Minding the Gap: Design of Seismic Separations

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Everyone knows how to design a seismic joint. Or at least it seems so due to the small amount of published information on the topic. In practice, there is frequently a gap not just between structures but also between the code and its application. The goal of this article is to narrow that gap and improve the uniformity of these designs across our profession.

ASCE 7-22 has three separate criteria for the design of structural separations (also known as seismic joints) and elements that cross separations. The applicability of each criterion is not always clear, and their requirements may seem contradictory. The three criteria discussed in ASCE 7 are for structural separations (Section 12.12.2, Figure 2), structural members spanning between structures (12.12.3, Figure 3), and nonstructural elements spanning between structures (Section 13.3.2, Figure 4). Mindfully considering each section, including interactions between the sections, will produce a wholistic and unified design for both the structures and elements that cross the gap between the structures.

Minimum structural separation distances need to be provided when two independent structures are located close to one another and there is a possibility of the structures colliding during an earthquake, resulting in what the code refers to as “damaging contact,” more commonly referred to as “pounding.” The minimum widths of structural separations including seismic joints are prescribed in ASCE 7, Section 12.12.2 – Structural Separation. Commentary Section C12.12.2 introduces the term “seismic joint” as an alternative to “structural separation.”
At locations where one structure is close to another independent structure on the same property, or where a structural separation occurs within a building, the maximum inelastic response displacement δ_DE is calculated for each structure at the location of the separation per Equation (12.8-16).

The required minimum separation between the structures (δ_SS) is calculated using the Square Root of the Sum of the Squares (SRSS) of the Design Earthquake (DE) Displacements in accordance with Equation (12.12-2).

Use of the SRSS method to calculate the structural separation between seismically independent structures has remained unchanged in ASCE 7 through several code cycles and is based on the underlying assumption that it is unlikely that the independent building structures will displace completely out-of-phase. This check is performed at the DE hazard level; damaging contact will be possible, if not likely, at the MCER hazard level. A smaller separation is permitted where an inelastic analysis is performed justifying the smaller value.

Where floors in adjacent buildings do not align, the edges of floor plates may laterally impact columns along their height, which is a more hazardous condition than when the floors in the buildings are aligned. When this condition occurs, the engineer should consider the architectural detailing in the area of potential impact to decide if damage will remain largely nonstructural or if it is possible that structural damage to the columns will occur, in which case a seismic separation larger than the code minimum may be warranted.

Section 12.12.2 states that a new building should be designed so that it does not displace across a privately owned property line, however, where an existing building already exists on the property line, designing a new building so that it does not displace across the property line will not be sufficient to avoid damaging contact. This is because the existing building located on the property line will sway across the line during an earthquake. This condition should be discussed with the building owner, and an engineer should consider providing a minimum structural separation based on Equation (12.12-2), as if the buildings were located on the same property, so that the buildings do not impact during smaller earthquakes than would be otherwise intended by the building code.

When displacements of an existing building on a neighboring property have not been directly calculated, it is usually acceptable to assume that the drift of the existing building is 3% per the recommendations of ASCE 41-23, Section 7.2.15.1, unless the adjacent structure can be characterized as a rigid shear wall structure with rigid diaphragms, in which case ASCE 7 Section C12.2.2 suggests that a drift of 0.42% can be assumed. The ASCE 41 recommendation is a conservative estimate if the existing building is not badly deficient—it is likely that the displacements for the existing building could be reduced if they are quantified using a more detailed method. Where ASCE 41-23 is the design standard, ASCE 41-23 Section 7.2.15.2 contains additional criteria for the evaluation of the separation.

Lastly, it may be desirable to slightly oversize the gap to avoid construction tolerances or thermal movements compromising the displacement capacity of the joint. An additional 1-inch separation is adequate to account for slab placement tolerance because one slab will be placed first, and the slab that is placed second will have a nearby hard edge from which to measure. This additional inch also allows for small amounts of other movement that is typically unquantified, such as thermal movement and shrinkage. Where thermal and shrinkage movements at the separation are expected to be significant, as may be the case in open parking structures, these movements should be quantified and included in the separation width calculations directly as required by ASCE 7 Section 2.4.4.

Where a structural element such as a pedestrian bridge spans between independent structures, the element must be supported such that there is a load path for both gravity and lateral loads while the relative movement of the structures is accommodated. One method to achieve this is to provide a fixed support at one end of the element, and a vertical-only support at the other end of the element. The vertical support is detailed to allow the element to translate and rotate freely in the horizontal plane. Another method of support is to use a pin connection at one end and a roller support at the other end, which allows the element to pivot to accommodate movements perpendicular to the span, and slide to accommodate movements in the direction of the span.
The amount of horizontal movement required to be accommodated by the supports is called the maximum anticipated relative displacement and is defined in ASCE 7 Section 12.12.3 – Members Spanning Between Structures. This Section has more conservative requirements than the requirements for structural separations because of the risk of local collapse if an element becomes unseated from its support.

To calculate the displacement requirements of the support, the maximum anticipated relative displacement is calculated using the Maximum Considered Earthquake Displacement (δMCE) per ASCE 7 Section 12.8.6. Then the total movement is calculated by summing the absolute value of the displacements at both support locations.

As described by this process, the maximum anticipated relative displacement is as defined below. Use of the factor R applied to the displacement value instead of Cd is intended to correct for an underprediction of displacements that has been shown to occur when the Cd factor is used.

maximum anticipated relative displacement= δ_MCE1+δ_MCE2

The term δ_e is defined as the elastic displacement that is computed under design earthquake forces and is not limited to horizontal movement. Where vertical movements and rotations occur due to the earthquake response, these movements should be considered in the design as well.

The movements prescribed for nonstructural elements crossing structural separations are greater than the movements used for determining the seismic separation itself and are less than the movements used for structural elements crossing seismic joints, suggesting that their importance to building performance also lies somewhere in the middle. However, the acceptance criteria used with these movements leaves much more room for the judgment of the responsible design professional. For nonstructural systems in general, ASCE 7 Section 13.3.2 simply states that these movements “shall be considered.” The commentary helpfully clarifies that this vague language is intentional, and that damage as a result of these movements may be acceptable.

ASCE 7 Section 13.5.2 states that architectural components are required to accommodate seismic relative displacement requirements if failure of the element represents a life safety hazard. Typically, the only architectural component crossing a seismic joint is the seismic joint cover, which is discussed in detail in the following section.

ASCE 7 Section 13.6.2 emphasizes that mechanical equipment in a building is required to be designed to accommodate seismic relative displacements if the component importance factor Ip is greater than 1.0. The component importance factor is defined in Section 13.1.3 and is 1.5 for components that are required to function for life safety purposes after an earthquake. So, while allowing damage might be one way to accommodate the movements, the mechanical designer is responsible for determining whether such damage represents an impermissible life safety hazard.

The seismic displacements required for design of nonstructural components that cross structural separations are governed by ASCE 7 Sections 13.3.2 and 13.3.2.2. These displacements are calculated as the absolute sum of the building displacements on either side of the separation, similar to Section 12.12.4. Movement can occur either perpendicular or parallel to a separation, which is important to make clear on the construction documents for the designers of the nonstructural systems. Displacements are amplified by the seismic importance factor from ASCE 7 Table 1.5-2 to provide enhanced seismic performance of nonstructural components within important facilities. Seismic Relative Displacements (DpI) at a structural separation are required to be calculated as shown below. Note that the subscript in Equation (13.3-8) is the capital letter “I,” which is intended to show that the importance factor is included within the parameter.

D_pI=D_p I_e (13.3-8)

D_p=|δ_xA |+|δ_xB | (13.3-11)

While some nonstructural systems may be exempt from accommodating seismic displacements, any nonstructural system crossing a seismic separation should be designed to accommodate service level movements without distress or loss of function. Possible sources of service level movements include: the 10 or 50-year wind or seismic loads, thermal changes (especially in unconditioned structures), and concrete shrinkage. One approach to determining serviceability movements would be to remove the Cd factor from Equation (12.8-16). The resulting displacement corresponds to the sum of the elastic drifts in both structures on either side of the joint, and represents the expected movement of the buildings at their design strength.

As referenced before, architectural components typically stop on either side of the seismic separation and a joint cover bridges the gap. Of most interest to the structural engineer are the seismic joint covers and associated fire-resistive joint systems at the floors and roof as these may affect the size of the gap between structures. In the extreme, the structural separation required to accommodate these joint systems can impact not just the edge of concrete, but also column locations, and even building layout grids. Thus, it is important to discuss this issue with the architect early in the project so that the appropriate design criteria can be applied and changes to the structural layout are not required late in the project.

The design or selection of the joint cover starts with establishing its intended functionality for both seismic and in-service conditions. ASCE 7 Section C13.5 provides insight into the performance needed to maintain life safety conditions, such as avoiding falling hazards or blocking egress paths. Joints that are part of a fire-resistant assembly must also comply with Section 715 of the International Building Code (IBC) which requires that such joints be capable of accommodating “expected building movements.” The IBC does not define what expected movements are, but we believe this is commonly understood to include nonstructural movement criteria from ASCE 7.

Where failure of a seismic joint cover creates a life safety hazard, or the building is required to remain operational following an earthquake, the joint cover should be sized for the criteria discussed in the nonstructural section, which will likely govern the width of the structural separation where the joint cover is placed.

Where failure of a seismic joint cover is not a life safety hazard, it is not required to accommodate the ASCE 7 Chapter 13 nonstructural movements and the joint cover can be sized to maintain the minimum separation as determined by ASCE 7 Equation (12.12-2). When this criterion is used, the cover may be damaged following an earthquake. As a result, these joint covers should also have movement criteria established for serviceability that meets the owner’s expectations.

With the criteria established, a joint cover can be designed or a commercial product selected. Many different types of joint covers exist, ranging from simple cover plates to complex assemblies that create a seamless appearance. Not all joint covers can close to zero width during seismic movement, and there may be other material such as fireproofing or hardware that occupy space within the structural gap and that would prevent the separation from closing fully.

As a result, determination of the structural gap width requires working with the architect to understand the type of joint cover that is wanted and what the displacement capabilities of the cover are. If the type of joint cover cannot be known early in the design, the final width of the structural separation may need to be modified once the joint cover is selected, and the location of walls and columns near the joint should accommodate some increase in the width of separation.

If failure of a seismic joint cover is a life safety hazard only due to disruption of the fire rating, it may be preferable to provide a separate architectural detail that can maintain a fire rating while accommodating the nonstructural displacement requirements, instead of increasing the width of the structural separation. In this condition, it would be reasonable for the fire-rated detail to accommodate the maximum displacements that can occur, which is limited by the as-built separation when the gap is closing, but per Equation (13.3-8) when the separation is opening or displacing along its length. This will, however, be subject to the Authority Having Jurisdiction’s interpretation of expected building movements as applied to IBC Section 715.

Minding the gap in seismic separations requires paying attention to much more than just the physical space—it requires minding the gaps between the various code requirements, owner expectations, and practical constraints. In addition, it requires bridging the gap between the structural code provisions and the design professionals responsible for the nonstructural elements crossing these joints. ■