This comprehensive guide focuses on the importance of base stiffness and base connections, along with the potential impact of foundation settlement on steel portal frames. Clause 5.1.2.4 of BS 5950: Part 1 provides guidelines on the stiffness of the base of a column, and the base stiffness should be treated as a beam stiffness. Other base support options, such as vertical and horizontal support options of rigid, free, and spring stiffness, are also discussed. Foundation settlement can have a significant impact on the bending moments of a frame, and isolated settlement can be more damaging than overall settlement. The article also provides an example of a triangulated roof truss supported by semi-rigid bases and highlights the benefits of analyzing with nominally pinned bases. By following the guidelines outlined in the article, designers can model and analyze base connections accurately and minimize the impact of foundation settlement on a structure’s bending moments.
Understanding Base Stiffness: A Crucial Aspect Of Soil-Structure Interaction
Discover the significance of base stiffness and base connections, along with the potential effects of foundation settlement on structures, in this comprehensive guide. From understanding the guidelines outlined in Clause 5.1.2.4 of BS 5950: Part 1 to exploring different base support options, such as rigid, free, and spring stiffness, this article covers it all. You’ll also gain insight into modeling and analyzing base connections and receive expert advice on minimizing the impact of foundation settlement on a frame’s bending moments. Plus, we’ll showcase an example of a triangulated roof truss supported by semi-rigid bases and highlight the benefits of analyzing with nominally pinned bases. Don’t miss out on this valuable information!
Importance of Base Stiffness and Different Base Connections
Clause 5.1.2.4 of BS 5950: Part 1 gives guidelines on the stiffness of the base of a column. If the column is firmly connected to a strong foundation, the base stiffness is assumed to be equal to the column’s stiffness. But if the connection is not very strong, the base stiffness can be assumed to be only 10% of the column stiffness. If the connection is a pin or rocker, the base stiffness is taken as zero.
It’s important to note that the base stiffness should be treated as a beam stiffness, not a column stiffness. To achieve the required base stiffness, a dummy beam needs to be connected to the base. The dummy beam should have the same length and inertia as the column, and its end should be fixed remotely from the column. This will help to model and analyze the interaction between the soil and the structure.
To avoid any confusion caused by the moment at the end of the dummy member, the remote end of the dummy beam can be pinned. Additionally, the length of the dummy beam can be reduced to 0.75 times the column length. This method is especially useful when there is little information available about the foundation stiffness.
Other Options for Base Support
Steel portal frames are popular in construction due to their strength, versatility, and cost-effectiveness. However, designing the base connection between the column and foundation can be a challenge both in modeling and practical implementation.
There are two types of base connections: a rigid base with dummy member inertia equal to column inertia, and a nominally pinned base with dummy member inertia of 10% of the column inertia. For portal frames, the base stiffness can be modeled as 10% of the column stiffness for the ultimate limit state and 20% of the column stiffness for the serviceability limit state. Separate analyses will most likely be required for both limit states.
The column base connection to the foundation is often difficult to define the distinction between pinned and fixed bases in practical details. Portal frames are typically analyzed with pinned bases, as the cost of moment-resisting foundations often exceeds the savings in frame weight achieved by using fixed bases.
In practice, it’s rare to find column base connections that are immediately identifiable as pinned. Instead, alternative base details are often used, which allow for easier adjustment and plumbing of the column. However, it’s important to note that such bases may still be able to resist moments and provide stability during a fire, particularly when the column is located near a site boundary.
The Building Regulations define when a building must incorporate this requirement. For boundary columns, the base detail must be capable of resisting moment, although such bases are generally modeled as pinned for the frame analysis.
Effects of Foundation Settlement on Structure
Let’s explore other options for base support, as well as the impact of foundation settlement on a structure. Besides rotational base fixity, there are also other base support options like vertical and horizontal support options of rigid, free, and spring stiffness. For more detailed information on these options, we suggest checking out Reference 16.
Foundation settlement can have a significant impact on the bending moments of a frame. Isolated settlement can be more damaging than overall settlement, and finding detailed advice on this matter can be challenging. Settlement of isolated foundations can dramatically affect the bending moments of a frame. If you have information about foundation details and soil properties, you can use spring supports to model the compressibility of the soil. However, you may also need to use horizontal releases to reflect reality. Keep in mind that any support other than the foundation can cause the structure to “spread,” and you may need to release one or more supports to reflect this.
Example of Triangulated Roof Truss and Semi-Rigid Bases
Consider a roof truss that is supported by two columns. To prevent compression in certain panels of the bottom boom, it is necessary to model the supports with a horizontal release when designing the truss. Even when the bases of a structure are semi-rigid, analyzing with pinned bases is recommended, as this typically produces conservative bending moments. It’s essential to take into account the impact of fixity on foundation costs before specifying fixed bases, which can quickly become prohibitively expensive. Most nominally pinned bases can be advantageous in reducing sway deflections, and it is important to follow the provisions of Clause 5.1.2.4 and avoid assuming full fixity in the analysis.
Conclusion and Recommendations
In conclusion, understanding the significance of base stiffness and base connections is crucial in designing safe and cost-effective steel portal frames. By following the guidelines outlined in Clause 5.1.2.4 of BS 5950: Part 1, designers can model and analyze base connections accurately and minimize the impact of foundation settlement on a structure’s bending moments. Alternative base details can be used for easier adjustment and plumbing of the column, but it’s important to ensure they can still resist moments and provide stability during a fire.
When analyzing a roof truss, it’s recommended to use pinned bases, even if the bases of the structure are semi-rigid, as this typically produces conservative bending moments. Additionally, it’s essential to consider the impact of fixity on foundation costs before specifying fixed bases, which can quickly become prohibitively expensive.
To avoid any confusion caused by the moment at the end of the dummy member, the remote end of the dummy beam can be pinned. Furthermore, spring supports can be used to model the compressibility of the soil, but horizontal releases may also be necessary to reflect reality.
Overall, by taking these factors into account, designers can ensure that their structures are safe, cost-effective, and comply with building regulations. For more detailed information on base support options, the effects of foundation settlement, and other related topics, check out Reference 16.
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