure affecting a major portion of the structure and totally collapse. Several buildings have collapsed in this fashion in recent years, and the possibility of progressivecollapse is a source of continuing concern. Several alternative methods to deal with the problem of design for the prevention of progressivecollapse are reviewed. A computer analysis program capable of tracing the behavior of framed structures through collapse is explained. Of particular note is the capability to remove selectively any member in the structure and determine if collapse will result. Several examples using interactive computer graphics techniques in applying the collapse resistant design procedures are presented. The debris which is derived from part of structure damaged becomes great dead weight to impose remaining structure and induce totally collapse. (American Society of Civil Engineers, 2007)
Key Words: architect, buildings, collapse, construction, design
The place and priority of buildings to man’s existence and survival as he lives and carries out his activities within them is important. Despite this fact, maintenance of the existing housing stock in habitable condition still remains a great problem to be solved in Nigeria among other countries in the World, (Olagunju, 2011). Buildings, either as temporary, permanent or monumental structures needs to be properly planned, designed, constructed and maintained to obtain the desired satisfaction, comfort and safety. The desired satisfaction, comfort and safety tend to be threatened when the building failed to perform any of its principal functions of satisfaction, safety and stability. Building failure may be as a result of a total or partial failure of one or more components of a building structure. Building failure is a defect or imperfection, deficiency or fault in a building element or component. It may also be as a result of omission of performance. The degree of building failure can therefore be related to the extent or degree of deviation of a building from the “as – built” state which is in most cases represent the acceptable standard within the neighbourhood, locality, state or country. (Ikpo, 1998). Failure in building could also be of two types, namely, cosmetics and structural failure. The former occurs when something has been added to or subtracted from the building, thus affecting the structural outlooks. The later affects both the outlook and structural stability of the building (Ayininuola and Olalusi, 2004). In line with the above assertions, building collapse can simply be defined as a total or partial/progressive failure of one or more components of a building leading to the inability of the building to perform its principal function of comfort, satisfaction, safety and stability.
Many other choices and suggestions have been proposed by many structural engineers and blast experts and with continued research more other alternatives are to be expected in the near future. The challenge exists in making decisions about the best solutions because of the built- in uniqueness that are to be met for each project. Also, there is little to no official designstandards or guidelines available for engineers to follow to assist their decisions. Instead, the engineer must be competent in blast resistance and progressivecollapse research in order to
Researchers have suggested that seismically designed structures are more resistant to progressivecollapse conditions [3,4,5]. By designing seismically resistant structural elements, such as the connections, the structure’s ductility, robustness and ability to perform catenary action is also enhanced. Steel structures with complete lateral force-resisting systems capable of resisting wind and seismic loads specified by building codes are able to resist credible blast loads without lateral instability and collapse. However, explosive charges detonated in close proximity to structural elements can cause extreme local damage, including complete loss of loading carrying capacities in individual columns, beams, girders and slabs . GSA developed simplified guidelines for the design of such systems. These guidelines specify that certain elements of the frame be proportioned with sufficient strength to resist twice the dead and live load anticipated to be present, without exceeding inelastic demand ratios, based on theory related to the instantaneous application load on an elastic element. Under general progressivecollapsedesign guidelines, structural members are permitted to experience flexural inelasticity based on allowable values proposed in seismic guidelines as the amplified loading occurs for a very short period, and the long-term loading following removal is a static condition. This is the reason that most of the recent studies found in the literature employ the non-linear static (push-down) analysis. However, GSA and DoD guidelines do not require the evaluation of the plastic strength of the frame supporting the weight of the structure in a static condition, while they should.
Abstract— Progressivecollapse of buildingsare generally triggered by a local failure due to accidental actions, followed by subsequent chain effect of the structures which may result in wide range failure or even collapse of the entire buildings. To study the effect of failure of load carrying elements i.e. columns on the entire structure; 10 storey moment resistant regular RC building is considered. The buildings are modelled and analyzed for progressivecollapse using the structural analysis and design software SAP2000. The frame is subjected to loading as described by General Services Administration (GSA) guideline for carrying out linear static analysis.Linear static and nonlinear static analysis is used to evaluate the potential for progressivecollapse of RC buildings.The results include the variation of bending moment of beams and evaluation of demand capacity ratios (DCR), hinge properties and % load attempt.
Such sequential failures can propagate through the structure. If a structure loses too many members, it may lead to partial or total collapse. This type of collapse behaviour may occur in framed structures, such as buildings (Griffiths, et al . 1968, Burnett, et al. 1973, Ger, et al . 1993, Sucuoglu, et al . 1994, Ellis and Currie 1998, Bazant and Zhou 2002), trusses (Murtha-Smith 1988, Blandford 1997), and bridges (Ghali and Tadros 1997, Abeysinghe 2002). On the morning of 16 May 1968, Mrs. Ivy Hodge, a tenant on the 18th floor of the 22-story Ronan Point apartment tower in Newham, east London, struck a match in her kitchen. The match set off a gas explosion that knocked out load-bearing precast concrete panels near the corner of the building. The loss of support at the 18th floor caused the floors above to collapse. The impact of these collapsing floors set off a chain reaction of collapses all the way to the ground. The ultimate result can be seen in Figure 1(a): the corner bay of the building has
The attention of structural engineers was first drawn after the accidental collapse of the Ronan Point tower in Canning Town, UK on May 1968. The cause of the collapse was a human error gas explosion that knocked out the precast concrete panels near the 18th floor causing the floors above to collapse.
Structural progressivecollapse has been the focus of extensive research during the past few years because of the increasing rate of victims resulting from natural disasters (e.g., earthquakes and hurricanes) or human-made disasters (Example: bomb blasts, fires and vehicular impacts). Structural designers have traditionally focused on optimizing the cost of constructed facilities while meeting code requirements. Unfortunately, most of the structures have been designed to resist gravity loads and lateral loads resulting from wind or moderate earthquakes. The structural behavior of a constructed facility when subjected to loads beyond conventional design is not typically addressed.
In addition, a memorandum published by the Federal Highway Administration (FHWA) with the subject “Clarification of Requirements for Fracture Critical Members”, defined a Fracture-critical Member (FCM) as “a steel member in tension, or sub-element within a built-up member that is in tension whose failure is expected to result in the collapse of the bridge.” (Lwin 2012). Moreover, referring to the National Bridge Inspection Standards (NBIS) and AASHTO Manual for Bridge Evaluation (MBE) Second Edition (2010), identifying FCM is required to study the redundancy rating and to evaluate a bridge’s structure. Furthermore, AASHTO LRFD Bridge Design Specifications Sixth Edition (2012) describes an FCM as “a component in tension whose failure is expected to result in the collapse of the bridge or the inability of the bridge to perform its function” to emphasize the inability of a bridge to safely carry some degree of traffic (live load) when in a damaged stage. Although the definition indicates that the failure of an FCM may lead to structural collapse, the required loading level for a collapse to occur remains unclear (the live load level might be less than the full design live load for the strength limit state load combination). The definition leaves much to engineering judgment and there are disagreements about what type of members should be classified as FCMs.
Buildings are vulnerable to progressivecollapse if one or more columns are lost due to extreme loadings; which underlines the importance of establishing the likelihood of progressive of structures in order to avoid catastrophic events. Published design guidelines and codes are now available to design engineers for mitigating progressivecollapse or minimizing the damages caused by progressivecollapse of a Sasani and Kropelnicki (2008) made a 3/8 model of a ed and tested and compared with a detailed finite element model of the structure. Many different details were analyzed to determine the adequacy of the structure. The finite element model (FEM) was compared to a demand capacity ratio (DCR) method and determined that the DCR method is overly conservative. Giriunas (2009) did a study involving the comparison of real building behavior to that of a computer model he developed on the computer program SAP2000. Giriunas placed strain gauges throughout various es in the structure to gather physical data of the building’s response to the loss of a sequential set of columns. While his experiment dealt with a steel framed structure, the information provided by his study gives great insight into the steps used to ther experimental data and how to use it to determine the credibility and accuracy of a specific analysis method. This paper presents important specification of GSA guidelines for progressivecollapse analysis. Linear static, linear dynamic en followed for progressivecollapse analysis.
Some studies (Habibi et al., 2012; Gouverneur et al., 2013) have indicated that integrity reinforcement of slabs may enhance the behavior of steel and RC structures. Few studies have emphasized on membrane action of slabs (Keyvani et al., 2014) and increasing of structural members’ catenary action by GFRP (Qian and Li, 2015) or steel cables (Astaneh-Asl, 2003) to resist against progressivecollapse. A feasible proposition would be to consider alternative fall-back parameters such as secondary load carrying mechanisms that can help to reduce the severity of the collapse, should it actually occur (Qian et al., 2014). Author has proposed some methods to prevent progressivecollapse by using diagonal steel cables for existing steel structures (Izadifard, 2013), horizontal steel cables for RC buildings (Ghanbari, 2010) and hat trusses seated on top of existing or new steel and RC structures (Badinrad, 2013). Also beam-column connections retrofitting to enhance the survival capacity of the steel framed structures subjected to a blast or impact is proposed by changing the partial-strength shear- resisting joints to the full-strength moment-resisting joints (Liu, 2010; Nikfar, 2013).
Numerous studies were found in the literature highlight- ing the response of steel frames under progressive col- lapse. Earlier studies accounting for dynamic redistribu- tion of forces in a progressivecollapse scenario were carried out by Mc Connel (1983) , Casciati (1984) , and Pretlove (1986) . Mc Connel (1983)  investi- gated the progressivecollapse failure of warehouse racking, where local failure was initiated by truck colli- sion or static overload. Several analytical studies of pro- gressive collapse were conducted for simple buildings [4,5] to validate analytical procedures and focus on ob- taining fundamental aspects of the progressivecollapse behavior. Progressivecollapse resistant-design in steel frame buildings was studied by Gross and McGuire (1983) . In his study, the behavior of 2-D moment resisting steel frames with the loss of one of the columns or increased load on the beams representing fallen debris was examined numerically. Pretlove (1986)  studied the dynamic effects that occur in the progressive failure of a simple uniaxial tension building and concluded that a building that appears to be safe under static load redis- tribution may actually be unsafe if the transient dynamic effects were taken into account. In another study, Pret- love (1991)  carried out experimental and numerical