Current Revision posted to Structural Analysis and Design Wiki by jeremyhoesly on 7/25/2013 9:28:10 PM
(12.20.2012) Structural Engineer Webcast: Thin Slab Floor Systems for High Rise Towers Part 2 Attendee Questions
This webcast was originallypresented on Dec. 20th, 2012, hosted by ZweigWhite’s Structural Engineer magazine and sponsored by Bentley Systems.
You can view the recording online (Click Here!).
The following are the product-related questions and corresponding answers from the eSeminar.
- Q: Did you encounter many problems having to reconfigure stair wells or emergency exits?
A: The north stairwell had to move in plan several times at the lower floors to accommodate the varying functions. For this stairwell the wall framing had to be light framed fire-rated partitions. The stair shaft locations transitions were handled by moving the location of the stair shaft opening through the floor. In most cases the floor slab system was able to accommodate this without the addition of beams. The south stairwell was relatively consistent through the building allowing our office to frame the stair walls in concrete and use them as part of the lateral load resisting system. - Q: Was a fire protection analysis done on the effects of thinner slabs?
A: Does fire spread faster from floor to floor and result is concrete failure sooner? Even thinner concrete slabs and walls have a high resistance to fire. The thinner 6 inch-thick concrete slabs and walls achieve a 3 hour fire rating, satisfying the fire fire-rating requirements for a high rise building. - Q: How was the camber achieved - both in the formwork AND in the placing/finishing operations?
A: There were four areas of the residential floor slabs that required camber. In these areas the contractor made adjustments to the flying form system to accommodate the camber. During the placing/finishing operation the elevation of the floor had to be set from screeds set on the formwork to help assure that the camber was cast in to both the top and bottom surface of the slabs in this area. - Q: What was the total time, concept to completion?
A: The construction of the project took three years. The design of the final concept for the project started about one year prior to construction. - Q: Elevation Control is mentioned at least a couple times. What was the process to ensure that the Survey team provided the proper accuracies required? How was the horizontal and vertical control established and verified? Did you end up with a long-term creep lunge due to the transfer of the diagonal partition walls?
A: The contractor shot both vertical and horizontal dimensional readings throughout the course of construction.The diagonal walls were lengthened in to the units so that they were located only slightly off center of the supporting columns below. In addition, most of the diagonal walls had an offsetting wall on the other side of the building to help further balance the eccentric loads. - Q: Was progressive collapse analysis performed for this project?
A: The actual security measures for the project are confidential. We can however say that the structure is much more robust than other hotels in Waikiki. - Q: Is there now a source to read up on thin-plate beams?
A: The Post-Tensioning Institute has several publications on flat plate concrete floor systems. - Q: What software did you use for the lateral design of the structure?
A: A combination of RAM Concept integrated in to an ETABS lateral model. - Q: Did you include the weight of the steel plates in the reinforcing weight for the building?
A: The steel plates in the composite link beams were a separate line item in the structural quantities but would not add much weight to the overall reinforcing weights for the project as not all of the link beams required steel plate reinforcing. - Q: What did you use for slab deflection criteria? Did you have trouble with PT draping tolerances for such a thin slab?
A: We tried to keep long-term slab deflection around 3/8 inch. In general, the live load deflection was typically under L/500 due to the post-tensioning in the system. The primary concerns on PT draping tolerances were other building trades such as electrical and plumbing subcontractors moving tendons after they had been placed. The project had Special Inspections performed for the reinforcing steel and tendons along with the General Contactor’s quality assurance program. - Q: Is there now a source to read up on thin-plate beams?
A: The Post-Tensioning Institute has several publications on flat plate concrete floor systems. - Q: Did you encounter floor vibration issues and if so how did you address these?
A: There were some concerns about vibration perceptibility at the parking levels. Since parking in this building is primarily by valet the client did not feel additional vibration mitigation measures were necessary. - Q: Is the 6.1 PSF for reinforcing steel for the horizontal elements only or does it include column and wall elements? Does the 6.1 PSF include foundation steel?
A: These were quantities for the horizontal, vertical and foundation elements provided to our office from the General Contractor. - Q: What type of fundation? How did the movement of elevator affect the structure?
A: The foundations were deep auger-cast piers. The location of the elevator did result in some torsional forces under seismic loads resulting in some increase in wall and foundation reinforcing quantities. - Q: Are slab bending moments due to lateral loads included in the Concept analysis?
A: A separate calculation of bending moments due to lateral loads was performed. - Q: What special requirements if any were there for the concrete?
A: The mix design was modified to gain early strength for the stressing of post-tension tendons. - Q: What strengths of concrete were used?
A: Wall and column concrete strengths varied from 8,000 psi at lower levels to 4,000 psi in the upper floors. The typical floor concrete strength was 5,000 psi. - Q: What curing methods were used? What was the slab strength?
A: The primary curing method selected by the General Contractor was a fluid applied curing compound. The typical floor concrete strength was 5,000 psi. - Q: Cost of the project?
A: We were not aware of the actual costs but would assume a total cost of approximately $390 million. - Q: Did you have a lot of torsion and lateral drift restrictions?
A: The torsion and lateral drift restrictions followed code and industry standard requirements. - Q: At any location were slabs tapered on the ceiling side between supporting elements?
A: The typical floor slab was stepped to a thicker banded slab area down the center of the building. - Q: Are there punching shear problems?
A: Studrail reinforcement was needed for many of the slab/column joints to meet code punching shear requirements. - Q: Can you give us any reference for the new (Beam with Steel Plate) System or design concept?
A: You can find references online from the University of Hong Kong. In the U.S., studies are being performed at UCLA - by Dr. John Wallace with sponsorship from the Pankow Foundation. - Q: What aspects for crack control have to be taken into account? How useful was RAM-Concept for that?
A: There are two primary types of cracks to be concerned with. In post-tensioned slabs you can have a combination of shrinkage and restraint cracking. These are typically addressed through detailing rather than software analysis. The Post-Tension Institute has several publications providing guidance on this. RAM Concept provides information on the expected flexural stresses in the slab system. As part of the interactive design process the post-tension forces are adjusted to meet code limits on flexural stress intended to ensure that tensile strength in the concrete is not exceeded. One of the benefits of post-tensioned systems is that the compressive forces imparted in to the slab design leads to a system where there should be no flexural cracking in the slab system. - Q: Why was there no basement?
A: Being located close to the ocean the water table was only a few feet below finished grade. A basement was initially considered but the costs of dewatering for an extensive basement were prohibitively expensive. - Q: Was axial shortening in columns and walls affecting the slab design?
A: One advantage of the thinner slab system is its lighter weight and flexibility. Axial shortening in columns and walls did not impact the slab design. - Q: With all of the rigid lateral elements at each level did you have to provide delay ties to the PT slabs to account for slab shrinkage?
A: Delayed pour strips were utilized to help minimize restraint cracking. - Q: How many floors were supported during curing upper floors?
A: The re-shoring was typically a minimum of three supporting floors. - Q: How closely was placement of the reinforcing monitored?
A: Code-required Special Inspections were performed during reinforcement placement. On average there was an inspector on the job full time during construction of the structural frame. - Q: Does the salt affect the structure (Proximity to the sea)?
A: While the project is located near the ocean the prevailing trade winds are off-shore, reducing some of the impact of air borne chlorides. The project did however utilize a combination of enhanced concrete mix, epoxy coated rebar in exposed areas and waterproofing to provide added resistance to corrosion. - Q: Were there any tsunami design considerations? If so, what are they and how did they affect the design of the buildings?
A: Tsunami load was not a direct design load case for the building. It has been recognized however that taller concrete buildings can resist tsunami loads due to their inherent lateral strength. In the case of a tsunami it is expected that the lower floor partitions would break away and the tsunami waves would pass through the building without impacting the structure above. - Q: Any details at wall for relief of the restrain from PT force of the slab?
A: Delayed pour strips were utilized to help minimize restraint cracking. - Q: How were the ducts handled?
A: Close coordination was required to fit mechanical ducts in to dropped ceilings located in the bathrooms and kitchens towards the interior of the units. - Q: Any long-term lateral displacement, due to the shear imposed by the transferred diagonal walls?
A: The diagonal walls were lengthened in-to the units so that they were located only slightly off center of the supporting columns below. In addition, most of the diagonal walls had an offsetting wall on the other side of the building to help further balance the eccentric loads. - Q: Some special requirement for first elevate slab due to shortening and shrinkage?
A: Delayed pour strips were utilized to help minimize restraint cracking. - Q: How large are the long-term deflections? Were there any special measures besides the pre-cambering such as delayed removal of formwork?
A: Camber was utilized to try to keep long-term deflections under 3/8 inch. - Q: How do I view this webcast again or share it with a colleague?
A: This event along with the live Q&A were recorded and it is available online.