By Daixi Yang, P.E.
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Hudson Site Block and Tower is a mixed-use project in Downtown Detroit. The Tower consists of condominiums and a hotel and has 49 levels above the ground, with Level 2 to 10 being concrete on metal deck supported on composite steel beam levels and Level 11 to 49 being a mixture of flat post-tensioned concrete plates and reinforced concrete slabs with formed drop-down beams. The exterior façade is a glass curtain wall system. During the construction phase, the perimeter of the building is enclosed with a guided climbing system for full perimeter protection, access stairs, and material handling. The reshoring system accounts for the force couples imposed at the perimeter by the climbing system considering the construction sequencing of the concrete floor being poured. This article will discuss the concrete floor reshoring design on the steel levels and the reinforced concrete and post-tensioned concrete levels.
Common practice in modern multistory concrete construction is to have the freshly cast floor placed on formwork temporarily supported by a shoring and reshoring system until the concrete has developed sufficient strength and is able to carry its own self weight. A shoring and reshoring system usually has one level of shores and multiple levels of reshores. Shores are vertical or inclined support members designed to carry the weight of the formwork, concrete, and construction loads above (ACI 347.2R-05) and transfer the loads to reshoring levels underneath. Shores and forms are removed once the casting floor concrete attains sufficient load-carrying capacity. Reshores are shores placed snugly under a stripped concrete slab or other structural member after the original forms and shores have been removed from a full bay, requiring the new slab or structural member to deflect and support its own weight and existing construction loads prior to the installation of reshores. Reshores distribute casting floor self-weight and construction loads among several levels underneath or to the ground to reduce the substantial construction loads on the shoring level and avoid excessive stress, deflection, or cracking in a reshoring level slab due to overloading. An efficient reshoring system will reduce construction material and labor cost.
Three assumptions made in the reshoring design of Hudson Site Tower were:
- Shores/reshores are closely spaced and their loads are treated as distributed load.
- Shores and reshores are infinitely stiff compared to the slab.
- Reshores are installed snug-tight so as not to transfer the self-weight of the floor and a portion of the design live loads through the reshore posts.
The floor reshoring operation procedure is assumed to follow ACI 347R-14 method:
- Installation of shores and formwork followed by placement of reinforcement and concrete for the upper slab.
- Removal of the live load immediately following placement and during curing of the slab.
- Removal of the level of shores and formwork, allowing the slab to deflect.
- Removal of the reshores at the lowest interconnected level.
- Placement of the reshores in the story from which the shores and forms were removed.
Since floor plates from Level 2 to 10 are concrete on metal deck supported by steel beams and from Level 11 and above are concrete slabs, Hudson site tower has two reshoring case scenarios, reshoring legs on steel floors and reshoring legs on concrete floors. The reshoring design is based on ACI 347.2R-05 but is modified for simplicity. Below are the steps for reshoring design that have been used:
Step 1–Calculate the casting level shoring load demand:
The deadload is the casting level floor self-weight with 16 psf of superimposed shoring formwork load estimated from the shoring plan provided by the shoring engineer. The live load is 50 psf construction live load. The casting level shoring load demand is equal to the ultimate load of dead load and live load.
Step 2–Calculate the shoring/reshoring level slab load capacity:
The shoring and reshoring level slab has a design capacity which is usually called out on the Structural Engineer of Record’s load map. These are the loads that these slabs can support after supporting their own self weight. Reshoring/shoring construction load shall be deducted from the design capacity. During reshoring, these slabs support 10 psf of superimposed reshoring formwork load estimated from the shoring plan provided by the shoring engineer. Ideally, the reshoring levels are expected to eliminate the additional live load to maximize their load carrying capacity. However, to be conservative, a quarter of full construction live load is considered present on the reshoring/shoring level slab. The capacity might need to be reduced based on the slab curing age. Shoring/reshoring level slab load capacity would be the ultimate load of the slab design dead and live loads reduced by the 10 psf reshoring formwork dead load and the 12.5 psf construction live load.
Step 3–Find the number of preliminary reshoring levels:
The shoring posts transfer 100% of the cast level shoring load demand to the shoring level slab, and then the shoring level slab is assumed to take the portion of the demand that is equal to its load carrying capacity. The rest of the shoring load demand is then transferred by the reshoring posts to the first level of reshoring. This transfer continues until all the cast level shoring load demand is transferred and taken by the reshoring slabs. Following the load path, one can determine the number of reshoring levels.
Step 4–Calculate the stiffness ratio of shoring/reshoring slab:
Since reshoring posts transfer the floor casting load down from floor to floor, all the supported slabs are expected to deflect the same amount. Due to the stiffness variation among those slabs, the reshoring load applied to these slabs has to be modified. For concrete floors, the deflection of a slab is inversely proportional to its cracked or uncracked moment of inertia multiplied by its elastic modulus. The reshoring load applied to each reshoring concrete floor is then likewise distributed proportionally. For concrete on metal deck levels, since the material is not homogeneous and the steel beam depth varies, the stiffness ratio can be determined by using the ultimate design load capacity. The reshoring load applied to each reshoring concrete on metal deck floor is then distributed proportionally to their ultimate design loads.
Step 5–Refine number of reshoring levels:
With distribution of shoring load demand to reshoring levels with respect to the thickness of slabs, the number of reshoring levels would be refined.
Step 6–Check localized effect:
On concrete metal deck reshoring levels, most of the reshoring posts land on the deck spans instead of on the steel beams directly. By overlaying the shoring post layout on the reshoring level structural plan, we categorize the loading case to single point loads supported by single span concrete on metal deck and two or more point loads supported by single span concrete on metal deck. Then the shear and flexural demand is checked against its capacity provided by the deck manufacturer’s catalog.
To check the moment demands under reshoring load at each slab against its as-built moment connection capacity, we built a RAM structural model. Where the posts land on slab openings, double channel waler beams are selected to span across the openings and transfer the loads to adjacent steel beams. On concrete levels, we checked the punching shear under reshoring posts.
Step 7–Select reshoring posts:
Reshoring posts are selected by their maximum reshoring load carrying demand based on unbraced lengths.
On mechanical floors, deep reinforced concrete beams create slabs steps. To save the construction labor and material cost, we targeted a maximum of four levels of reshoring in the design. Leg to leg reshoring underneath those deep beams would have resulted in more than four levels of reshoring. Widening the reshoring bay underneath the last level of slab reshoring engaged more slab area to provide more reshoring capacity.
A full-area enclosure-guided climbing system around the perimeter of the building is used for full perimeter protection and material handling during construction. The floor support for each protection screen creates a resultant force couple from the active pin; horizontal force provides lateral support. The point load resultant force couple requires upward shoring and downward shoring. Horizontal reactions might require additional reinforcement around the active pin. The protection screen reshoring will happen simultaneously with floor reshoring.
We recommend having protection screen active pins placed two levels down from the casting level so there is a fully cured level above. This avoids excessive upward load imposed on post-tension concrete floors by upward shoring installation.
Below are the steps for protection screen reshoring design that have been used.
Step 1: Determine protection reshoring point load capacity at the level where active pins are located (“Active Pin Level”).
Protection screen SC2 on Level 12 serves as an example. The two loading cases that we consider are:
When Level 14 concrete is being poured and the concrete is wet, the Level 13 cantilevered span must take the floor reshoring load and active pin upward reshoring load, while the Level 12 cantilevered span must take the floor reshoring load and active pin downward reshoring load. Therefore, this creates the worst negative flexural reaction for Level 12.
When Level 14 concrete is ready to pour, since there is no floor reshoring load to counterbalance the upward active pin upward reshoring load, Level 13 experiences the worst positive flexural reaction.
We then compare the flexural and shear demand of Loading Case 1 with the floor reshoring load with design capacities using one-way slab action to be conservative. For Loading Case 2, we checked if the slab self-weight can balance the upward reshoring load. Additional flexural and shear capacity can be used for active pin reshoring.
Step 2: Determine protection reshoring point load capacity at floor reshoring levels below.
Following the same procedure as Step 1, one can calculate the additional point reshoring load for the protection screen that the reshoring floors can take in addition to supporting its own self-weight and floor reshoring area load.
Step 3: Determine protection reshoring point load capacity at non-floor reshoring levels below.
We assume 10 psf of superimposed dead load and 50 psf of construction live load for active floor loads on these levels below the last level of reshoring. Following Step 1 and 2 would compute the protection screen point load reshoring capacity.
Step 4: Check localized effect in structural FEM model.
We used a structural FEM model to check the load from previous steps and optimize the reshoring design. We built six RAM Concept models for each “Active Pin Level”:
- Baseline model that reflects the as-built slab condition.
- Loading Case 1 at “Active Pin Level.”
- Loading Case 1 at level above “Active Pin Level.”
- Loading Case 1 at level below “Active Pin Level.”
- Loading Case 2 at “Active Pin Level.”
- Loading Case 2 at Level above “Active Pin Level.”
The reshoring point loads at each level can then be tuned for slab localized action and deflection.
The most important consideration for floor and protection reshoring is to provide a safe and conservative design at the construction site. An economic and efficient reshoring design plays an important role in delivering a high-rise structure successfully. ■
References:
American Concrete Institute. (2014). Guide to Formwork for Concrete (ACI 347R-14).
American Concrete Institute. (2005). Guide for Shoring/Reshoring of Concrete Multistory Buildings (ACI 347.2R-05)