Chloe Zhang
Story drift, also known as inter-story drift, is a deflection of a single story relative to the base or ground level of a structure. It is important to understand story displacement to comprehend story drift. The drift index varies significantly over the height of the structure, and the maximum interstory drift index is usually more than twice the global drift index. To evaluate the story drift, it is necessary to create a centre of gravity for each story as a node. The story drift is then determined at the top and bottom edges of the story. There are four loading cases to consider when determining the design story drift per Chapter 12.8.6 of ASCE 7-10. The story drift can be calculated and reported based on calculated displacements in the X-direction.
| Characteristics | Values |
|---|---|
| Story drift determination | Create a center of gravity for each story as a node |
| Story displacement | Deflection of a single story relative to the base or ground level of the structure |
| Seismic drift | Depends on the code and country but are usually given directly in the code |
| Seismic design | Provide life safety (strength and ductility) and damage control (serviceability drift limits) |
| Plastic hinge development | Studied via elastic-plastic time history analysis |
| Plastic story drift | Depends on the story height, usually represented as "h" or "H" |
| Inter-story drift | Calculated based on displacements in the X-direction |
| Maximum story drift | Calculated for each mode shape individually for each direction |
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What You'll Learn
- Understand story displacement, the deflection of a single story relative to the base or ground level of the structure
- Consult building code documents for your area, such as ASCE 7-16
- Calculate inter-story drift using specific software, like RISA or SkyCiv
- Consider seismic design forces and loading cases, especially for taller structures
- Account for the redundancy factor (rho) and the Direct Analysis Method
Understand story displacement, the deflection of a single story relative to the base or ground level of the structure
To understand story drift, also known as inter-story drift, we must first understand story displacement. Story displacement is the deflection of a single story relative to the base or ground level of the structure. As we move up the structure, we can expect higher total displacement values. In SkyCiv Structural 3D, these values are shown as the X, Y, or Z-axis displacements, depending on the specific structure's lateral directions.
Story drift is the deflection of a single story relative to the previous story. Story drift is relative to the nearby levels, so we can expect a different-looking graph when comparing story drift to the height of the structure. The calculation of story drift is simple: to find the story drift of level "X", subtract the story displacement of level "X-1" from the story displacement of level "X". For example, the story drift of level 4 is the total story displacement of level 4 minus the story displacement of level 3. Story drift is sometimes shown as a ratio, dividing the story drift value by the story height.
When considering the limits and allowable story drift, the type of force causing the deflection is important. Seismic forces have more rigorous limits, which depend on the code and country but are usually given directly in the code. For wind loads, ASCE 7 does not impose an allowable drift limit, but there are common practices and commentary that engineers should consider. Common values of inter-story drift range from H/200 to H/600, with the most common limit being H/400.
The story drift ratio, as required by the code, must be checked against the limit of 2.0% under earthquakes. The story drift ratio around the intermediate level of the building is typically more critical than at the top. Additionally, the importance of story drift is in the design of partitions and curtain walls.
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Consult building code documents for your area, such as ASCE 7-16
When determining plastic story drift, it is essential to refer to the building code documents applicable in your region, such as the ASCE 7-16. The ASCE 7 standard is a comprehensive resource that provides requirements and guidelines for structural design. It covers various load types, including dead, live, soil, flood, wind, snow, rain, atmospheric ice, and earthquake loads, as well as their combinations. The ASCE 7 standard is designed to be incorporated into building codes and serves as a valuable reference for engineers and designers involved in construction projects.
ASCE 7-16, specifically, offers provisions for lateral drift determination, which is an essential aspect of structural analysis. Lateral story drift refers to the displacement of a single story relative to the base or ground level of a structure. This displacement can be caused by various forces, including seismic activity and wind loads. By consulting the ASCE 7-16 provisions, engineers can understand the allowable limits for lateral drift and ensure that their designs meet the required safety standards.
The ASCE 7 standard has undergone several revisions, and it is crucial to refer to the latest version or the edition specifically mandated by the local building codes. For instance, the ASCE 7-22 edition, released in 2022, addresses the design loads and associated criteria for buildings and other structures. It includes provisions for wind loads, with specific attention to tornadic winds of up to 135 mph. This information is vital for designing structures that can withstand high wind speeds and ensuring the safety of occupants and the structural integrity of the building.
In addition to the ASCE 7 standard, there may be other relevant building code documents specific to your region or country. These documents could include local or national design codes, standards, and guidelines that supplement or override the ASCE 7 provisions. Therefore, it is essential to consult with local authorities or structural engineering professionals familiar with the applicable codes in your area to ensure compliance with all relevant standards.
By referring to the ASCE 7 standard, particularly the ASCE 7-16 edition, and other pertinent building code documents, engineers can make informed decisions about structural design, including the determination of plastic story drift. These resources provide valuable insights into the allowable limits of lateral drift, ensuring that structures are designed to withstand the anticipated loads and displacements, thereby enhancing the overall safety and resilience of the building.
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Calculate inter-story drift using specific software, like RISA or SkyCiv
RISA is a software that can be used to calculate inter-story drift. To calculate inter-story drift for a particular direction, the displacement at the lower level is subtracted from the current story displacement. For example, to calculate the X-direction drift for story 2, the X displacement for story 1 is subtracted from the X displacement for story 2.
RISA also provides a Drift Definitions spreadsheet that defines where the drift calculations will be performed. Drift calculations can be performed at specific elevations, but these must be manually entered by the user in the spreadsheet. These elevations are defined with respect to the vertical axis of the model.
Additionally, RISA provides a Story Drift spreadsheet that lists the drift for all defined diaphragms, elevations, and definitions that exist in the Drift Definitions spreadsheet. The results are reported in the same order as they appear in the Drift Definitions spreadsheet.
Another software that can be used to calculate inter-story drift is SkyCiv Structural 3D. In SkyCiv, the story displacement, which is the deflection of a single story relative to the base or ground level of the structure, can be easily found as users can view their structure's displacement values after analysis.
To calculate the story drift in SkyCiv, the story displacement of a particular level is subtracted from the story displacement of the level below it. For example, the story drift of level 4 is equal to the total story displacement of level 4 minus the story displacement of level 3. Story drift can also be shown as a ratio by dividing the story drift value by the story height.
It is important to note that for seismic drift, there may be additional provisions based on the code being used, so reviewing the relevant code documents is necessary when considering story drift in this case.
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Consider seismic design forces and loading cases, especially for taller structures
When designing taller structures, it is crucial to carefully consider seismic design forces and loading cases to ensure structural integrity and safety. Seismic loads can have a significant impact on the behaviour and stability of a structure, and their effects become more pronounced as the height of the structure increases.
The first step in determining the seismic design forces is to identify the Seismic Design Category (SDC) of the structure. The SDC takes into account the risk category, life safety considerations, and the essential nature of the structure. This classification helps determine the required lateral forces, which include a percentage of the dead load, dead plus live load, and wall weight for different connection types.
Next, the fundamental period of the structure, denoted as Ta, needs to be determined. This value can be user-defined or approximated using methods outlined in ASCE 7-16. The fundamental period represents the inherent vibration characteristics of the structure and is crucial for subsequent calculations.
The seismic design base shear, V, is then calculated using parameters such as the design spectral response acceleration (Sd1), the fundamental period (T), and the mapped maximum considered earthquake spectral response acceleration (S1). Once V is determined, the forces are distributed along the height of the structure using guidelines from ASCE 7-16.
To ensure the structure can withstand seismic forces without failure, it is essential to consider the amplification of forces in certain elements. The variable Ωo is introduced as an amplification factor to prevent premature failure and ensure full energy dissipation and ductility. This factor is applied to elements such as connections, bolts, welds, and anchor bolts, ensuring they are stronger than the maximum anticipated forces in the braces.
Additionally, the response modification coefficient, R, is an important consideration in seismic design. It allows for an elastic design while taking into account the overstrength and ductility of the lateral force-resisting system. The R factor triggers specific requirements, including detailing, modal analysis, force amplification, and collector force increases.
By following these steps and considering the relevant codes and standards, such as ASCE 7-16, IBC, and ACI 318-11, engineers can effectively design taller structures to withstand seismic forces and ensure the safety and stability of the building.
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Account for the redundancy factor (rho) and the Direct Analysis Method
To determine plastic story drift, it is essential to understand story displacement, which is the deflection of a single story relative to the base or ground level of a structure. The drift index varies significantly over the height of the structure, and it is influenced by factors such as the type of force causing the deflection and the story height. Seismic forces, for example, have more stringent limits compared to wind loads.
When using the ASCE and IBC codes, the Story Drift calculations for strength level combinations account for the inelastic deflection, known as the Cd factor. This is achieved by amplifying the joint deflections at each level by Cd/I, as outlined in Section 12.8.6 of ASCE-7. The value of I, the importance factor, is based on the risk or occupancy category and is used to amplify the seismic forces applied to the structure.
To negate the impact of the redundancy factor, rho (ρ), the joint deflection results are divided by rho before performing drift calculations. This step is outlined in Section 12.3.4.1, Item 2 of ASCE-7-16. The redundancy factor, ρ, is either 1.0 or 1.3, depending on whether an individual element can be removed from the lateral force-resisting system without significantly compromising the remaining structure. If removing an element causes a reduction in story strength of more than 33%, or introduces extreme torsional irregularity, the redundancy factor is affected.
The introduction of the redundancy coefficient in building codes was a response to the observation of structures damaged by the 1994 Northridge earthquake. It was found that economic pressures had led engineers to design structures with minimal redundancy, particularly in steel frame and concrete shear wall buildings. The redundancy coefficient aims to encourage the design of structures with greater redundancy and a higher number of elements to resist lateral forces.
In summary, when determining plastic story drift, it is crucial to consider the relevant building codes, the influence of the Cd factor on inelastic deflection, and the role of the redundancy factor (rho) in ensuring adequate structural redundancy.
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Frequently asked questions
Story drift, also known as inter-story drift, is the deflection of a single story relative to the base or ground level of the structure.
Story drift is the difference in total displacement at the top and bottom of the story. It is determined in the corresponding centres of gravity. You can use software such as SkyCiv Structural 3D to calculate story drift.
The limits for allowable story drift depend on the kind of force causing the deflection. For wind loads, ASCE 7 does not impose a limit, but for seismic loads, the limits depend on the code and country.
