Performance Based Design is a method for designing structures with predictable performance for initially considered loads. This approach is used to design a new building or evaluate an existing structure. In performance-based design, civil engineers first determine the specific performance of structures in consultation with the owner and then proceed to design or evaluate the existing structure.
Performance-based design is often used in the context of seismic loading. Assessment of the existing structure or design structure against likely seismic loading is completed first. However, in modern construction, performance-based design is now used in wind and earthquake design, blast analysis and design, and progressive collapse analysis.
Furthermore, the behavior of non-structural elements is also taken into account in the performance-oriented design of structures.
Due to the dynamic behavior of the loads to be considered, such as wind and blast loads, civil engineers have now started to consider these types of construction.
Explaining this concept better, we first select an occupancy level at which the structure must behave at a load initially determined by the owner. For example, consider a magnitude 7.5 earthquake. We then consider the load/acceleration values relevant for performance-based earthquake design. Now we know the inputs and outputs. We then change the structural stiffness to utilize these loads and outputs. This can be referred to as performance-based seismic building design.
In summary, the following steps can be identified in the performance-oriented construction planning of a new building.
- Set performance goals for specific inputs to consider.
- Proceed to the first draft
- Check whether the desired results were achieved.
The deformation of the structure is assessed for drift by monitoring the behavior of structural and non-structural elements. Drift limitations are set forth in guidelines such as FEMA 273 and FEMA 356.
Applicable restrictions apply not only to vertical elements but also to other elements.
The guidelines provided in FEMA 356 can be used to define performance levels. They can be specified in terms of the lateral deformation/deviation of the building or as the formation of the hinges.
The occupancy level has three defined states based on the rotation of the element. They are specified according to the rotation of the hinge. The following figure from FEMA 356 shows the formation of occupancy levels according to global change.
As stated in FEMA 356, there are three levels of occupancy.
- Immediate purchase
- Elevator safety
- Collapse prevention
Occupancy levels are also defined based on the deformation of the structure as discussed previously. The definition of the occupancy levels of the structural elements is based on the rotation of the hinge.
Although the behavior of the elements is as shown in the figure above, a simplified behavior is considered in the structural analysis. The displacement force cure given in the Sap2000 structural analysis software manual is shown in the figure below.
The variation of each occupancy level is shown in the figure above. Furthermore, collapse does not mean that they completely collapse. There is some stiffness in the structural member after it suddenly loses its stiffness, as indicated by the CDE area in the figure above.
The behavior of the element is expressed by the rotation of the hinge. The hinge rotation can be specified in the following figure. Specifies the relative rotation and levels at which the hinge displays each occupancy level.
The figure above was extracted from the Sap2000 software.
The rotation of the element or hinge is defined by the moment-curvature diagram. If we can generate the bending moment of a section, this can be used to generate the hinge rotation curve. Sap2000 offers this possibility. There are also other software that offer these options.
When the profile is created using Profile Designer in SAP2000, the moment-curvature diagram required to create the hinge is created automatically. This allows us to define the hinge.
Let's discuss each occupancy level.
Occupancy levels are defined to identify the behavior of structural elements and determine their status. Occupancy levels represent the state of the element from operational to failure.
As mentioned above, the occupancy rate is an indication of the degree of structural damage. Furthermore, it can be considered as a level of security for the building’s residents. The higher the occupancy rate, the greater the risk to people in the building.
Let's discuss each occupancy level in detail.
Power level for immediate occupancy
- As the name suggests, we can enter the building immediately after a sudden event such as an earthquake or explosion.
- Structural damage is less
- There are no major cracks in the structure, they are small.
- There are no permanent deformations in the structure.
- Mechanical systems such as elevators, fire alarm and protection systems, etc. are working correctly.
- Electrical systems function properly even when the building is immediately occupied.
- The increase in structural stresses is not significant compared to their limit values. The slight increase may have resulted in small cracks.
- Minor injuries to occupants, but no serious injuries or danger to life
- Steel structures also feel busy and slight runoff is expected.
Life Safety Performance Level
- Additional deformation of the structural element leads to this level of occupancy. If we consider the same structure, a load greater than that applied is necessary to reach the immediate occupancy level so that the structural elements behave at the occupancy level that guarantees life safety.
- In this limit state, large cracks can be observed in the structure
- The framework is repairable.
- Repairs incur significant costs.
- It may be that the structural elements have lost their rigidity due to cracking.
- Non-structural components may fall onto the stage
- Partitions can be damaged/broken by the effects of lateral loads .
- Falling non-structural components and other objects can injure occupants.
- With this occupancy rate, deaths are to be expected.
- However, it can happen that people fall with serious injuries.
- Permanent deformation of components may occur.
- Significant damage to construction technology such as elevators can occur.
- Additionally, steel structures can also feel stress significantly.
- Assess the condition of the structure and carry out the necessary repairs for the occupation of the building.
Collapse prevention performance level
- The structure did not collapse. We design structures so that, when they reach this limit stage, they do not collapse.
- These types of damage arise because the structure is subject to very high lateral loads or because its rigidity is insufficient to support the lateral loads.
- Even if the structure has not collapsed, some parts or structural elements may have failed and collapsed.
- The structural damage is significant.
- The structure is not repairable and cannot be reused.
- The occupants could have fallen into the water. There may be injuries. However, the lives of most prisoners are safe.
- Buildings are designed to behave in this limit state when exposed to a lateral load that is too high to evacuate occupants.
- If the building reaches this occupancy level, it runs the risk of collapsing. To prevent further deaths, an evacuation must be carried out immediately.
- A similar extent of damage can also be expected in steel structures. However, the type of damage may vary as the material changes. Connection errors are normally expected.
Nonlinearity of Materials in Performance-Based Design
Considering material nonlinearity is one of the main objectives when designing structures for higher loads that may be identified as unusual. Earthquakes and explosive loads are rare events to which the structure is exposed.
If the structure is to behave in the linear range under such high loads, very high stiffness will be required to support these loads. This will significantly increase costs for the building owner.
If there are more, why aren't they being used?
Beyond the linear region, as discussed previously, there is considerable structural rigidity.
We can allow the structure to act in these areas because we know the structural behavior under such extraordinary loads.
If we know the expected load and structural behavior, we can use them effectively and efficiently.
For example, a structure may be required to perform at the safety limit when known loads are applied. Therefore, structural and construction analyzes are carried out. Elements that resist lateral loads must be provided with the necessary rigidity.
The figure above shows the stress-strain diagram of concrete. As shown in the figure, stress varies with strain based on the confinement of the concrete .
If the concrete is limited, the element can support additional loads. Performance-oriented design takes these types of characteristics into account to get the best out of the design.
