2.6.1. Cracking Coefficient and Effective Flexural Stiffness of Concrete Walls
The cracked moment of inertia for structural walls including precast and tilt-up wall panels can be calculated using different provisions permissible in codes and standards. This design professional has several choices to select from when making a decision as to which methods to use and that depends partially on the wall application and its required behavior under service conditions. This Technical Article discusses and compares the commonly used provisions in ACI 318-14 to calculate the cracked moment of inertia. It also reveals the considerable variation that exists for this design parameter as well as its impact on the strength and serviceability of the structural member under consideration.
2.6.2. Shear Wall Seismic Design Considerations
Shear wall reinforcement detailing to meet seismic design provisions is required by codes and standards. It is also dependent on seismic design categories and other factors. The reinforcement computed by spWall needs to be distributed and detailed further by the design professional to ensure that seismic design and detailing required by the relevant code are implemented. Such requirements are explained further in various references and code documents. Seismic Detailing of Concrete Buildings from the Portland Cement Association (PCA) contains a comprehensive summary of the seismic detailing requirements in ACI 318.
2.6.3. Retaining Wall Design Considerations
Reinforced concrete cantilever retaining walls consist of a relatively thin stem and a base slab. The stem may have constant thickness along the length or may be tapered based on economic and construction criteria. Cantilever retaining walls whether precast in a factory or formed on site can be analyzed and designed using the engineering software programs spWall and spMats. This Design Example and Case Study explores the traditional hand solution for a cantilevered retaining wall and provides a comparable solution with the finite element method (FEM) using spWall. Using spWall extends the flexibility of the end user to model nonstandard condition of support and loading for earth retention wall systems. Similarly, spWall effectively models the behavior of highway sound walls, soil nail walls, and tie-back anchored retaining walls with focus on understanding the support reactions and wall out-of-plan displacements with active or passive restraints.
2.6.4. Tank Wall Design Considerations
Reinforced concrete tanks are used widely to collect and contain liquids from wastewater stations, process facilities, agricultural and environmental plants. This Case Study focuses on the design considerations for rectangular concrete tank walls and base slab with emphasis on the variation of the loading cases and load combination required to adequately address all stage of construction and operations.
Many designers depend on simplified conservative assumptions for analysis of tank walls or rely on design coefficients such as the tables provided in the “Rectangular Concrete Tanks” Design handbook by the Portland Cement Association (PCA). In spWall tank wall design conditions and loading scenarios can extend the limits of hand solutions and provide the end user with control over support locations, load distribution, placement of openings and much more flexibility to examine the behavior of the complete tank structure. This is particularly relevant in the design for durability and liquid tightness where specialized load factors and load combinations are necessary to meet stricter functionality or serviceability conditions such as that of ACI-349 or ACI-350. More information about this topic can be found in this Article.
2.6.5. Deep Beam & Transfer Girder Wall Design Considerations
The spWall program is typically used for the analysis and design of structural reinforced concrete walls. Additionally, the program can be used to analyze and design deep beams, transfer girders, coupling beams, corbels, pile caps, and other non-standard concrete elements with geometric discontinuity. The Strut-and-Tie Model (STM) is a tool for the analysis, design, and detailing of reinforced concrete members. It is essentially a truss analogy, based on the fact that concrete is strong in compression, and that steel is strong in tension. Truss members that are in compression are made up of concrete, while truss members that are in tension consist of steel reinforcement. STM provides a design approach, applicable to an array of design problems that do not have an explicit design solution in the body of structural codes. This method requires the design professional to consciously select a realistic load path within the structural member in the form of an idealized truss. Rational detailing of the truss elements and compliance with equilibrium assures the safe transfer of loads to the supports or to other regions designed by conventional procedures. While solutions provided with this powerful analysis and design tool are not unique, they represent a conservative lower bound approach.
Deep beams such as transfer girders can be analyzed and designed by any procedure satisfying equilibrium and geometric compatibility. Two established methods are widely used. The Strut-and-Tie Model or Method (STM). This method gives the engineer considerable control over modeling choices and ways to assert engineering judgement. Finite Element Method (FEM) is another method for analyzing reinforced concrete deep beams, particularly useful for irregular beams and walls with variable thicknesses, openings, and other features that limit the use of STM or significantly complicate the STM truss and calculations. Many reputable commercial FEM analysis software packages are available on the market today such as spWall. Using FEM requires critical understanding of the relationship between the actual behavior of the structure and the numerical simulation since this method is an approximate numerical method. FEM is based on several assumptions and the engineer has a great deal of decisions to make while setting up the model and applying loads and boundary conditions. The results obtained from FEM models should be verified to confirm their suitability for design and detailing of concrete structures. This Design Example discusses in detail the analysis and design of deep beam (transfer girder) using STM and FEM.
spWall program offers Deep Beam Templates to enable the user to select from a set of pre-defined templates and edit them for quick model generation.
2.6.6. Shear Wall Lateral Displacement and Stability in High - Rise Buildings
Lateral displacement of shear walls in high-rise buildings and structures is a critical design parameter with a direct impact on the functionality and serviceability of a building and its occupant comfort. In tall structures lateral displacements also must be controlled to preserve content and operability withing permissible tolerances. This Case Study experiments with the displacement computation and magnitude investigated in spWall using different Cracking Coefficient Equations. Study indicates that using reduced stiffness in critical region (heel of shear wall) has significant effect on the lateral displacement of shear walls and need to be considered. The reduced stiffness can be calculated using different ACI 318 provisions or using more detailed analysis.
2.6.7. Alternative Method for Out-of-Plane Slender Wall Analysis Limitations
Concrete walls analyzed using simplified methods in structural concrete standards must meets and comply with a number of conditions for such methods to be suitable. This Technical Article is intended to assist design professional with such conditions in wall systems and reviews options for concrete walls spanning a number of floors with and without openings. Special attention is paid to the proper calculation of moment magnification due to second order effects and exacting the location of maximum deign moments after magnification.
2.6.8. Multi-Story Tilt-up Wall Considerations
Multi-story structural tilt-up walls frequently used in low-rise and mid-rise construction present a number of additional considerations beyond what is commonly covered in codes and standards for simply supported single span structural walls. In this Design Example and the Technical Article, the limits of applicability of simplified alternative analysis method is explored along with additional considerations for multi-span continuous wall. In continuous wall models, the maximum positive moments don’t necessarily occur at midspan. Also, second order effects lead to moment magnification that may further shift the location of the maximum design moments in each span. Many other issues arise with panels with cantilevered spans, variable thickness and width, irregular openings, non-standard boundary conditions, high compressive loads, non-standard concentrated load, and intermediate support at ground floor slab locations.
2.6.9. Lifting, Shoring, and Bracing of Wall Panels
Wall panels in earth retention structures, precast and tilt-up buildings, and deep foundations regularly require wall analysis during lifting, shoring, and bracing. Such analyses may be for short term erection and construction load conditions or long-term service conditions with requiring stabilization. spWall can be used to model and analyze a variety of these construction scenarios by using a comprehensive set of boundary conditions and nodal restraints to investigate the wall behavior, strengths, and required reinforcing. Contact StructurePoint support to discuss specialty examples and case studies from our archives including specialty applications of tiebacks, walers, and rakers/struts installed as part of a shoring system to resist earth and water pressures.
2.6.10. Insulating Concrete Forms (ICF) Walls
Insulating concrete form (ICF) is a concrete formwork system made of rigid thermal insulation that stays in place as a permanent interior and exterior substrate for walls, floors, and roofs. This Case Study focuses on the design of ICF walls using the engineering software program spWall. The ICF wall under study is in a typical four-story residential building with prestressed precast hollow core concrete plank floor with a 2-inch concrete cast in place topping. A review of the out of plane wind load displacement and support reactions along with the required reinforcement is included.