Beam Mechanism Failure in Multistory Steel Frame Structures

Abstract

Engineers must ensure that building structures are not in danger of collapse, so analyses always include safety factors that create redundant yet materially inefficient buildings. This has been common practice for most of structural history, but today, growing concerns for carbon emissions force designers to cut material usage while retaining the same level of safety. Processes opt into one of two processes: an overall lighter unit or stiffening specific internal systems to encourage a load path. The problem with either of these options lies in progressive collapse in the event of structural damage. If one column is lost, stresses propagate either until equilibrium or a larger collapse occurs. Progressive collapse remains a popular research area to identify specific vulnerabilities, often with numerical models for a visualization of each stress state and redundant capacity. Previous studies used analytical and experimental performance to observe the critical effects of losing an external versus internal column and the role of other components, such as joints, joists, and composite slabs, to carry additional loads. However, designs and analyses are bound by assumptions that govern model behavior. To understand the sensitivity and limits of these assumptions, this thesis predicts the performance of steel moment-frame structures of varying bay geometries, proposing deflection fields to inform modern practice in all phases of project development. Instead of numerical simulations, the process follows an analytical approach based in the fundamental methods of equilibrium and the conservation of work and energy. By designing sections for their elastic capacity, their operational performance is directly linked to their failure response. This suggests the dominance of design preferences in stability, even with changes in beam spans or floor loading. Results support an optimal span ratio for plasticity under two-way load distributions that favors bay geometry ratios (L1/L2) between 1 and 2 but varies based on failure locations and how many columns have been lost. This also emphasizes the weaknesses out of plane as span ratios range from 0.5 to 1. Project layouts can utilize the free strength provided by bay geometries as part of the structural design process. If large deflections or span lengths are expected, beam depth and section thickness should increase together to ensure beams utilize their full plastic capacity to achieve additional redundancy from catenary action. Overall, the thesis demonstrates that such considerations in the early design stage can enable steel structures to achieve greater safety with less material.