Integrated Buildings & Structures

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.

I have also question about two staged analysis. 

• To perform two staged analysis, if any one of  section a-e of section 12.2.3.2 to be satisfied or if the structure falls under any one of point a-e , we need to do a two staged analysis?
• For example my upper portion stiffness is 10 times higher than the lower portion (point a), but the upper portion period is less than 1.1 time  the overall portion, should i stiff perform the two staged analysis.
• Second Question is, 12.2.3.2 , To calculate the stiffness of the upper portion, the code allows to calculate the base shear/ elastic displacement at top of that portion. This means 25 story tower on two story basement obviously less stiffer than the same tower with 10 stories. Is this is the correct way to calculate the stiffness? 

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.

Hi Sobana,

The two-stage equivalent lateral force procedure is an optional method that can be used only when the structure meets all the conditions listed in Section 12.2.3.2. 

If any of the conditions are not met, this method cannot be used, and the entire building must be analyzed as one system.

When checking stiffness, the displacement is measured at the top of the part being studied, and the base shear is calculated at its base. The ratio of base shear to displacement gives the stiffness of that part.

The lower part of the structure should be analyzed while taking the reactions from the upper part before its own stiffness is evaluated.

The stiffness calculation approach is sound and consistent with the intent of the two-stage equivalent lateral force procedure.
What's your concern here?

I have also question about two staged analysis. 

• To perform two staged analysis, if any one of  section a-e of section 12.2.3.2 to be satisfied or if the structure falls under any one of point a-e , we need to do a two staged analysis?
• For example my upper portion stiffness is 10 times higher than the lower portion (point a), but the upper portion period is less than 1.1 time  the overall portion, should i stiff perform the two staged analysis.
• Second Question is, 12.2.3.2 , To calculate the stiffness of the upper portion, the code allows to calculate the base shear/ elastic displacement at top of that portion. This means 25 story tower on two story basement obviously less stiffer than the same tower with 10 stories. Is this is the correct way to calculate the stiffness? 

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.

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.

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.