This is the final of a series of eleven articles on diaphragms. In this series I have presented a basic discussion of diaphragm behavior and code requirements, with some practical insights. Here are the articles in the series, with their links:
- A History of Diaphragms
- Types of Diaphragms
- Characteristics and Modeling of Flexible Diaphragms
- Classifying Rigid Diaphragms
- Special Considerations for Rigid Diaphragms
- Classifying Semirigid Diaphragms
- Characteristics and Modeling of Semirigid Diaphragms
- Impact of Mesh Size in Semirigid Diaphragm Analysis
- Impact of Diaphragm Properties in Semirigid Diaphragm Analysis
- Loads on Rigid, Semirigid, and Flexible Diaphragms
- Diaphragms: Conclusions and Recommendations
Historically the influence of the diaphragm and the distribution of the lateral forces to the frames was performed manually. Software was unavailable or computers lacked the capacity to perform more rigorous analyses. For simplicity diaphragms were treated as either flexible or rigid, they were rarely rigorously investigated as semirigid. As analysis and design techniques have improved, as there is a tendency to push structures to their capacity limits in order to achieve more economical structures, and with structures becoming more architecturally challenging, it has become increasingly obvious that those old assumptions are in many cases inadequate and unsafe. As a result, the codes have become more reluctant to permit the use of the rigid diaphragm assumption, allowing it only under a narrow set of conditions. Fortunately, software has become more accommodating and computers have become more capable of handling the large analytical models created by the inclusion of semirigid diaphragm elements.
Take care to correctly categorize each diaphragm: Flexible, Semirigid, or Rigid. Improperly implementing the diaphragm model in your analysis model can result in significantly erroneous results.
I didn’t include much discussion about the requirements for diaphragms under wind loads. While they are less stringent than for seismic loads, I recommend that for simplicity you use the same diaphragm model for wind loads as for seismic loads.
The rigid diaphragm assumption still provides the simplest and fastest approach; take advantage of that when you can. Use good judgement, and only do so in limited situations where you are confident it is appropriate, but consider the IBC’s more liberal definition of diaphragms, in which all diaphragms can be classified as either Flexible or Rigid.
Any diaphragm can be analyzed as Semirigid, even if it qualifies as Flexible or as Rigid. It may be easier to model it as Semirigid than to go to the effort to determine what category it is in. And if done correctly the results will be more accurate than if either the Flexible or Rigid assumption is used.
When modeling a diaphragm as Semirigid, use as large a mesh as reasonable: 4 ft or even 8 ft. The analysis time is substantially reduced, with little impact on the accuracy of the results.
When modeling a diaphragm as Semirigid, use reasonably accurate material property values, but don’t waste time in an effort to have a high degree of precision. Be practical. In most cases the distribution of forces through the diaphragms to the frames isn’t particularly sensitive to the diaphragm properties. If the values you use are close, that is good enough.
If you are interested in more information on this topic, some time ago I presented a webinar in which I covered these topics in more depth. It is available on-demand, free, from Bentley Systems' website: Modeling and Analysis of Diaphragms.
The most valuable recommendation that I can give you is to use your analysis tools to experiment and investigate. Create various models and see for yourself how changes in the types of diaphragms and changes in the diaphragm properties impact the analysis results. For the various variations in the model, look at and compare displaced shapes, building periods, frame story shears, and drift, among others. Doing so will give you confidence in your analytical results even when you don’t know the properties exactly; you will work more practically and become more productive, while providing better designs.
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