12 Mar Secret #11: SEAU Recap – Two Stage Analysis (Stage 2 – Podium)
Here are a few take aways (adapted from the presentation by Phil Miller, SE*) on the podium 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
Materials
Locally in Utah post tensioned concrete deck on concrete columns and walls is the most common type of podium, but other types include:
- Reinforced concrete deck on concrete columns and walls
- Concrete over metal deck on either steel beams, steel columns and masonry walls
Post tensioned (PT) concrete uses strands of steel cables that are anchored at each end creating compression in the slab when the cables are tensioned. The PT cables can be either bonded (grouted with concrete) or unbonded (protected with plastic sheathing). A ‘strand’ consists of 7 wires (1/2″ or 0.6″ diameter) wound in a helix pattern. A ‘tendon’ consists of 1 or more strands that are grouped together. Oftentimes, the tendons are internal to the concrete slab but there are instances such as retrofits that tendons may be external.
The PT cables cause the slab to go into compression which helps reduce flexural tension and increase shear strength. The catenary cable analysis is closely related to the moments within the concrete slab. This allows for high profiles at supports and low profiles at mid-span to cause lift to occur at the mid-span.
Modeling Tips
The loading and analysis of a PT podium design is often complex that software should be used for the analysis. The following software packages are often used for PT Design:
- Adapt Floor Pro
- Bentley RAM Concept
- Dublal RFEM
- CSI SAFE
- Risa Floor ES
Whichever software you use for your analysis, it is important to understand how best to represent the loads from the structure above. You can import your loads (area, line, point, etc.) from another analysis software to the geometry of your PT podium slab design or you may need to input them in yourself.
Line Loads can be used to simplify reactions from the structure above when there are small openings. Using engineering judgement, the max size of opening before point loads are used can be determined by the design professional… 4 feet? 6 feet? 8 feet?
Point loads at shear walls due to overturning should be included in the design, though they don’t often govern. The PT podium slab design strip design may not be governed by the seismic point loads because there are live load reductions in the load combinations that occur during seismic events. Verify this for yourself and omit them where applicable.
The ‘PTI Guide Specification’ is an excellent resource that will help you with a basic understanding in PT design, including design criteria, explanations, and practical design examples. While the book ‘PTI DC20.701: Design, Construction and Maintenance of Cast-in-Place Post-Tensioned Concrete Parking Structures’, which was recommended pertains to parking structures, the information found within still largely transfers over to the PT podium design. It is also a great resource on ‘load balancing’ which are important to account for in member design. Check out Phil’s presentation for a few slides on load balancing.
Basic Layout Guidelines
Here are some good rules of thumb that can help as you layout your preliminary PT podium:
- Typical Column to Column Layout – 30′ spacing (16″x24″ columns or larger)
- Typical Slab spans – span/depth ratio of 40 to 45 (transfer slabs can be 12″ to 16″ thick)
- Typical Slab Precompression – 150 to 250ksi (350+ksi is rare); One tendon imparts 26.7kips after losses are considered
- Typical Tendon Layout – over columns in one direction (banded) and then distributed in the other direction (uniform)
- Shrinkage Cracking Will Occur – 3/4″ to 1″ per 100′ is typical
- Closure Strip/Expansion Joint Placement – Greater than 250′ = closure strip; Greater than 325′ = expansion joint
- The slab like a big cookie that doesn’t cook as quickly in the middle(I like cookies)
- Leave open as long as possible
The sooner that the shear walls, closure strips, and expansion joints are located, the sooner that the analysis and design can begin. These are ideally laid out in schematic design, but that doesn’t mean that the design won’t change later… nothing is ever certain.
Podium Layout Example
(image credit — Phil’s Presentation)
One Last Thing…
When dealing with anything in design and construction, mistakes can be made but there are always options to fix those issues. If you need to fix or core drill a PT slab, remember that rebar scans are contextual. What is shown in the scan can either be a #4 rebar or 1/2″ diameter tendon. Mistakes can happen very quickly and it can be catastrophic if a tendon is cut instead of a reinforcement bar.
“Respect the physics of PT. [The force] is not something to be messed with”
– Phil Miller, SE
If you missed it, go back to Stage 1… CLICK HERE
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If you have any additional topics or subtopics that you would like me to cover, please contact me at josh@thehipsterengineer.com
*This information is adapted from Phil Miller’s presentation entitled ‘Two Stage Analysis: Podium Design – Part 2’ given at the Structural Engineers Association of Utah (SEAU) Conference in Layton, Utah on February 21st, 2018. “Structural” Phil received his BS and MS degree from the University of Utah (UU). Phil Miller, SE is a licensed structural engineer in both the states of Utah and California. He began his career in 2006 and is currently a Principal at Dunn Associates, Inc. (www.Dunn-SE.com). He is passionate about PT design, mentoring and structural design software. He is also very actively involved in SEAU as not only a member, but also a board member from 2015-2017.
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: Podium Design Presentation
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