Civil Engineering Association

Ponding of Concrete Deck Floors

Floor construction consisting of concrete over metal decking and supported by steel beams and girders is a frequently employed structural system. When temporary shoring is not used, the steel framing and decking deflects during placement of the concrete floor slab. If the concrete were placed to the specified uniform thickness, the result would be a floor surface defined by the deflected shape of the supporting members. The purpose of this paper is to present an interim report concerning studies toward an ultimate objective of predicting concrete volumes required to produce an acceptably level slab which is placed over a flexible substrate. As concrete is placed, the supporting system deflects. As more concrete is placed to compensate for the deflection, additional displacements occur. The situation may be considered analogous to the rainwater ponding phenomenon of roof systems. However, there are notable differences between the rainwater ponding phenomenon and the concrete placement operation. Concrete is plastic, not liquid, consequently it does not seek a constant level. Also, the concrete placement process is controlled by man and rainwater deposition is not.

Practical Application of Energy Methods to Structural Stability Problems + Errata

Simple energy-based formulations can provide convenient solutions for, and fresh insights into, many common structural stability problems. This is true even today, when practicing engineers have ready access to sophisticated computer programs for first- and second-order structural analysis. In general, the direct energy-Lased formulation of a stability problem involves balancing the strain energy resulting from deformation of the structure with the work done by applied loads. The critical load corresponding to any assumed buckled shape can be determined in this manner. In many practical situations, this is a surprisingly simple and straightforward process. At the most basic level, energy concepts can be used to assess whether a particular deformation pattern in a structure is a potential buckling mode: If the deformation causes the applied loads to move in the direction in which they are applied, i.e., if the loads do positive work, the deformation is a potential buckling mode, otherwise it isn't, regardless of the magnitude of loading. This simple concept can be used to determine whether and where bracing is needed in a structure. This paper will begin with a brief reintroduction to the energy-based formulation of buckling problems. The energy formulation will be applied to certain structure types for which buckling solutions, by other means, are readily available. This will show how the energy approach can provide fresh insight into the behavior of structures, by furnishing simple physical explanations for some of the characteristics of familiar solutions. A general procedure for stability analysis based on energy concepts will then be presented, and illustrated with the help of practical examples.

Effective Length Factor for the Design of X-bracing Systems

X-bracing systems made with single angles are quite common in steel structures. In current practice, the design of X-bracing members may be performed in one of two ways. The first method is to ignore the strength of the compression diagonal in resisting the imposed loads. Conversely, the second method recognizes the contribution of the compression diagonal. This method requires overall buckling of the full diagonal in the out-of-plane direction as well as buckling of one half diagonal about the weak principal axis to be investigated in calculating the effective slenderness ratio. However, for all the hot-rolled single equal leg angles (listed in Ref. 4), the radius of gyration about the weak axis, Z-axis, is greater than half of that in the out-of-plane direction. Accordingly, buckling of full diagonal in the out-of-plane direction governs the strength of single-angle compression diagonals. The purpose of this paper is to provide designers with some recommendations regarding the effective length factor to be considered in the design of X-bracing systems. The recommendations were drawn from experimental and theoretical study of full-scale X-bracing specimens. For more details, refer to the original report by the authors.

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Structural Engineering for the 80's and Beyond

Shortly after its founding in 1921, the American Institute of Steel Construction took the step that established its future. It is described in the preface to the Institute's first specification: "In 1923, AISC undertook the work of promoting uniform practice in the industry, and in order that its efforts would not be interpreted as being unduly influenced by commercial interests it selected a committee from among the leading talent in the academic, engineering and architectural professions to prepare a Standard Specification on the Design, Fabrication, and Erection of Structural Steel."1 The committee, consisting of two practicing engineers, an architect, and two academicians, was indeed an eminent one, and did its job well. The specification produced the same year was less than nine pages long, but met its objectives and gained the respect of the engineering profession. The consistency and integrity with which the principles of that statement have been applied over 61 years are remarkable. I know of no other standard sponsored by a private industry that has achieved and maintained the worldwide recognition the AISC Specification has.

Effective Width of Composite L-Beams in Buildings

The latest AISC design specifications permit the use of wider effective widths for steel-concrete composite exterior beams (or L-beams) than previous editions. This sometimes results in more flexible beams which quite often support deflection-sensitive nonstructural components. An analytical study of composite L-beams was conducted within a practical range of span and spacing for buildings. Both the conventional stress-based and rational stiffness-based definitions of effective width were considered. Effective widths were computed from the results of finite element analysis of three dimensional models permitting longitudinal slip between the concrete slab and the steel beam. It was concluded that the AISC effective width criteria for L-beams tends in an unconservative direction; a stiffness-based formula dependent on the spacing/span ratio is proposed.