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.

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Mahmoud Hassan M.ASCE
Senior Lead Structural Engineer
Cairo
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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? 

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Sobana Swaminathan
Principal Structural Engineer
Dubai
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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?

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Mahmoud Hassan M.ASCE
Senior Lead Structural Engineer
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Original Message:
Sent: 10-10-2025 04:28 PM
From: Sobana Swaminathan
Subject: Scaling of RSA Results to Match ELF Base Shear for Towers with Podiums/Basements (ASCE 7)

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? 

------------------------------
Sobana Swaminathan
Principal Structural Engineer
Dubai
------------------------------

Thanks

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Sobana Swaminathan M.ASCE
Principal Structural Engineer
Dubai
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Original Message:
Sent: 10-10-2025 06:16 PM
From: Mahmoud Hassan
Subject: Scaling of RSA Results to Match ELF Base Shear for Towers with Podiums/Basements (ASCE 7)

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?

------------------------------
Mahmoud Hassan M.ASCE
Senior Lead Structural Engineer
------------------------------


Original Message:
Sent: 10-10-2025 04:28 PM
From: Sobana Swaminathan
Subject: Scaling of RSA Results to Match ELF Base Shear for Towers with Podiums/Basements (ASCE 7)

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? 

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Sobana Swaminathan
Principal Structural Engineer
Dubai

Excellent topic - the interaction between tower and podium/basement structures often makes RSA results differ significantly from ELF base shear. Scaling RSA to match ELF can be quite challenging, especially for irregular geometries or mixed systems. Looking forward to insights on practical approaches and code interpretations for achieving consistent and rational scaling in such cases.

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Jaher Wasim S.E., M.ASCE
Managing Director & CEO
D8 Consultants Ltd. (D8CL)
jaher@...
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Mahmoud,

Is there something in ASCE 7 that drives the requirement to scale at top of podium?  My interpretation is that scaling is intended to be at the base of the structure, not intermediate levels.  I do feel that the mass of a podium can contribute to story shears above in real life, although the effect will be small if your podium has a very stiff lateral system per unit mass compared to the tower.  It does feel disproportionate to put the entire building base shear on the 15th floor.  Do you then design your podium for that same shear at every story, regardless of the seismic mass of the podium floors?  Puts you in a weird place with diaphragm design too, assuming zero story shear below the podium roof.

To your first two discussion questions, basement levels are a can of worms and I understand that there are different opinions on how to handle this, but I take the base to be top of foundations, not at exterior grade.  I wonder if the AHJ requirement for RSA scaling stems from an assumption that engineers are designing the tower from t/podium or t/grade up?  Without knowing the specific requirements, it seems like they would be automatically satisfied if you take the base at the lowest basement and analyze the entire building in one global model.

To your third question, my gut says that the point of RSA is to use your analysis of relative story stiffness and mass to anticipate distribution of forces across the full structural height.  RSA does push seismic force towards the top of your structure, just not quite to the extent of ELF.  So you would get more favorable and less realistic (i.e. unconservative) results by placing the cutoff higher in the building if you also scale at that higher floor.  Definitely curious to hear other perspectives though.

Anyone brave enough to make the case for designing from t/grade up when you have basement levels?  I've heard of this practice but never caught a compelling explanation.

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Christian Parker P.E., M.ASCE
Structural Project Engineer
Chicago IL
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