Practice Periodical on Structural Design and Construction

A simple low-cost friction base-isolation system is examined for its applicability in reducing seismic vulnerability of rural buildings. Four friction isolation interfaces, namely, marble–marble, marble–high-density polyethylene, marble–rubber sheet, and marble–geosynthetic, were studied. The friction properties of these interfaces were studied under static and dynamic conditions for a range of normal loads from 10 to 50 kN. The average coefficients of friction for all of these interfaces except marble–rubber were found to be in the range of 0.05–0.15. The effectiveness of these isolation systems was investigated both analytically and experimentally for a spectrum-compatible ground motion corresponding to the maximum credible earthquake for the most severe earthquake zone according to Indian standards for earthquake-resistant design. The analytical prediction of seismic response of buildings with such isolation was found to be in good agreement (within 19%) with the experimental observation. It was found that for marble–marble and marble–geosynthetic interfaces, more than 50% reduction in absolute response acceleration at the roof level could be achieved in comparison with the response of the fixed-base structure at the cost of increasing relative sliding displacements at the friction interface. However, these relative displacements were well within the commonly adopted plinth projection (75 mm).

Since the advent of mainstream computer applications, the structural engineering community has strongly embraced technology. Furthermore, the structural community is a leader in the advancement of new software, hardware, and applications within the market. Developments over the last 10 years have greatly expanded structural engineering beyond the traditional analytical methods; essentially, information and communication technologies are changing the way people do business. To understand how these technologies are changing the design and construction of structural solutions to problems in building projects, a study was conducted to develop a deeper understanding into these areas. Industry experts across the United States were interviewed and surveyed on these directions to look at how structural engineering is changing and what to keep in mind when implementing technology on projects. The focus of this paper will be twofold. First, it will focus on information and communication technologies, in particular, building information modeling). Second, it will focus on structural analytical technologies that encompass analysis and design methods. Trends, advantages, and disadvantages, as well as future directions that practitioners are taking are discussed.

Reinforcing cages for large columns requires shoring during the fabrication and erection stages of construction before concrete is placed to form a permanent load-bearing structure. Many column-reinforcing cage collapses have occurred, resulting in injuries and fatalities because the cages were not properly supported in the temporary condition. Guying systems are commonly used to stabilize large reinforcing cages during construction to avoid such accidents. This paper presents a two-dimensional first-principle stability analysis of reinforcing cages that are either pinned or fixed at the base, and supported by one level of guy wires along the height of the cage. The effect of lowering guy-wire attachment points on the stability of cages is also included because this construction practice is reported to be used in some locales. Critical buckling-load diagrams are determined that account for the relative stiffness of the guy wires with respect to the reinforcing cages and the effect of pretensioning the guy wires. These diagrams are used to provide plausible explanations for validating or refuting current construction practices as far as guy-wire angle of inclination and pretensioning, and rebar cage-base support conditions are concerned.

In underground crude oil storage facilities, concrete plugs are mainly required to contain crude oil inside caverns and are designed as gas-tight to prevent movement of any oil or vapor across the tunnel, either in a horizontal or vertical direction. Plugs are retained in a key excavated in the rock surface, which provides sufficient bearing to the concrete mass under sustained loading conditions. Construction of plugs involves mass concreting under the typically hot and humid conditions found in underground construction. Therefore, the construction of plugs requires an engineering design that takes into account not only design loads and reinforcement, but also the systematic cooling arrangement and efficient grouting mechanism so that the basic purpose of the plug design is not defeated. This paper discusses the design and construction aspects involved in the construction of large tunnel and shaft plugs for an underground crude oil storage project.

Seismic design of wood-frame single-family dwellings’ (WFSFD) lateral force–resisting systems requires determination of the stiffness of horizontal diaphragms and shearwalls. During design, sizes and locations of shearwall openings are often changed, altering shearwall stiffness and loads and requiring a significant redesign effort. Rigid and tributary area method analyses are examined for different geometries of L-shaped WFSFD and include stiffness reductions for roof geometry and pitch. These methods are applied to historic earthquake damage reports and compared using rigid, semirigid, or flexible horizontal diaphragm analyses useful in design practice. Most WFSFD should be designed using an envelope method because they contain a mix of horizontal diaphragm types owing to the effects of roof pitch and geometry on diaphragm stiffness. Cases occur where determination of semirigid or flexible diaphragm behavior is difficult because the analysis results are contradictory or unclear. This suggests that semirigid modeling or an envelope method is prudent. The use of simple rigid plate, flexible plate or semirigid plate methods can be practical for analyzing WFSFD with a reasonable level of detail and effort.

As a passive energy dissipation device, the fluid viscous damper (FVD) is effective for mitigating wind or seismic load-induced vibrations. In this paper, the effects of FVDs for a cable-stayed bridge under randomly generated earthquake excitation are investigated. The FVD is modeled as a simplified Maxwell model, which consists of a linear spring in series with a nonlinear dashpot. The pile is modeled as a beam on the nonlinear Winkler foundation, and the soil–pile interactions are simulated by using continuously distributed hysteretic springs and viscous dashpots placed in parallel. Three random ground motions are generated from the earthquake risk assessment based on the seismotectonics and seismicity analysis of the bridge location. The seismic response of the cable-stayed bridge with FVD considering soil–structure interactions (SSIs) is obtained by solving the equations of motion in the time domain using a direct integration method. Parametric studies are conducted for the two key parameters in the simplified Maxwell model, namely, the damping coefficient $C$ and the damping exponent $\alpha$. The results indicate that the FVD is very efficient in reducing the displacement response of the cable-stayed bridge and the bending moment of the tower while simultaneously limiting the shear force in the tower. Compared with the linear FVD, the nonlinear FVD has a better performance in reducing the seismic response of the cable-stayed bridge. The maximum displacement response of the deck and the tower, when implementing the nonlinear FVD, could be reduced up to 79 and 77%, respectively, whereas the maximum reduction of the corresponding displacement response with the linear FVD drops to 51 and 50%, respectively. The reduction of the bending moment of the tower using nonlinear FVD could be up to 56%, whereas the maximum reduction of the corresponding response using linear FVD is 40%. After the parametric study, an optimal design of the FVD is chosen and implemented in the final bridge design.

The surface-mining industry provides essential sources of energy in North America and has a major impact on the economy. Little information and few guidelines are available in current codes of practice to feed the engineering demands of the mining industry. Design of mobile mining facilities is a cost-effective concept that has recently evolved in the industry to increase production and meet market demands. This paper illustrates challenging issues that are encountered in the design of mobile structures that will be of interest to practicing engineers and construction contractors. A brief description is first provided to describe the major facilities used in surface mining plants. The economic aspects and cost savings that result from using mobile facilities are also highlighted. The transportation procedure and some of the major design requirements are briefly discussed. Recommendations are provided to compute the center of mass and the structural analysis procedure. The paper contains numerous industrial illustrations to aid the presentation and to describe the challenges encountered in the engineering and construction phases. Unfortunately, there are no design rules or guidelines available to practicing engineers for this type of heavy steel construction.

Drive-in storage racks, made of cold-formed steel, are the main racking system used in industry for storing goods. The storage space utilization of drive-in racks is high at the price of the weak downaisle stability. This paper presents a general analysis of the lateral stiffness of drive-in racks in the downaisle direction. A total of 24 static tests of drive-in rack systems have been conducted under single-point horizontal force. The effects of some constructional details, such as bracing configuration, cargo pallets, and the number of bays, are considered. The experimental results indicate that either top plan bracing or back-spine bracing can change the load transfer through rack structures and increase the lateral stiffness of racks. By acting as the horizontal bracings of adjacent columns and reinforcing the rotational stiffness of column bases, the stored pallets are also the beneficial factor for improving the lateral stiffness of racks. According to the special structural characteristic of tested racks, the simplified models and calculation formulas are proposed for the lateral stiffness of tested racks with bracings and without goods. To consider the influence of local deformations in bracing member connectors, sectional area–reduction factors of bracing members are introduced in the simplified calculation method. The lateral stiffness of a complete rack system can be used to determine the lateral horizontal restraint at the top of each rack column, which has large effect on the load-carrying capacity of columns in drive-in racks. In the last part of the paper, three drive-in rack examples with typical section properties and overall dimensions are presented to illustrate the computational procedure of horizontal deflection of drive-in racks.

The glazing and glazing systems of a building are the first part of a structure to be loaded in an external accidental and/or deliberate explosions; thus, it offers the first line of defense against these extreme loading conditions. Glazing fragments or shards remain a primary source of injury to occupants of buildings after explosive events. As a result, utmost effort must be placed in designing the glazing systems of a building against blast loading to minimize injury and damage. This paper provides an overview of blast loading, the types of glazing products, glazing construction, glazing systems, design objectives, approaches, performance and response criteria, and the code provisions related to the blast-resistant design of glazing and glazing systems used in buildings. In addition, test methods and retrofit strategies for glazing systems are succinctly discussed, and future research areas are highlighted.

Structurally hardened safe rooms are a proven strategy for enhancing life safety during tornadoes and other severe wind events. Performance criteria for safe rooms include resistance to both wind pressure and impact from wind-borne debris. This paper presents the results of a test program evaluating debris impact resistance of light-gauge steel decking for use as roof cladding on safe rooms. Particular attention is given to the modes of damage caused by debris impact and strategies for controlling and mitigating such damage. Recommendations are provided for detailing lap joints and bearing connections and for selecting appropriate support spacing. This information will aid design engineers in detailing light-gauge steel roof decks for use in tornado safe rooms. Recommendations provided in this paper are also applicable to other scenarios where steel roof decks are at risk of wind-borne debris impact.

Special concentrically braced frames (SCBFs) are among the most common steel structures for resisting earthquake loads in high seismic regions. Concentrically braced frames (CBFs) are elastically designed as one vertical truss system to resist lateral loads through axial brace members when they are introduced. The explicit capacity-design approach has been fully incorporated into the newest seismic provisions for structural steel buildings. One new analysis section is added into AISC 341-10 to address the inelastic responses of SCBFs. Two separate structural analyses and one additional analysis are required for SCBFs in AISC 341-10. These analysis requirements significantly increase design efforts in typical design offices, and a comprehensive study to demonstrate how such an explicit inelastic design procedure would (or would not) significantly improve seismic performance of SCBFs appears to be justified. This paper summarizes the seismic design of three SCBFs with different heights, namely, 4, 10, and 20 stories, at different sites. Two bracing configurations, chevron and X-bracing over two stories, are considered for each braced frame. Step-by-step design procedures on typical SCBFs are completed under AISC 341-05 and AISC 341-10. The design results of studied frames are compared to discover the general trends for low-rise and medium-rise CBFs between the two design provisions.

Personnel or ore-carrying conveyances traveling up and down mine shafts can experience a loss of control or encounter other emergency situations where its travel must then be slowed down and stopped. The methods to achieve this depend on how much damage to the conveyance and the impacted supporting structure can be tolerated and accepted by the design engineer. This paper examines several simplified methods of determining braking and impact forces of traveling conveyances. The first method assumes that no damage to the supporting structure occurs. The second method assumes that a portion of the impacting force is absorbed by elastomeric material placed on top of the supports. The third method gives an approximate way of estimating the damage to the impacted structure necessary to completely stop the conveyance. Numerical examples are presented to show the proposed procedures. These approximate methods should be considered as a first attempt to estimate the response of a complex problem.

Many buildings suffer unexpected damage to the extent that continued use requires repair. As the extent of damage increases, so do questions regarding the necessary extent of the repairs. Are upgrades required? What code provisions apply? What is the difference between a repair and an alteration? Because building codes change over time, either they must require all existing buildings to be modified every time a change is introduced, or they must treat proposed and existing construction differently. Given the costs associated with upgrading existing buildings, model codes in the United States have taken the prudent position that, with a few exceptions, provisions apply only to construction that follows their adoption, whereas existing construction need only meet the provisions in effect at the time it was built. This notion is referred to as grandfathering, and it has many applications outside of the construction industry. Although upgrading older construction may at times be prudent and/or desirable, based on specific circumstances, it is seldom required to maintain code compliance. The action of repairing damage to an older building provides opportunities to incorporate improvements consistent with more modern construction. Sometimes such upgrades are worth the cost, and sometimes they are not. For this reason, U.S. model building codes have historically treated the repair of significant, unexpected damage differently from the repairs normally associated with routine maintenance. This paper examines the model building code provisions related to the repair of damaged buildings from the inaugural editions of the three U.S. model building codes, dating as far back as 1927, to the 2015 International Existing Building Code. Some fundamental themes found in all of the model codes discussed in this paper include the following: provisions governing repair are separate from others, grandfathering, and explicit allowance for like-kind repair for common circumstances. In the most recent model codes, like-kind repair and repair that does not make the building less conforming than it was previously are often explicitly allowed. Significant repair-related upgrades are now triggered only in limited circumstances.

High levels of modularization have yet to be generally achieved by the construction industry. To suggest how such levels might be achieved, this study identified needed changes or adaptations in conventional project work processes. It did so by identifying and analyzing differences in how modular projects should be planned and executed. Nineteen research team members and two academic researchers identified, classified, and grouped 107 differences in how modular projects should be planned and executed. Later, three modularization case studies were conducted, and the findings were subsequently validated by an external validation committee. At the topic level, nearly 37% of the planning differences pertained to one of four topics: planning and cost estimating; modularization scoping, layout process, and plot plan; basic design standards, models, and deliverables; or detailed design deliverables. The industry may need to pay attention to these four topics to achieve higher levels of modularization. The scope of this research primarily includes the industrial subsector.

The equations commonly used in practice for the design of beams are based on the assumption that the members are restrained against lateral displacement and twist at the ends of the unbraced length. Beams suspended by wire rope or other flexible elements, such as lifting beams, are not restrained as such. Thus, the buckling behavior of suspended beams can be expected to vary from that predicted by the common design equations. This note examines the lateral–torsional buckling behavior of suspended I-shaped beams. A correction factor by which existing beam-design equations can be modified to provide a practical means of accounting for the buckling behavior of suspended beams is evaluated. The use of this correction factor for I-shaped beams has been tested by comparison to published experimental data.

In an earlier paper, allowable bending fatigue stress was expressed in terms of ultimate tensile strength. For many reasons, it is useful to express allowable bending fatigue stress in terms of hardness. Hardness is occasionally used to obtain a quick approximation of tensile strength. The rail grades are named by hardness because they are directly related to the wear properties. Many metal alloys are heat treatable, and the hardness, rather than the tensile strength, is known. In this technical note, the allowable bending fatigue stress is expressed in terms of hardness. The applicability of the derived relation on American Railway Engineering and Maintenance-of-Way Association steel, welded rail joints, and head-hardened rail sections is discussed, which has implications in rail-section design. This note provides an addition and supplement to the earlier paper.

The latest bridge design manual from the Precast/Prestressed Concrete Institute (PCI) provides preliminary design charts for selecting the girder size and prestressing strands for a given span length and beam spacing but only for $f_c^{\prime }$ = 55 MPa and 0.6-in. (15-mm) diameter strands. This single concrete strength and strand size may limit the use of the charts, particularly in states considering high- and ultrahigh-performance concrete. Accordingly, this paper presents a simplified procedure to develop preliminary design charts for prestressed concrete bulb-tee girders based on the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) bridge design specifications considering service load stress limits and flexural strength. The procedure is demonstrated for a BT-72 section and new LRFD charts are generated to compare the effects of higher concrete strength and larger strand size on bridge girder design. The benefits and limitations of the approach and recommendations for future implementation are discussed.