Seismic Design Considerations for Elevators Installed in the U.S. under IBC
More than 40 U.S. states have replaced their legacy building code with the International Building Code (IBC). In this paper, we will explore the impact of this transition as it relates to the elevator seismic requirements under the ASME A17.1/CSA B44 Elevator Code (See Section 8.4).
Prior to the 2013 code, elevator component seismic force levels were determined by either seismic zone or ground motion. However, for jurisdictions under IBC, this long standing approach of needing only one value to determine elevator component seismic force level is no longer valid.
Transition of Seismic Design criteria in Model Codes
The intent of the seismic design criteria in model codes is to minimize property damage and maintain function during and after an earthquake. This seismic design criterion has evolved to the point where, under the IBC as incorporated in the 2013 version of the elevator code, the traditional “Seismic Zone” approach used in elevator design and installation is no longer applicable. The criterion used in the IBC is called “Seismic Design Category.” For the United States’ building industry,
this transition has been going on for a number of years. Table 1 (page 3) shows the building code’s evolution during this transition from Seismic Zones to Seismic Design Category.
The elevator code retains the seismic zone approach by allowing equivalence to or comparison with a seismic zone, given a ground motion parameter, during this transition period. This equivalence is based on the Affected Peak Velocity Acceleration Parameter (AV). However, the transition period is over, a fact which is readily apparent with the publication of the 2013 elevator code. So what does this mean for those jurisdictions who adopted the IBC Seismic Design Category? See Table 2 (page 3) for the comparison between Affected Peak Velocity Acceleration and Seismic Zone. This comparison has been in the A17.1 elevator code since 2000 and continues to be in the 2013 elevator code.
Where the new code has been adopted, the elevator manufacturer/installer must obtain a number of seismic parameters in order to determine the applicable force levels to be applied to the installed elevator equipment.
Seismic Design parameters in the IBC
Under the IBC, which references ASCE 7, there are a number of seismic parameters that the elevator manufacturer/installer must know in order to bid, design, specify, layout, and install the elevator equipment in a building designed under IBC. These parameters are needed before the elevator manufacturer/installer can determine if Section 8.4 of the elevator code will or will not apply to the installation. These parameters are also specified in ASCE 7, American Society of Civil Engineers – Minimum Design Loads for Buildings and Other Structures. You can see the design parameters in detail in Table 3 (page 3).
To assist elevator manufacturers/installers in acquiring the required seismic parameters in conformance with IBC and the 2013 elevator code, a Seismic Requirements Data Form is available on the NEII web site. Member companies can use this form to request the required seismic data from the building designer.
When does section 8.4 apply?
Legacy building codes allow force level calculations based on either seismic zone or ground motion Av. Where IBC has been adopted, force levels must be based on a number of seismic parameters (aka seismic design) as dictated in Section 8.4. For the elevator manufacturer/installer, the first concern is whether or not Section 8.4 applies to his/her particular project.
As listed in the first two sections of Table 3, Seismic Design Category and the Component Importance Factor, A17.1-2013/B44-13 requirements 8.4(a)(1) and 8.4(a)(2), respectively, are the key factors used to determine if the Elevator Seismic Requirements do or do not apply to the installation. (As a rule, A17.1/B44, Section 8.4, Elevator Seismic Requirements are considered applicable where either of the following exist
- 8.4(a)( (1) Seismic Design Category C with Component Importance Factor, Ip, of 1.5 as defined by IBC (see 1.3, building code)
- 8.4(a)( (2) Seismic Design Category D or greater as defined by IBC (see 1.3, building code)
A determination that seismic design is not required occurs where either of these conditions apply:
- Buildings with Seismic Design Categories A or B,
- Buildings with Seismic Design Category C where the Component Importance Factor is 1.0.
A17.1-2013/B44-13 Incorporating IBC - How does section 8.4 apply?
If the Section 8.4 requirements do apply, the elevator manufacturer/installer is required to determine the Elevator Seismic Design Forces FP and FV and other parameters as given in Section 8.4.14. For the United States these forces and parameters are based on IBC with reference to ASCE 7. These seismic calculations and parameters are provided below in Table 4.
A17.1-2013/B44-13 Incorporating IBC - What section 8.4 requirements are impacted?
Elevator equipment installations under IBC have parameters differ from the traditional seismic zones approach. Some of the 8.4 requirements that are impacted where there is a difference in the determination and application of normal and seismic forces between the zone approach and the IBC approach are given below in Table 5.
A17.1-2013/B44-13 Incorporating IBC - What is the impact to rail bracket spacing?
Under IBC, the permissible seismic force per pair of rails is determined from the horizontal force FP based on WP instead of directly from the Component Operating Weight WP. The guide rail bracket spacing will now decrease as a function of vertical location within the structure, i.e., the higher the bracket is located in the building, the closer the bracket spacing should be. This decrease in bracket spacing occurs due to the amplification factor [1 + 2(z/h)] that is applied to WP. (See Equation FP in Table 4.)
As examples, bracket pairs installed at the building base will have an amplification factor of 1 applied to WP while bracket pairs installed at the roof level will have an amplification factor of 3 applied to WP. Intermediate bracket pairs will then fall somewhere between 1 and 3. To determine the required bracket spacing for various rail sizes, see Figures 22.214.171.124-1 through 126.96.36.199-7 in the 2013 elevator code.
The FP value as given in Table 4 is needed to determine the vertical bracket spacing for each bracket pair. The actual force value to be applied to Figure 188.8.131.52-1 through 184.108.40.206-7 vertical axis is 2.93 x 0.7 x FP. Further, these calculations include the amplification factor [1 + 2(z/h)] and as such will vary as a function of the vertical location of the guide-rail bracket relative to the building base. In order to perform these calculations, the person preparing the layout drawing must have the building base (b) and height (h) information.
This is critical data called for on the Seismic Requirement Data Form. From the base and height information, the person preparing the layout will determine the location of the bracket (z) relative to the base (b). It is at this point that the amplification factor can be known and the FP value for each rail pair determined. Given FP, the person preparing the layout can now determine rail bracket spacing. (See the appropriate Figure 220.127.116.11-1 through 18.104.22.168-7 in the 2013 elevator code).
Given parameters FP and FV, the F x-x and F y-y normal forces are also calculated and provided on the layout drawings. Without completing the above steps, one cannot prepare layout drawings that comply with IBC and the 2013 elevator code. Without this calculation, it is also not possible to determine the precise number of car and counterweight bracket pairs for the installation.
A17.1-2013/B44-13 Incorporating IBC - Are there additional impacts to layout drawings?
For jurisdictions enforcing IBC, the information required on elevator layouts relative to the normal forces Fx-x and Fy-y is determined by a different method (See Requirement 22.214.171.124.1). Here, these normal forces are calculated based on Horizontal Seismic Force FP and Vertical Seismic Force FV instead of Component Operating Weight WP. (See Equations FP and FV in Table 4). The calculations for these normal forces are given in Table 6.
As with rail bracket spacing consideration, these normal force calculations also include the amplification factor [1 + 2(z/h)]. In order to perform these calculations, the person preparing the layout drawing must have the building base (b) and height (h) information. This is critical data called for on the Seismic Requirement Data Form.
To date, more than 40 states in the United States have replaced their legacy building code with the IBC.
Eliminating counterweight derailment detection
Under IBC, the seismic zone approach no longer applies when determining whether or not a displacement switch (counterweight derailment) and the associated operation required by 126.96.36.199.1 may be eliminated. Instead, this determination is based on the calculation of seismic force FP. If all the conditions given in Table 7 are met, then the manufacturer installer may opt out of providing counterweight derailment detection.
Alternatively, the option to not provide counterweight derailment detection can be made without having to calculate FP. This can be done using only the data given on the Seismic Requirement Data Form. The seismic parameters required to make this determination are Seismic Design Category (SDC) Component Importance Factor IP and Spectral Response Acceleration SDS. If all the conditions given in Table 8 are met, then the manufacturer installer may opt out of providing counterweight derailment detection.
It is also important to note that under IBC, there are a number of places in Section 8.4 where the determination of the seismic design forces first requires calculating Horizontal Strength Level FP and Concurrent Vertical Seismic Force FV using the equations given in Table 9. Having a means to determine zone equivalence may be useful in earlier bidding and evaluating requirements.
Table 10 gives a rough zone equivalence within the parameters of Seismic Design Category (SDC), Component Importance Factor IP and Spectral Response Acceleration for Short Period SDS.
As more and more buildings are being constructed under IBC, it is critical for elevator manufacturers/ installers to align themselves with the new IBC seismic requirements as applied in the A17.1-2013/B4413 elevator code.