Practice Periodical on Structural Design and Construction

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Base-isolation systems have been increasingly used in concrete and steel structures for seismic design, but their application to woodframe structures remains largely unexplored with only several hundred buildings isolated, primarily post-and-beam buildings in Japan. This paper examines the incorporation of friction pendulum (FP) bearings into the performance-based seismic design (PBSD) for woodframe structures using nonlinear time-history analysis. Specifically, a regressive equation for the selection of the bearing radius (R) based on the fundamental period and the seismic weight present on each FP bearing is developed. Six generalized models representative of 1- to 6-story woodframe buildings were analyzed and used to develop a regressive design equation. Based on rigorous multi-incremental dynamic analysis (M-IDA), a regressive linear relationship between the spectral acceleration and bearing radius of the FP bearing was developed. An illustrative application to a 4-story residential woodframe building is presented to illustrate use of the approximate design expression. Furthermore, the expression allows one to achieve a PBSD using base isolation with no specific need to consider the detailing of the superstructure configuration with the exception of the elastic period.

The distinguishing characteristics of a tied-arch bridge have long been regarded as an elegant solution for the long-span crossing. The successfully completed 47.9 m (157 ft) long and 24.4 m (80 ft) wide parabolically shaped Halsted Street Bridge over the Chicago River North Branch Canal demonstrates that a short-span tied-arch bridge can be done economically with attention to the steel details that accommodates both constructability and durability. The thoughtful steel detailing also ensures sufficient design redundancy and improves cost competitiveness.

Two deck floors, with different lateral stiffnesses, were constructed to measure lateral loads generated by occupants that are needed for design of residential or commercial exterior deck-type floors. Load was measured with two load cells attached at each end of the ledger representing the attachment to the building. The decks were supported on rollers, simulating minimal lateral resistance from columns. Occupants performed two different types of dynamic actions, in two orthogonal directions, at occupancy live load levels of 0.48, 0.96, 1.44, and 1.92 kPa (10, 20, 30, and 40 psf). The first loading cyclic was a side-to-side swaying motion that could result from a deliberate synchronized movement such as dancing. The second was an impulse-type load, which simulated horseplay-type activities. The difference in stiffness between the two decks resulted in a significant change in the human–structure interaction. Significant inertial effects were observed when the flexible deck was loaded parallel to the ledger. A design lateral load of 0.57 kPa (12 psf) of plan area was recommended, which conservatively includes inertial forces from a flexible deck.

Precast concrete floor systems represent a major component of the cost and weight of precast concrete buildings. Hollow-core (HC) planking is considered the most common precast concrete floor system for residential and commercial buildings because of their economy, light weight, structural capacity, and ease of production and erection. However, the high thermal conductivity of HC planks hinders their use in radiant heated floor and roof applications where thermal insulation is needed. This paper presents the development of a precast/prestressed concrete sandwich floor panel that consists of an internal wythe of insulation and two external wythes of concrete similar to those used in sandwich wall panels. The main difference between the sandwich floor and wall panels is the design of shear connectors between concrete wythes to achieve full composite action under ultimate loads while simultaneously having an adequate creep resistance under sustained loads and acceptable deflection under live loads. Truss-shaped glass fiber-reinforced polymer ties, known as NU ties, were used as shear connectors because of their structural and thermal efficiency. The proposed floor panels have comparable weight and capacity to HC planks while being more thermally efficient and can easily be fabricated in standard casting beds with typical equipment, thus eliminating the high initial investment required for HC production. This paper presents the design, detailing, production, and testing of three 7.9-m-long (26-ft-long), 1.22-m-wide (4-ft-wide), and 200-mm-thick (8-in.-thick) specimens with 50-mm-thick (2-in.-thick) composite topping. The three specimens represent two different panel designs. One specimen has solid concrete ends and was designed for a total superimposed service load of 9.6 kPa (200 psf), whereas two specimens are fully insulated and designed for a total superimposed service load of 4.8 kPa (100 psf). The performance of the three specimens under flexure and shear loadings indicated that the proposed sandwich floor panels can achieve full composite action and have satisfactory structural performance and acceptable deflection characteristics.

Timber substructure bridges supported by three, four, or five piles were commonly used in Alabama in the past for secondary and county highway bridges. Because of the large number of such bridges in the state, the Alabama DOT recently developed a screening tool to assist its engineers in assessing the adequacy of bridge pile bents for extreme flood and scour events. The evaluation procedure employed in the screening tool is presented in this paper through the following sequence: (1) preliminary or general checks; (2) possible kick-out or plunging failure; (3) bent pile buckling failure; (4) transverse pushover failure from combined vertical gravity and lateral floodwater loadings; and (5) bent upstream pile beam-column failure from combined vertical gravity and lateral debris raft loadings. This paper discusses the theoretical background for the equations used to evaluate each failure mode in the timber screening tool, in addition to describing the flow of the screening process.