Concrete and steel structure calculation and design

Author: Chief Editor: Zhai Ailiang et al.
Publisher:
Publish Date: 2000-03-01
Features: Synopsis This book is written according to the new code, with a total of ten chapters. The main content includes the basic calculation formulas, design methods, and application of formulas for reinforced concrete structural members. It also introduces some issues in the design calculations of plain concrete structures and masonry structures. This book can serve as a reference for technical personnel in water conservancy and hydropower engineering to learn the new code, as well as a textbook or reference for relevant majors in colleges and universities. Excerpt: Chapter Reinforced Concrete Structural Design Methods Structural design is the process of determining the required section sizes, reinforcement conditions, and construction measures for the functional requirements of structural members under predetermined loads and material properties. The purpose of design is to achieve a sufficiently reliable structure that meets all functional requirements with the least possible consumption of labor and materials, based on existing technology. Section Limit States of Structural Design I. Functional Requirements of Structures (1) Safety Structures should be able to withstand various actions that may occur during normal construction and use without failure; in the event of accidental events, they should maintain the necessary overall stability, such as during strong earthquakes, explosions, or impacts, where although the building structure may suffer local damage, it does not collapse. (2) Serviceability Structures should exhibit good performance during normal use. For example, they should not exhibit excessive deformation or vibration that affects normal use, nor should they develop wide cracks. (3) Durability Under normal maintenance conditions, structures should meet all functional requirements within the predetermined service life. For example, they should not suffer severe steel corrosion or serious weathering and aging of concrete, which could affect the service life of the structure. The above functional requirements are collectively referred to as the reliability of structures, which is the ability of a structure to perform its intended functions within a specified time (currently stipulated as 50 years in China) under specified conditions (such as normal design, construction, use, and maintenance). There is a trade-off between the reliability and economy of structures. Scientific design methods require finding an optimal balance between reliability and economy to ensure that the structure is both economical and reliable. II. Classification of Limit States of Structures The working condition of a structure during its service life is referred to as the working state of the structure. When the entire structure or a part of it exceeds a specific state and cannot meet a certain functional requirement as specified in the design, this state is called the limit state of that function. A structure is considered reliable or effective if it can meet a certain functional requirement and work well; conversely, it is considered unreliable or inoperative. Clearly, the criterion for distinguishing whether the working state of a structure is reliable or unreliable is the limit state. The limit states of structural functions can be divided into two categories: limit states of load-bearing capacity and limit states of serviceability. (1) Limit State of Load-Bearing Capacity When a structure or member reaches its maximum load-bearing capacity or undergoes deformation that is no longer suitable for continued service, it is in the limit state of load-bearing capacity. It is considered that the limit state of load-bearing capacity has been exceeded when any of the following conditions occur: 1) The entire structure or a part of it loses equilibrium as a rigid body, such as the overturning of an eaves overhang or the sliding of a retaining wall; 2) Structural members or connections fail due to stress exceeding material strength or due to excessive plastic deformation that makes them unsuitable for continued service; 3) The structure transforms into a mechanism and loses its load-bearing capacity; 4) The structure or member loses stability due to reaching a critical load, such as a column being buckled. The limit state of load-bearing capacity is related to the overall or partial failure of the structure, which may result in significant loss of life and property. Therefore, it must be strictly controlled, and all structures and members must be calculated based on the limit state of load-bearing capacity to ensure sufficient reliability. (2) Limit State of Serviceability This type of limit state refers to the state where a structure or member reaches a specified limit value for normal use or durability. It is considered that the limit state of serviceability has been exceeded when any of the following conditions occur: 1) Deformation that affects normal use or appearance; 2) Local damage that affects normal use or durability, such as excessively wide cracks; 3) Vibration that affects normal use; 4) Other specific states that affect normal use. When a structure exceeds this type of limit state, it cannot function normally, affecting its durability and serviceability, but it generally does not lead to personal injury or significant economic loss. During design, reliability can be slightly lower than that of the limit state of load-bearing capacity. Typically, structures and members are first designed based on the limit state of load-bearing capacity and then checked based on the limit state of serviceability. Section Basic Concepts of Probabilistic Limit State Design I. Action Effects and Structural Resistance (1) Actions and Action Effects Action refers to all causes that produce internal forces and deformation in a structure. Action effects refer to the internal forces and deformation produced in a structure under the influence of various actions, denoted as "S". When internal forces and deformation are produced by loads, they are referred to as load effects. Actions and action effects are generally random variables and are approximately linearly related, expressed as: S = CQ (1-1) where S — Design value of the combination of load effects; Q — A certain load; C — Load effect coefficient. For example, a simply supported beam subjected to a uniformly distributed load q with a clear span of l0 has a mid-span bending moment of M = 1/8ql02. Here, M is the load effect, q is the load, and 1/8l02 is the load effect coefficient. Load effects are one of the bases for proposing predetermined functional requirements for the structure and are also the main basis for design. (2) Structural Resistance Structural resistance refers to the ability of a structure or member to withstand internal forces and deformation. It is a function of material properties, cross-sectional geometric characteristics, and calculation models. Structural resistance is also a random variable.