Dr. Ahed Habib
This article explores non prestressed concrete footing design per ACI 318-19, emphasizing its significance, design considerations, and common practices.
Concrete footings are a vital component in structural engineering, as they play a critical role in distributing loads from structures to the soil below. These footings are designed to transfer different types of loads from the structure to the soil, such as dead load, live load, wind load, and seismic load. By doing so, they ensure that the building stays stable under various conditions.
Concrete footings vary in type, with some common ones being spread or pad footings, strip or wall footings, and pier footings. The choice of footing type depends on the structure's design requirements, the site's soil conditions, and the applied loads.
In recent years, technological advancements have made it easier to design and analyze concrete footings with structural analysis tools like ClearCalcs that integrate ACI 318-19 guidelines automatically.
This article explores nonprestressed concrete footing design per ACI 318-19, emphasizing its significance, design considerations, and common practices.
Table of Contents:
In general, footings serve as a critical intermediary between a structure and the ground upon which it stands. They transfer loads from the structure to the underlying soil or rock.
These loads, such as dead load, live load, wind load, seismic forces, etc., if not appropriately distributed, can result in structural failure, making the design and placement of footings paramount.
The effective distribution of these loads ensures that no point underneath the structure undergoes stress (or bearing pressure) that exceeds the soil's capacity, which could lead to undesirable outcomes such as excessive settlement, tilting, or even complete failure.
To ensure stability and durability of the structure, footings must be designed with a deep understanding of the loads exerted by the structure and the soil's bearing capacity.
Concrete footings can be broadly categorized into several types, each with its unique application and design considerations. The most common types include:
Figure 1. Isolated footing
Figure 2. Combined footing
Figure 3. Pier footing
The American Concrete Institute (ACI) provides the essential guidelines for designing these footings through ACI 318-19. The ACI 318-19 code (Building Code Requirements for Structural Concrete), an update from the earlier ACI 318-14, provides comprehensive guidelines for designing concrete footings.
The code provides detailed specifications on concrete design, including considerations for compressive strength, flexural capacity, development length, and other essential parameters. The ACI 318-19 specifications ensure that structures can withstand various forces, such as seismic and wind forces.
The code outlines the importance of accurate load calculations, effective depth considerations, and appropriate reinforcement details in concrete footing design.
Key terms and aspects covered in the code include flexural reinforcement, axial load considerations, shear strength, spacing of reinforcement, and more. Concepts such as one-way shear, two-way shear, modification factors, and strength reduction factors have specific guidelines in ACI 318-19, ensuring that structures can effectively resist external and internal forces.
By adhering to ACI 318-19 guidelines, structural engineers ensure that their designs for concrete footings and other structural elements meet internationally recognized best practices. This guarantees the safety and stability of structures and instills trust in stakeholders, from builders to occupants.
Accurately determining the loads that act upon footings is the preliminary and most crucial step for designing safe and efficient footings, ensuring that the structure remains stable under varied conditions.
Typically, several loads are involved in the process of calculating loads on footings. Example of these loads includes:
In addition to these loads, the ACI 318-19 requires computing multiple load combinations as detailed in the ASCE 7 to ensure the safety of the footings when these loads are applied simultaneously.
Some of the basic load combinations in the ASCE 7 are listed as follows:
where D is the dead load; L is the live load; Lr is the roof live load; S is the snow load; R is the rain load; W is the wind load; Wt is the Tornado load
Prior to the design of footings, understanding the nature and characteristics of the underlying soil is paramount.
Soil investigation provides valuable data on the type, strength, and other properties of the soil, enabling engineers to design footings that efficiently transfer loads without causing undue settlements or failures.
The soil-bearing capacity, often denoted as
, refers to the maximum load per unit area that the soil can support without failure. The computation involves consideration of factors like soil type, depth, moisture content, and more. Common soil types include:
Determining soil-bearing capacity involves laboratory tests like the triaxial or unconfined compression test and field tests such as the standard penetration test. Accurate determination ensures footings are designed for optimal size and depth to prevent undue settlements or bearing failures.
The sizing of concrete footings is pivotal in ensuring that the loads from the structure are effectively distributed to the underlying soil. The dimensions of footings play a crucial role in accommodating the different types of loads, such as dead loads (permanent or stationary loads like the weight of the building), live loads (transient loads like occupants or furniture), wind loads, and other dynamic forces. For a typical footing, the dimensions are commonly determined using the following formula:
is the axial load imposed on the footing from the structure;
is the allowable bearing capacity of the soil;
is area of the footing.
For a rectangular footing, the area can then be used to calculate each dimension by assuming one of them and then computing the second one. It is worth noting that the axial load is calculated for the corresponding load combination (as per the investigated design problem) in the unfactored (service) load case. In this regard, the ASCE 7 can be used to obtain suitable load combinations that consider the dead loads, live loads, wind loads, and seismic loads.
As per the ACI 318-19, flexural reinforcement, shear reinforcement, and axial load capacity play vital roles in ensuring safety and serviceability. In this regard, reinforcement, typically in the form of steel bars or "rebar," significantly enhances the tensile strength, shear capacity, and overall resilience of concrete footings.
The inherent properties of concrete, particularly its impressive compressive strength but relatively weaker tensile strength, make reinforcement an indispensable aspect of concrete design.
Technically, the primary purpose of incorporating reinforcement in footings is to counteract the tensile stresses that may develop due to the applied loads. Steel reinforcements fill this gap and ensure that the footing remains intact and functional under various loading conditions. During the design process, the reinforcements are selected by checking the section's resistance against one-way shear, two-way shear, and flexural actions.
The effectiveness of reinforcement depends on its proper placement and spacing within the footing. According to the ACI 318-19 guidelines:
An example of the reinforcement details for an isolated footing is given in Figure 4.
Figure 4. An illustration of the reinforcements' detail in an isolated footing
Designing concrete footings is a long process, and even experienced engineers can sometimes overlook critical aspects. Here is a list of some common mistakes in concrete footing design:
Staying updated with the latest guidelines, utilizing reliable computational tools, and thorough peer-review processes are essential to mitigate these errors.
Compliance with established standards is not just a best practice but a crucial necessity given the established regulations worldwide. To ensure compliance with ACI 318-19, professionals often refer to a detailed checklist (see Table 1 for summary).
Table 1: Summary of Design Requirement for Concrete Footing in ACI 318-19
|Design Requirement||Reference in ACI 318-19|
|Accurate calculation of loads such as dead, live, seismic, and wind.||Chapter 5: Loads|
|Appropriate selection and integration of materials based on their compressive and tensile strengths.||Chapter 19: Concrete: Design and Durability|
Chapter 20: Steel Reinforcement Properties, Durability, & Embedments
|The positioning, spacing, and size of reinforcement bars.||Chapter 25: Details of Reinforcement|
|Soil's bearing capacity based on comprehensive soil investigations.||Chapter 22: Foundations|
ASTM Testing Standards and AASHTO Standards (external source)
|Adherence to the specifics of seismic design, strength reduction factors, and flexural reinforcement.||Chapter 13: Foundations|
Chapter 18: Earthquake-Resistant Structures
Chapter 22: Sectional Strength
Chapter 21: Strength Reduction Factors
Chapter 24: Serviceability
This list encompasses a variety of critical points, including verifying the accurate calculation of loads such as dead, live, seismic, and wind. It also ensures the appropriate selection and integration of materials based on their compressive and tensile strengths. The positioning, spacing, and size of reinforcement bars, as defined by the code, are also examined, guaranteeing that the footing can aptly handle applied shear forces and axial loads.
Furthermore, the checklist emphasizes that foundation designs align with the soil's bearing capacity based on comprehensive soil investigations. Lastly, ensuring adherence to the specifics of seismic design, strength reduction factors, and flexural reinforcement, among other parameters, completes the compliance list. However, compliance is not just about checking off items on a list. It is an evolving process, largely because codes and standards are not static entities.
As advancements in research, technology, and construction methods come to the fore, design codes like ACI 318-19 undergo revisions. These updates incorporate the latest knowledge, addressing new challenges, or incorporating innovative solutions.
For a structural engineer, staying updated with the latest revisions is a matter of adherence and a commitment to excellence and safety. An updated code often brings about more refined and nuanced approaches to design, emphasizing a more effective and efficient integration of components.
ClearCalcs is a powerful structural design tool that simplifies complex calculations. With its user-friendly interface and time-saving features, like dynamically linking load reactions between two or more calculators, it streamlines the structural design process, saving valuable time for structural engineers and designers.
The ClearCalcs spread footing design calculator is designed to comply with the ACI 318-19 code.
This calculator enables professionals to perform spread footing designs quickly and accurately. The user-friendly interface makes it easy for engineers to define footing and column geometries, catering to a variety of columns ranging from wood and steel to generic columns with steel base plates.
An innovative feature of this calculator is the dynamic linking of load reactions between structural elements, ensuring that footing and column designs are intricately linked and vetted for adequacy.
Moreover, the traffic light checks provide an instant overview of the design's compliance, covering critical aspects such as factored moment capacity and development lengths.
ClearCalcs strip/wall footing design calculator enables engineers to design wall footings that are compliant with the ACI 318-19 concrete code.
Users can specify the wall footing's geometry and integrate various loads such as vertical, lateral, shear, or moment loads. The software also takes into account the self-weight of the wall footing and the soil above it.
One of the standout features of ClearCalcs is its ability to incorporate the latest building codes and standards that align with the IBC 2021. The traffic light checks ensure that the wall footing design meets crucial criteria such as bearing capacity and serviceability loads.
ClearCalcs concrete pier footing design calculator is designed to make the process of specifying pier diameters, concrete strengths, post and connection types, and defining loads in both the X and Y axes effortless for engineers.
Its versatility makes it relevant for diverse design scenarios. The ASD and LRFD load combinations tables are in compliance with ASCE 7 and ACI 318-19 design code requirements.
Instant results are comprehensive, including checks for bearing capacity under service loads, as well as two-way or punching shear design.
Structural engineering emphasizes the stability and longevity of buildings by relying heavily on the proper design and analysis of concrete footings.
In the United States, the design of this foundational element, responsible for transferring the loads from structures to the soil beneath, is governed by the guidelines outlined in the ACI 318-19 code.
Ensuring adherence to these standards is about compliance and guaranteeing safety, bearing pressure consistency, and serviceability. With varying types of loads like dead, live, wind, and seismic, the intricacies of designing concrete footings are substantial.
The use of technology, such as the ClearCalcs platform, aligned with the rigorous standards of ACI 318-19, offers structural engineers a precise and efficient means of designing footings, ensuring that structures stand firm and secure.
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