08 Mar Secret #10: SEAU Recap – Two Stage Analysis (Stage 1 – Wood)
Here are a few take aways (adapted from the presentation by Jared Cope, SE*) on the wood portion of two stage analysis that you should understand…
This is a two part series on two stage analysis:
- Stage 1 — the wood basics: what, why, systems & criteria, design considerations, and common issues
- Stage 2 — the podium basics: materials, modeling tips, and guidelines
What is Two Stage Analysis?
Two stage analysis is a method to analyze a structure separately. This is allowed when the top portion is flexible and the bottom part is rigid. The top flexible portion can be analyzed as part of ‘Stage #1’. The bottom rigid portion is analyzed as ‘Stage #2’. The top portion is analyzed in the same way, whether the bottom podium is present or this structure is built on grade. There is additional criteria beyond a flexible top portion and rigid bottom portion, found in ASCE 7-10 Section 12.2.3.2.
Why Use Two Stage Analysis?
This method of analysis allows for the separation of the structure which the design more simple. Typically the seismic response modification factors ‘R‘ vary between the top portion and the bottom portion. Since the bottom podium portion of the building is more rigid, when the ‘R‘ factor is applied to the entire structure it causes higher loads at the upper levels. Under typical analysis, the mass from the concrete podium would also have to be considered in the upper levels which causes increased forces. With the ‘two stage analysis’ the top flexible structure uses its own ‘R’ factor and only the mass from the flexible structure contributes to its lateral design. Then the adjusted seismic base shear from the upper portion (flexible) is applied at the top of the lower portion (rigid).
Building structures that are suited for this analysis procedure include multi-family housing, hotels, assisted living facilities, and residential housing above retail/parking. These building types are all developer driven and great options for two stage analysis due to the following:
- Construction costs
- Limited land requirements
- Future phases of construction
- Mixed use zoning (retail, residential, etc.)
- Convenience of parking and residential in the same structure
Systems and Criteria
The lateral force resisting systems that are permitted in the two stage analysis method include:
- Wood shear walls on concrete/masonry podium
- Metal stud shear walls on concrete/masonry podium
- Other light frame shear walls on concrete/masonry podium
The criteria for the two stage analysis is found in ASCE 7-10 Section 12.2.3.2. The gist of the criteria include:
- Stiffness of upper portion MUST be 10x that of the lower portion
- Period of the upper portion MUST be 1.1x that of the lower portion
- Upper portion to be analyzed separately using separate values for ‘R‘ and ‘rho‘
- Lower portion to be analyzed using different ‘R‘ and ‘rho‘, with a ratio of ‘R/rho‘ where the loads transfer
- Permitted analysis procedures
- Upper portion – Equivalent Lateral Force (ELF) or Modal Response
- Lower portion – Equivalent Lateral Force (ELF) ONLY
Design Considerations
One major item to consider when determining whether or not two stage analysis is used is if at least ONE side of the lower podium structure is not at grade level. So if there is a lower rigid basement but all sides of the structure are at grade level, then a two stage analysis is not to be performed. Instances where one side may be exposed include exposed podiums, and sloped grades.
Here are some more items to consider in your design:
- Aspect ratios of wood shear walls (NDS 4.3.3.4.1 & 4.3.4)
- Penalties for stiffness and strength of shear wall capacities under wind and seismic loading
- Drift design and expansion joints
- Expansion joints at garages occur at the top of the podium
- Sometimes expansion joints are up to 12″ at the top of a 5 story structure
- Wood shrinks over time
- Continuous hold down rods at ends of shear walls need to account for shrinkage via take-up device
- Causes issues for plumbing and drains if no expansion devices are used
- Fire rated walls
- 2 hr fire rated walls are required at the exterior walls for 5 story (Type III) construction
- Use 2 layers of gypsum board and fire hangers for floor joists
Common Issues
A few common issues that might be found in construction include:
- Holes at floor joists
- Sill bolts out of place
- No vertical load transfer (missing posts, straps, etc.)
- Holes in shear walls
- No blocking at compression members
- Hold down in the wrong location
These are construction issues that engineers have to be ready to resolve with the contractor. Engineers need to provide quick responses when the project schedule hangs in the balance. However, this cannot hinder true and proper engineering judgement during design and throughout construction.
Wood Can Do Amazing Things
Multiple agencies, firms, and universities have come together to push the envelope of wood design as shown in the earthquake test video below. This is a 6 story wood structure that is being subject to seismic forces 1.8x those of the Northridge Earthquake. Typically the upper wood portion is 5 stories but 6 stories of wood is okay, however there are various penalties that need to be taken into account.
“If a wood structure is designed well, it performs well in a structural event”
– Jared Cope, SE
Watch how this wood structure performs by watching below…
Stay tuned for Stage 2… CLICK HERE
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*This information is adapted from Jared Cope’s presentation entitled ‘Two Stage Analysis: Wood on Podium Design – Part 1’ given at the Structural Engineers Association of Utah (SEAU) Conference in Layton, Utah on February 21st, 2018. Jared Cope, SE began his career in construction prior to studying engineering at Utah State University (USU). He is a licensed Structural Engineer and Senior Associate at Dunn Associates, Inc. (www.Dunn-SE.com). He specializes in wood design and has helped oversee numerous two-stage analysis projects since 2006.
I was not able to cover everything that was discussed, so I will may have to periodically come back and revisit these thoughts another day. In the meantime, the presentation can be found here: Wood on Podium Design Presentation
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