Concrete Footing Design to ACI 318-19

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:

• The Role of Footings in Supporting Structures.
• Introduction to ACI 318-19 and Its Significance.
• Design Considerations for Concrete Footings.
• Common Mistakes to Avoid.
• Time-Saving Tools to Analyze and Design Concrete Footings to ACI 318-19.

Understanding the Basics

The Role of Footings in Supporting Structures.

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.

Footing Types and Forms

Concrete footings can be broadly categorized into several types, each with its unique application and design considerations. The most common types include:

• Isolated Footing: Typically square or rectangular, these footings support individual columns. They distribute the load over a larger area, ensuring the soil's bearing pressure isn't exceeded.

Figure 1. Isolated footing

• Combined/Wall Footing: This runs continuously along the length of walls or supports multiple columns and helps distribute the load of the wall and any superimposed loads to the ground.

Figure 2. Combined footing

• Pier Footing: Essentially column footings but at a greater depth, pier footings transfer loads to a deeper, more stable stratum when surface soils are weak or collapsible.

Figure 3. Pier footing

TIP: Get a detailed analysis of the various footing types and their specific applications.

Introduction to ACI 318-19 and Its Significance.

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.

Design Considerations for Concrete Footings

Load Calculation

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:

• Dead Load: This refers to the permanent static load exerted by the structure itself, including the weight of walls, roofs, and other permanent elements. Mathematically, it can be derived from the product of the volume and density of each construction element.
• Live Load: Variable or dynamic loads introduced by the occupancy or use of the structure. This could encompass loads due to people, movable equipment, furniture, etc. Standards like ASCE 7 provide guidelines for estimating live loads.
• Wind Load: These loads arise from the force of the wind acting on the structure's surfaces. Depending on the location and height of the structure, these can have a significant impact, necessitating detailed analysis.
• Seismic Load: In regions prone to earthquakes, the seismic or earthquake-induced loads are critical. These loads are dynamic and can exert both lateral and vertical forces on the structure.

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:

• D
• D+L
• D+(Lr or 0.7S or R)
• D + 0.75L + 0.75(Lr or 0.7S or R)
• D + 0.6(W or WT)
• D + 0.75L + 0.75(0.6(W or WT)) + 0.75(Lr or 0:7S or R)
• 0.6D + 0.6(W or WT)
• 1.4D
• 1.2D+1.6L+(0.5Lr or 0.3S or 0.5R)
• 1.2 D+(1.6Lr or 1.0S or 1.6R) + (L or 0.5W)
• 1.2D + 1.0(W or WT) + L + (0.5Lr or 0.3S or 0.5R)
• 0.9D + 1.0(W or WT)

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

Soil Investigation and Bearing Capacity

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 $q_u$, 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:

• Clay: Characterized by small particle size and low permeability, clayey soils exhibit significant volume changes with moisture variations.
• Silt: Silt soils have intermediate particle sizes and can be susceptible to liquefaction under seismic actions.
• Sand: Granular and with good drainage properties, sandy soils offer higher shear strength but can be prone to shifting under loads.
• Gravel: With large particle sizes, gravelly soils offer high shear strength and good drainage.

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.

Designing Footing Dimensions

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:

$A=P/q_{all}$

where $P$ is the axial load imposed on the footing from the structure; $q_{all}$ is the allowable bearing capacity of the soil; $A$ 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.

Reinforcement and Reinforcement Layout

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:

• The reinforcement should be adequately covered by concrete to ensure protection against environmental factors and to provide bond strength. The minimum reinforcement cover is defined based on the exposure condition, aggregate size, and rebar diameter.
• Spacing of rebars is critical. It should be neither too close (to prevent congestion and ensure proper concrete flow) nor too far apart (to provide the required strength). Spacing is governed by the size of rebars, concrete footings' dimensions, and structural design requirements.

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

Common Mistakes to Avoid

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:

• One common error is making incorrect assumptions regarding soil characteristics. Assuming a uniform soil profile, rather than conducting a thorough soil investigation, can lead to significant discrepancies between the assumed and actual bearing capacities, potentially compromising the footing's stability.
• Manual calculations are the basis of foundational designs and are susceptible to human errors. Missing out on load combinations or miscalculating the resultant loads can lead to under-designed or over-designed footings.
• Utilizing outdated design codes, such as referencing ACI 318-14 when ACI 318-19 is the applicable standard, can also introduce errors and non-compliance in the design.
• Oversights in reinforcement details, like incorrect spacing, improper reinforcement ratio, or inadequate development length, can compromise the structural integrity of the footing.
• Furthermore, neglecting specific load types, such as seismic or wind loads in areas prone to such forces, can lead to catastrophic failures.
• Lastly, not accounting for modifications or adjustments like strength reduction factors, size effects, or modification factors based on the specific conditions and requirements can skew the design's accuracy.

Staying updated with the latest guidelines, utilizing reliable computational tools, and thorough peer-review processes are essential to mitigate these errors.

Compliance with ACI 318-19

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.

Time-Saving Tools to Analyze and Design Concrete Footings to ACI 318-19

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.

Spread Footing Calculator to ACI 318-19

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.

Wall Footing Calculator to ACI 318-19

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.

Pier Footing Calculator to IBC 2021, ACI 318-19

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.

Conclusion

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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Additional Resources

• ACI 318-19
• ACI 318-14
• ASCE 7
• The Foundation Engineering Handbook
• Foundation Engineering Handbook