Peer-to-Peer Standards Exchange

I'd like to open a discussion on the correct application of seismic response scaling in ASCE 7, specifically the requirement to scale Response Spectrum Analysis (RSA) results to match Equivalent Lateral Force (ELF) base shear for high-rise buildings that include basements or podiums.

Here in Dubai, ASCE 7 is the primary international reference for seismic design. However, some local authorities require designers to check and enforce RSA scaling at the podium roof or ground level, while still using the total mass of the entire structure (including basement/podium) for the ELF reference base shear.

This approach raises several concerns:

• Potential Overestimation of Tower Response:

  When scaling is applied at the podium or ground level using the full mass (including basement/podium), the RSA forces above the podium may be artificially amplified, since the basement/podium mass does not fully participate in the tower's dynamic response.

• Alignment with ASCE 7 Intent:

  ASCE 7 defines scaling relative to the base shear at the base level. Scaling at the foundation level with the total mass seems more consistent with this definition, especially where major story shear transfer occurs at the podium or ground level.

• Critical When Two-Stage Analysis Does Not Apply:

  If the two-stage analysis conditions of ASCE 7 are not satisfied, the tower and podium cannot be decoupled, and the level at which scaling is applied will directly affect design forces.

Key Questions for Discussion:

• Should RSA scaling always be checked at the foundation/base level using the total structure mass as the ELF reference base shear?

• Is there a valid interpretation that allows RSA scaling to be checked at a higher level (podium/ground) while still including the mass below that level in the ELF reference calculation?

• Alternatively, can RSA scaling be checked at a higher level but using only the mass above that level in the ELF reference base shear?

I'd appreciate your insights, practical experience, or any authoritative references (commentary, research papers, or local authority guidelines) that clarify the correct interpretation of this ASCE 7 requirement.

ASCE 7 does not specifically address the issues raised in your question.  Many engineers define the seismic base of the structure at the podium floor level, and consider only the mass of the structure above the seismic base in determining the base shear for scaling RSA solutions.

Two documents commonly used in the United States for the seismic design of tall buildings (using performance-based procedures) do address the issue.  The first of these is the Pacific Earthquake Engineering Research Center (PEER) Tall Buildings Initiative, Guidelines for Seismic Design of Tall Buildings.  The other is the Los Angeles Tall Buildings Structural Design Council publication "An Alternative Procedure for Seismic Analysis and Design of Tall Buildings."  Both documents are available for free download on the web.  Both documents suggest that the portion of the podium mass bounded by the projection of the tower through the basement level should be considered as part of the seismic mass and that the seismic base should be taken as the base of the podium.

I'd like to open a discussion on the correct application of seismic response scaling in ASCE 7, specifically the requirement to scale Response Spectrum Analysis (RSA) results to match Equivalent Lateral Force (ELF) base shear for high-rise buildings that include basements or podiums.

Here in Dubai, ASCE 7 is the primary international reference for seismic design. However, some local authorities require designers to check and enforce RSA scaling at the podium roof or ground level, while still using the total mass of the entire structure (including basement/podium) for the ELF reference base shear.

This approach raises several concerns:

• Potential Overestimation of Tower Response:

  When scaling is applied at the podium or ground level using the full mass (including basement/podium), the RSA forces above the podium may be artificially amplified, since the basement/podium mass does not fully participate in the tower's dynamic response.

• Alignment with ASCE 7 Intent:

  ASCE 7 defines scaling relative to the base shear at the base level. Scaling at the foundation level with the total mass seems more consistent with this definition, especially where major story shear transfer occurs at the podium or ground level.

• Critical When Two-Stage Analysis Does Not Apply:

  If the two-stage analysis conditions of ASCE 7 are not satisfied, the tower and podium cannot be decoupled, and the level at which scaling is applied will directly affect design forces.

Key Questions for Discussion:

• Should RSA scaling always be checked at the foundation/base level using the total structure mass as the ELF reference base shear?

• Is there a valid interpretation that allows RSA scaling to be checked at a higher level (podium/ground) while still including the mass below that level in the ELF reference calculation?

• Alternatively, can RSA scaling be checked at a higher level but using only the mass above that level in the ELF reference base shear?

I'd appreciate your insights, practical experience, or any authoritative references (commentary, research papers, or local authority guidelines) that clarify the correct interpretation of this ASCE 7 requirement.