Practice Periodical on Structural Design and Construction

The adoption of structurally superior forms, such as domes and vaults, in combination with cheap, locally available materials is an efficient solution to low-cost housing. Earthen building materials, in particular, offer many benefits, such as sustainability, excellent climatic performance, low cost, and low carbon footprint. Furthermore, earthen masonry shells are typically designed as pure-compression structures, thus eliminating the need for expensive and energy-intensive materials, such as steel. However, although these shells are designed for compressive stress only, many show substantial cracking damage, which may become hugely problematic because these pathways enable water to enter and saturate the underlying material. This is concerning because moisture can severely compromise the strength and durability of earthen masonry. In this paper, the advantages and difficulties associated with adopting unreinforced compressed stabilized earth block (CSEB) masonry shells for low-cost housing are reviewed, particularly from a South African context. Of particular focus are issues with the performance (e.g., strength and durability) of these structures. Subsequently, considerations are presented to improve the longevity of CSEB shells, which are based on a review of the literature as well as an evaluation of several full-scale masonry shells built in South Africa. The case studies considered in this paper reveal that cracking may be detrimental to the performance of CSEB shell housing structures, particularly in low-cost settings where maintenance and repairs are often deficient.

An office floor with a typical grid size of 10.8 × 8 m in a multistory building was considered for design with flat-slab, flat-plate, and beam-slab systems. In all cases, both RC and post-tensioned systems were used for the comparison of structural performance and cost together with safety. The slab systems were designed using three different codes of practice: Indian Standard (IS), British Standard (BS), and American Concrete Institute (ACI) standards. From the cost analysis, it is observed that the post-tensioned flat-slab system provides better cost reduction of approximately 16.5 and 20% in the case of IS and BS standards, respectively. In the case of ACI standards, the RC and post-tensioned flat-slab systems yield 17.7 and 12.5% cost reduction, respectively. From the study, it was concluded that the post-tensioned flat-slab system using ACI standards is considered to be durable because the tensile stress is restricted and designed as an uncracked section, whereas the IS and BS allow 0.2-mm crack width.

Failure of metal roof is the most commonly observed damage in cyclones. Significant numbers of metal roof coverings, corners, and eaves are damaged. International codes of practice specify the procedure for the calculation of structural capacity of purlins supporting standing-seam metal roof (SSMR) systems, but they do not specify the procedure to design the panel–purlin connection system, integrated by a clip called a halter. The panel profiles and fastening techniques adopted worldwide are so diverse that the strength of panel–purlin connection and the lateral restraint on purlin by diaphragm action cannot be generalized. However, the type of standing-seam system presented in this paper, viz., the clip lock system, is prevalent in many countries with uniformity in halter dimensions and seam heights. The details of an experimental investigation on prototype cold-formed steel (CFS) standing-seam clip lock system tested under wind uplift in a vacuum chamber are presented here. A vacuum chamber was fabricated specially for this purpose, as per standards. The behavior and failure modes of rigid purlin-panel system were studied. Experimental data on the uplift behavior of SSMR were gathered. This data were converted to design parameters through statistical evaluation. Three limit states of panel-purlin interaction failure were studied and a procedure for the integral design of SSMR halter-rigid purlin systems was developed, guided by the experimental results. On the design effect side, four wind-pressure cases, along with cyclonic wind pressure, were considered. The applicability of the experimental results to practical designs was quantified by a reliability-based analysis. Based on the values of reliability indices, the zones of failure were identified, and a rational design of variable halter length was proposed. The halter length could be adjusted as per the target reliability, leading to the concept of SSMR roof system with a uniform safety.

Today, the use of composite sections of concrete and steel is increasing. In this study, the behavior of two partially encased composite (PEC) columns is studied experimentally under pure compressive and bending loads. Then a simple analytical method is suggested for PEC columns by introducing all-steel columns, which are composed of four stiffener plates on the four flanges of the steel section. The sizes of the stiffener plates are found by equalizing the compressing strength as well as bending stiffness of the two sections. The accuracy and efficiency of the suggested method was assessed using the FEM in a software program by calculating and comparing the internal forces resulting from analyzing an irregular building. The results indicated high accuracy of the suggested linear static analysis method and meet the need to use finite-element simulations for the users and designers.

An upper-bound and lower-bound approach using a combination of the rebound hammer test or Windsor probe test and ultrasonic pulse velocity test or core test for estimating in situ compressive strength of existing concrete structures is presented. A bias factor for each of the nondestructive tests and core test was introduced to account for the overestimation of characteristic in situ compressive strength by rebound hammer test and Windsor probe test and underestimation of characteristic in situ compressive strength by ultrasonic pulse velocity test and core test. In situ concrete testing results of 34 existing concrete structures aged 4–73 years were analyzed to obtain a range of each of the bias factors. Illustrative examples using this upper-bound and lower-bound approach for estimating characteristic in situ compressive strength of existing concrete structures are presented.

Reuse of existing bridge foundations has been gaining momentum and acceptance among practitioners in the last few years. Proper characterization of the bridge foundation (embedment, load-carrying capacity, etc.) is necessary for bridge reuse when the original design conditions are changed because of scour, bridge widening, increased loading, or other changes to the structure. This paper presents rational means to analyze existing foundations using a hybrid analytical-numerical method so that reuse can be optimized. An integrated bridge load rating that considers the entire superstructure-substructure system can be established using the proposed method. Performing a quantitative integrated bridge load-rating analysis that considers geotechnical issues and soil-structure interaction for a substructure is essential to prevent functional or catastrophic failure under a live load. A reliable determination of the unknown pile embedment is also a prerequisite for a realistic integrated bridge load rating. The design criteria used when the bridge was originally constructed are important factors in the load rating process. Costly detours, repairs, replacement, or loss of life could be associated with inaccurate embedment estimates or use of criteria incongruous with the original design. The results of case histories show the importance of a realistic assessment of the pile embedment and of using appropriate geotechnical and structural design criteria when calculating the integrated load rating of bridges to advance the consideration of foundation reuse.

The objective of this article is to present new analytical expressions for lipped channels subject to combined shear and compressive loadings by accounting for rotational and lateral restraints. Very limited design guidelines are available to use in practice for channels subject to this load combination with partially restrained boundary conditions. North American codes provide expressions for the case of simply supported with free in-plane boundary conditions. The article also highlights the influence of flange and lip geometric properties on local web shear buckling stress. Results show an apparent decrease in critical shear stress when the channel longitudinal edges are fully restrained against lateral movement. The cost savings of the proposed design procedures are also highlighted.

In terms of weight and curing of mortar in the brace core, an all-steel buckling-restrained brace is considered an improved type of common buckling-restrained brace. In this study, a new type of all-steel buckling-restrained brace was introduced; it was made up of three parallel plates that were connected through a Z-shaped profile. Cyclic analysis was conducted through a software program for finite-element analysis. First, the numerical model was validated through an experimental sample, and the finite-element numerical model was introduced after achieving a desirable match with the behavior of the model. The variable parameters of this analysis were the overlap length of the plates and the section depth of the brace. Determination of the nonlinear modeling parameters of the brace was followed by comparison of two modes of the frame with typical convergent X-shaped braces and buckling-restrained braces for 5-, 10-, and 15-story structures through pushover and nonlinear time-history analyses. The obtained results suggest that the new buckling-restrained brace maintained the structural-life safety performance level.

Significant attention is given to the transport properties of the concrete covering the reinforcing bars (covercrete) while neglecting the statistical characteristic of its thickness resulting from variations in rebar placement. A statistics-based model was developed to evaluate the effect of variations in covercrete thickness on the equivalent service life of a structure. It was demonstrated that service life can diminish by ∼30% when the standard deviation of the covercrete thickness reaches 15 mm (0.6 in.), as sometimes observed in practice. Field investigation of covercrete thickness in five infrastructure projects identified two sources of errors: (1) error in the average covercrete thickness, which indicates poor control of the specified thickness, and (2) standard deviation of thickness that ranges from 7.3 to 16.4 mm (0.29 to 0.65 in.). As a result, the projects' equivalent service life ranged from 0.40 to 0.99, where higher values were obtained in projects in which the mean concrete cover thickness was much greater than specified.

The composite rigid-frame bridge replaces a specific concrete section of the midspan girder of a concrete rigid-frame bridge with a steel section. Although the added steel section improves the bending moment distribution of the rigid-frame bridge, it increases the stress level and structural deformation in a few specific points along the bridge girder. Therefore, the determination of the steel section length needs to consider the comprehensive mechanical effects caused by the replacement of the concrete section with the steel materials. This article presents the results of a study on the influence of steel section lengths on the structural mechanical behavior of rigid-frame bridges, including bending moment, bending stress, and structural deformation. Based on the results of the sensitivity analysis, this study optimizes the steel section length by using the bending strain energy as the objective function. Taking a composite rigid-frame bridge with a span combination of 84  + 200  + 84 m as an example, the structural optimization yields an optimal steel section length ratio of 0.55 (110 m), with the least bending strain energy and with acceptable magnitudes of bending moment, bending stress, and structural deformation. The outcome of this study provides guidance regarding the steel section length for the structural design of composite rigid-frame bridges.

The dynamic response of a machine-foundation system depends on several factors, such as (1) the soil dynamic properties, (2) geometric properties of the foundation, (3) amplitude of the applied dynamic loads, and (4) frequency of the dynamic excitation force. The main goal in machine foundation design is to keep the foundation response within a specific limit to enable the satisfactory operation of the machine. If the foundation response exceeds this limit, the foundation will adversely affect the performance of the machine and may damage the machine internals or cause it to function improperly. Furthermore, the excessive vibrations impose additional stresses on the machine resulting in increased unbalanced loading, and thus, to increased dynamic loads on the soil-foundation system. This paper presents the results of the dynamic analysis of a four-cylinder compressor foundation. The original design of the foundation was performed in the early 1960s and ignored the effect of the soil on the response of the foundation system; therefore, the foundation has been suffering from excessive loading. The foundation block supported a four-cylinder compressor, suction and discharge bottles, and a crank and driving motor with a total weight of approximately 974 kN (219 kips). The results of a three-dimensional (3D) finite-element model of a soil-foundation system was used to determine the dynamic response of the soil-foundation system and to assess the foundation response under applied dynamic loading resulting from the compressor crank. The dynamic analysis was performed by performing (1) eigenvalue analysis of the foundation block, considering the effect of the soil-foundation interaction, to determine the soil-foundation natural frequencies and modal participation factors and (2) a forced response of the foundation under an unbalanced crankshaft load applied to determine the forced response amplitude of the soil-foundation system.

Slotted tracks are commonly used in the construction of nonload-bearing infill walls in steel and concrete frame structures. These members are configured with closely spaced slots in the track flanges to accommodate the relative vertical displacements between adjacent floors and a variety of stud spacings. Engineered design of slotted tracks is based entirely on manufacturer published data, which in turn are largely based on testing. Published data typically give allowable design strength but often do not indicate the controlling limit state—strength or serviceability—and there is a general lack of information on safety and resistance factors. Based on a series on monotonic tests, it is shown that the setup used to evaluate performance and develop design values can significantly influence the reported design values. It is finally recommended that manufacturers, and the cold-formed steel industry, move toward development of a standard for evaluation and reporting of test-based design values.

Increased traffic loads and the creation of passages and openings in buildings by cutting a piece of slab or a portion of a wall require the strengthening of the weak or impaired member. Means are various: addition of a supporting beam, prestressing the section with external tendons, attaching steel plates or carbon-fiber-reinforced plastic (CFRP) strips to the weak face of a slab or wall. Adding a steel beam with the purpose of creating a composite action between the beam and slab was critically examined. Strength as well as rigidities of the modified structure were affected.