Revised; Originally Published: 2002 / 6 pp.

Revised; Originally Published: 2002 / 8 pp.

There is vigorous debate regarding possible changes in the spatial and temporal attributes of chaparral fire regimes within southern California. We employed a novel approach to reconstruct a multi-century record chaparral fire history and to evaluate the effects of climate on these fire regimes across three southern California National Forests. The research in this dissertation is presented as three related studies. The first focused on using fire scars and tree rings from isolated stands of bigcone Douglas-fir (Pseudotsuga macrocarpa), which we demonstrate as reflective of the temporal and spatial patterns of fire in the surrounding chaparral. We found many extensive fires were apparent in both the pre-and post-twentieth century period indicating that such events were a natural component of the system. The second study applied the same approach but the spatial extent of the project was expanded to examine fire histories at a regional scale. Our results confirm that widespread fire events have, for centuries, likely played a critical role in shaping the fire regime of southern California chaparral landscapes. We found that such events occurred on a multi-decadal interval and that interval lengths have nearly doubled since the turn of the century. The third study examined the relationship between antecedent climate and wildfires in chaparral landscapes across southern California. We found that acute drought, driven by antecedent cool season precipitation in the previous winter and spring, was a reliable indicator of increased wildfire activity in the past; however, we now find a contemporary system influenced by antecedent climate in the two years prior to the fire event and no immediate connections to climatic drivers is apparent in the year of the event. The broader results from these three studies indicate that some changes in fire return intervals have occurred in the modern era but widespread fires have been and remain an integral part of chaparral fire regimes. We hypothesis that land use in the 20th century has altered vegetation structure and composition so much so that chaparral fire regimes now respond differently to climatic cues than they had for the past 200-300 years.

No description available.

No description available.

No description available.

No description available.

No description available.

Modern composite steel frame structures possess a high degree of redundancy. This allows them to survive extreme fires without collapse as there are many alternative loadpaths which can be used to transfer load away from the fire affected part of the structure as demonstrated in the Broadgate fire. Subsequent tests carried out on the Cardington frame showed that it was not necessary to apply fire protection to all steel beams. It was possible to leave selected secondary beams without fire protection. In the event of a fire this results in large deflections due to thermal expansion and material degradation, however, in a fire where servicability requirements do not need to be met this is acceptable so long as life safety is ensured. The weakening beams and large deflections result in a change in the load transfer mechanism with load being carried through tensile membrane action in the slab. This thesis presents a method for calculating the membrane load capacity of composite floor slabs in fire. Extensive numerical modelling at the University of Edinburgh has shown that the temperature distribution through a structural member greatly effects the deflection and pattern of internal stresses and strains. Theoretical solutions were produced to calculate the structural response of laterally restrained beams and plates subject to thermal loads. The theoretical deflections and internal forces were shown to compare well with those from numerical models. To determine the membrane load capacity of concrete floor slabs in fire a three-stage design method was developed. Initially the temperature distribution through the slab was calculated for the design fire. From this the deflection of the slab and resulting stress and strain distributions in the steel reinforcement due to the thermal loads were calculated for the design fire. From this the deflection of the slab and resulting stress and strain distributions in the steel reinforcement due to thermal loads were calculated using equations from the theory developed previously. Failure of the slab was defined based on a limiting value of mechanical strain in the reinforcement, this strain corresponded to a limiting deflection. The load capacity of the slab at the limiting deflection was calculated using an energy method. When compared against results from numerical models the ultimate load capacity was shown to be accurately predicted. None of the fire test carried out on the Cardington structure reached failure. Although demonstrating the inherent strength of such buildings this was also a major short coming as it was not possible to define the point of failure. the design method developed was used to calculate the membrane laod capacity of four of the six Cardington tests. All four tests were shown to have had a significant reserve capacity with none being close to failure.

The behaviour of connections in fire has become of particular interest to the structural engineering community following the possible link of connection failure to the collapse of the World Trade Centre building 7 and the failures and huge distortion of some connections after the Cardington full scale tests. In order to widen the understanding of the complex behaviour of connections in fire this thesis discusses a number of specific issues relating to connections in fire and their influence on structural response. The first part of this work presents a finite element model for predicting connection temperature profiles. A parametric study is then carried out to investigate which factors have the greatest influence on temperature prediction. This method is compared to the currently available methods for connection temperature prediction presented in the Eurocodes: using a percentage of the beam mid-span lower flange temperature to estimate the temperature across the connection and a lumped capacitance method to calculate average joint temperature based on the mass of material and surface area. In each case, along with the predicted temperatures, the influence on connection material strength is also presented. The three methods have varying levels of accuracy. The finite element model provides detailed and accurate results due to the thorough consideration given to the input parameters. The percentages method gives reasonable estimates in the heating phase but is less accurate in cooling and the lumped capacitance method is only suitable for crude estimations. The remainder of the thesis is concerned with how a number of phenomena affect the overall structural behaviour of buildings: the inclusion of detailed connection models within larger, less complex, finite element models; the effects of connection rotational capacity and the composite beam-slab shear connection. A finite element model for isolated joints is presented in detail for a number of heating regimes and connection types. The influence of the bolt shear and tensile properties is considered in detail and the need for further testing on bolts at high temperatures is discussed. The model has the capacity to predict a number of failure modes and also shows a good comparison between experimental and theoretical deflected shapes. This connection model is then inserted into a large model. It is shown that whilst the inclusion of the shell connection has a small influence on the residual deflections of a structure after cooling when compare to a model where connections are simple and fixed, the difference between heating and not heating the connection does effect structural deflections. Following on from the previous full scale model, simple connections are then exclusively included where the connection rotational capacity is varied. Results show that there is not a large effect on the structural deflections or beam axial and shear forces when rotational behaviour is changed. However column bending moments are hugely increased during heating both in the fire compartment and away from it and fixed connections result in larger bending moment that pinned ones. Finally, the shear interaction between the slab and beams is investigated. The detailed development of both an ambient temperature and then an elevated temperature model of a beam-slab system including explicit shear studs are presented. A study is then carried out looking at the effects on deflections and beam forces when the strength and ductility of the studs are altered. It is found that more ductile studs with a high shear capacity are beneficial for reducing forces in beams and limiting their deflections. Finally the shear studs are included in the larger model used in previous chapters where results are similar to those seen in the beam-slab model, but are less pronounced.