International Concrete Abstracts Portal

Steel columns are commonly attached to concrete foundations with groups of cast-in-place headed anchors. Recent physical tests and simulations have shown that the strength of these connections can be limited by concrete breakout failure. Four full-scale physical specimens of axially loaded columns attached to a foundation slab were tested, varying the shear reinforcement configuration in the slab. All specimens were governed by concrete breakout failure. The tests suggest that adequately placed distributed shear reinforcement can increase connection strength and displacement capacity. Steep cone failures were observed to limit the beneficial effect of shear reinforcement. Calibrated finite element models were used to investigate critical parameters such as the extent of the shear-reinforced region and bar spacing. A design approach is proposed to calculate connection strength by adding the strength of the concrete and the distributed shear reinforcement. Design detailing is discussed.

The structural performance of four reinforced concrete pile cap specimens anchored by widely used proprietary high-strength threaded bars (HSTBs) and subjected to column uplift loads is investigated. The specimens were full-scale replicas of in-service foundations in terms of geometry, material properties, reinforcement details, and loading conditions. The study also included isolated pullout tests of HSTB anchorages. Two of the specimens were modified by increasing the embedment depth of the column anchor plate to assess effectiveness as a possible retrofit approach. Failure of all specimens was brittle without yielding of embedded reinforcing steel. Moreover, the ultimate pullout capacity of the anchors was not achieved in any of the specimens. ACI 318-19 provisions underestimated the concrete breakout capacity of all specimens and were unable to incorporate the beneficial effects of the column anchor plate modification as it only considered the anchor embedment length and edge distance as critical parameters for calculating strength.

Steel and precast columns are commonly designed to transfer moments to foundations through cast-in-place headed anchors. The concrete breakout failure mode is not routinely checked, even though recent tests have shown it can limit the connection’s strength. This paper describes how physical test data are used to calibrate finite element models of column-foundation connections to investigate critical variables. When designing column-foundation connections with cast-in-place anchors, both beam-column joint shear strength and concrete breakout failure strength should be calculated, with the connection strength taken as the smaller of the two values. Results suggest that properly detailed distributed shear reinforcement in the foundation can increase connection strength and displacement capacity if the connection is controlled by the concrete breakout failure mode. This effect is ignored by current building codes.

Reinforced concrete squat structural walls (SSWs) are a popular seismic force-resisting system used in low-rise buildings due to their high strength and stiffness. However, extensive studies have shown that rectangular SSWs have limited shear strength and drift ductility, primarily because their deterioration initiates from brittle compression failure of the diagonal concrete struts across the wall’s web. This research investigated a new reinforcing detail for SSWs to achieve substantially improved ductility and strength. While the current ACI Code requires a mesh of steel reinforcing bars to reinforce the SSW web, the new detail presented in this paper fortifies the SSW by multiple steel cages which contain vertical reinforcing bars enclosed by transverse hoops. These steel cages can be easily prefabricated at a shop to minimize on-site assembly work and time. Each steel cage is similar to that in well-confined columns, which increases concrete’s strength and ductility, therefore allowing a higher amount of vertical reinforcement for greater shear strength. Five specimens with an aspect ratio of 0.5 and reinforced according to either the ACI requirements or by the proposed details, and one specimen with an aspect ratio of 0.33 using the proposed details were tested. Similar to prior research results, SSWs using conventional details exhibited a fast strength deterioration at low drift ratios due to severe damage of the diagonal concrete struts under cyclic loading. Conversely, the proposed SSWs provide an increase of approximately 100% in the drift and ductility ratios, as well as a very gradual strength degradation and concrete damage progression. The proposed design allows SSWs to develop a ductile seismic behavior which is essential to safety against collapse, post-earthquake functionality and repairability, and seismic response predictability of structures. The enhanced ductility warrants a higher shear strength reduction factor, ϕ, of greater than 0.6 for SSWs as well as the diaphragms and foundations connected to them. In addition, a seismic response modification coefficient, R, of greater than 6 can be justified, which translates into a smaller seismic design base shear. These adjustments can ultimately lead to a more economical design of structures with SSWs.

Steel and precast columns are commonly designed to transfer moment loads to concrete foundations through cast-in-place headed anchors. In design office practice in the United States, connection strength has been evaluated considering mechanisms emphasizing joint shear, strut-and-tie modeling, and anchoring to concrete. For any given connection, the strengths calculated with these three methods can differ by a wide margin. The application of these methods, including possible enhancements that improve strength estimates, is described. Laboratory tests were performed to provide benchmark physical data to determine the applicability of various design methods. The test specimens consisted of full-scale interior steel-column-to-concrete-foundation connections located away from foundation edges, with details typical of current construction practice on the West Coast of the United States. Strength in both tests was governed by concrete breakout failure. Strategically placed reinforcement increased the strength and displacement capacity of anchored connections governed by breakout. Design recommendations are provided.