Post-tensioned slab-to-wall connection embed

Abstract

A slab-to-wall connection embed for use in post-tensioned concrete podium structures is disclosed. The embed includes a center plate with a slotted aperture, allowing longitudinal movement to accommodate shrinkage forces while maintaining seismic and wind resistance. Side plates are positioned on opposite sides of the center plate and are embedded within a concrete wall to provide structural anchorage. A bolt assembly, including a bolt passing through the slotted aperture, limits movement of the center plate along the longitudinal axis while allowing controlled slip. A headed concrete anchor extends from the center plate into the post-tensioned or reinforced concrete slab, securing the connection. The system allows immediate structural integration without the need for temporary connections, reducing construction wait times while improving long-term durability and lateral-force resistance.

Description

RELATED APPLICATION

[0001] This application claims priority benefit of U.S. Provisional Patent Application No. 63 / 776,857 filed Mar. 24, 2025, which is hereby incorporated by reference.TECHNICAL FIELD

[0002] This disclosure relates generally to structural engineering and construction technology, specifically to a fabricated steel connection used in post-tensioned concrete construction to improve slab-to-wall connections.BACKGROUND INFORMATION

[0003] In conventional podium construction, post-tensioned slabs shrink as they cure and mature, which can cause significant displacement. While the highest shrinkage occurs within the first few months after pouring, shrinkage continues for years. Additionally, temperature fluctuations cause concrete slabs to expand and contract.

[0004] The connection between a concrete slab and its supporting wall is critical for transferring seismic and wind forces to the building's foundation. FIG. 1, for instance, shows a post-tensioned slab construction 100 using current construction techniques for slab-to-wall connections. In general, the current practice entails welding embeds to connect slabs to walls. Specifically, FIG. 1 shows the use of rigid rebar dowel connections 102 between a post-tensioned concrete slab 104 and a concrete foundation wall 106. Rigid rebar dowel connections 102 are shown as evenly spaced along the length of concrete foundation wall 106, providing a highly rigid connection that prevents movement between post-tensioned concrete slab 104 and concrete foundation wall 106.

[0005] The rigid tie between post-tensioned concrete slab 104 and concrete foundation wall 106 resists seismic and wind forces. However, these conventional connections also restrain shrinkage forces, leading to wall cracks 108 in both post-tensioned concrete slab 104 and concrete foundation walls 106. As post-tensioned concrete slab 104 undergoes natural shrinkage and temperature-induced movement, it pulls against the rigid connection, generating tensile stresses that result in wall cracks 108.

[0006] While some cracks are superficial, significant cracking can compromise the building's structural integrity and lateral-force resistance. In extreme cases, connection failure can lead to safety hazards and costly repairs. FIG. 1 highlights the failure of conventional slab-to-wall connections, where welded embeds or dowels do not accommodate shrinkage displacement, resulting in stress buildup and cracking. This issue is particularly critical in podium buildings, where the slab serves as a transfer level supporting multiple stories of wood-framed construction.BRIEF SUMMARY OF THE DISCLOSURE

[0007] The post-tensioned slab-to-wall connection embed is a steel connection system designed to address the challenges associated with post-tensioned slab shrinkage and structural integrity in podium construction. This system consists of three steel plates connected with slotted high-strength bolts, allowing for controlled slip to accommodate shrinkage forces while maintaining seismic and wind force resistance. The embed is pre-installed into the concrete shear wall before the slab is poured, integrating seamlessly into the structure without requiring additional modifications after installation. The center plate, secured with headed concrete anchors, moves parallel to the wall to relieve shrinkage stresses but remains locked under seismic forces. The slip force is precisely controlled by the size and quantity of high-strength bolts, ensuring a stable and calibrated connection. Unlike traditional methods that either resist shrinkage forces completely or require delayed connections, this system allows immediate displacement for shrinkage and temperature changes while providing an immediate, fully functional lateral-force-resisting system.

[0008] This design offers several advantages over conventional slab-to-wall connections. By allowing the slab to move naturally without compromising structural stability, it prevents excessive stress transfer to the foundation and minimizes concrete cracking, which is a common issue in post-tensioned podium construction. Traditional connections are typically designed to handle either seismic or wind forces, but this system accommodates both simultaneously while still allowing shrinkage movement. Additionally, because the connection activates as soon as the slab is poured, it eliminates construction delays that typically result from waiting for shrinkage to occur before finalizing slab-to-wall connections. The system is also adjustable and scalable, making it adaptable to a variety of building designs and structural requirements.

[0009] This innovative connection method may improve construction efficiency, durability, and structural performance, providing an effective solution to a long-standing issue in post-tensioned concrete construction. Additional aspects and advantages will become apparent in the detailed description of embodiments, with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0010] To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced.

[0011] FIG. 1 is a fragmentary perspective view showing a conventional method of slab-to-wall connection, resulting in issues such as cracking due to restrained shrinkage forces.

[0012] FIG. 2 is a photograph of a podium under construction for a 5-on-1 building, in which a post-tensioned slab-to-wall connection embed is used in unseen embedded portions of shear walls.

[0013] FIG. 3 is a fragmentary isometric view of a slab-to-wall connection embed within a post-tensioned concrete podium structure, according to one embodiment.

[0014] FIG. 4 is a section view taken along lines 4-4 of FIG. 3, in accordance with one embodiment.

[0015] FIG. 5 is an exploded view of the slab-to-wall connection embed shown in FIG. 3 in accordance with one embodiment.

[0016] FIG. 6 is a side elevation view of a center plate of the slab-to-wall connection embed in its center position in accordance with one embodiment.

[0017] FIG. 7 is a side elevation view of a center plate of the slab-to-wall connection embed in its right-most position in accordance with one embodiment.

[0018] FIG. 8 is a side elevation view of a center plate of the slab-to-wall connection embed in its left-most position in accordance with one embodiment.

[0019] FIG. 9 shows a routine for installing a slab-to-wall connection embed in a post-tensioned concrete podium structure, in accordance with one embodiment.DETAILED DESCRIPTION OF EMBODIMENTS

[0020] FIG. 2 shows a post-tensioned concrete podium structure 200, which is a type of building construction where a reinforced concrete slab, known as a podium slab 202, is post-tensioned using high-strength steel tendons to improve structural performance. This slab 202 serves as a transfer level, supporting multiple lightweight upper floors while efficiently distributing loads to the foundation. The post-tensioning system consists of high-strength steel cables (tendons) embedded within slab 202, which are stressed after the concrete has gained sufficient strength. This process introduces compressive forces, counteracting tensile stresses and reducing cracking.

[0021] Podium slab 202 serves as a structural separator between lower levels 204, which often contain parking or commercial spaces, and upper wood or steel-framed residential floors (not shown). It transfers loads from the upper structure to concrete shear walls 206 and columns 208 below. These shear walls 206 provide seismic and wind resistance, making the connection between slab 202 and walls 206 a design consideration for structural stability. Proper slab-to-wall connections, described below, ensure that podium slab 202 can shrink without inducing excessive stresses while maintaining lateral-force integrity. Shrinkage and temperature forces are typically 10 or more times the required seismic or wind connection forces.

[0022] One of the main advantages of post-tensioning is that it allows for thinner slabs without compromising strength, leading to material savings and increased structural efficiency. Additionally, it minimizes long-term shrinkage and cracking, which are common issues in conventional reinforced concrete slabs. This makes post-tensioned podium structures ideal for mixed-use developments, residential apartments, and commercial buildings, particularly in “5-over-1” or “4-over-1” configurations, where a concrete podium supports multiple levels of wood-framed construction.

[0023] In the example of FIG. 2, post-tensioned concrete podium structure 200 is under construction, with the post-tensioned podium slab 202 being poured and finished. The embed components described below, which are first installed within shear walls 206 prior to pouring podium slab 202, ensure that podium slab 202 can move slightly under shrinkage forces while providing a strong, reliable connection for seismic and wind resistance. In the image, a concrete pour is in progress for the post-tensioned slab. A concrete pump truck with an extended boom is delivering fresh concrete to the upper deck of the structure. Construction workers are actively spreading and finishing the concrete surface.

[0024] The embeds would be evenly spaced along the length of shear walls 206, though they are not visible in this figure as they are cast into the center of the top portion of shear walls 206. The embed allows podium slab 202 movement parallel to shear walls 206, not perpendicular to it. When podium slab 202 moves perpendicular to the plane of shear walls 206, there is usually enough flexibility in shear walls 206 to accommodate the movement.

[0025] FIG. 3 shows a post-tensioned concrete podium structure 300, where a slab-to-wall connection embed 302 is installed. Embed 302 is cast into the center of a top surface of concrete shear wall 304 and is not visible once structure 300 is completed. Multiple embeds 302 would be evenly spaced along the length of concrete shear wall 304, providing a controlled slip connection between post-tensioned or reinforced concrete slab 306 and concrete shear wall 304.

[0026] To ensure proper installation, concrete shear wall 304 is poured first, embedding bottom section 308 of slab-to-wall connection embed 302 within wall 304. Crushable end fill block 310 maintains proper positioning and spacing of embed 302 during casting. Specifically, crushable end fill block 310 is positioned within an end gap between side plates 312 and a center plate 314 to prevent concrete from filling this space during casting. This ensures that center plate 314 has free space to slide longitudinally as needed, accommodating shrinkage movements without causing excessive stress on the connection.

[0027] Reinforcing bar dowels 316, made from ASTM A706 Grade 60 steel, are welded to side plates 312, providing additional reinforcement and securing embed 302 within concrete shear wall 304. ASTM A706 Grade 60 steel is a low-alloy, weldable reinforcing steel designed for applications requiring high ductility, controlled strength, and seismic resistance. It has a specified yield strength of 60 ksi (60,000 psi) with a controlled upper limit, ensuring predictable performance in structural applications. Unlike ASTM A615, which has higher carbon content and is more difficult to weld, ASTM A706 Grade 60 is formulated for weldability, eliminating the need for preheating or post-weld heat treatment. It is commonly used in reinforced concrete shear walls, slabs, and seismic-resistant structures, where rebar must absorb and dissipate forces without brittle failure.

[0028] In FIG. 3, four reinforcing bar dowels 316, made from ASTM A706 Grade 60 steel, welded at downward angles on each side of embed 302 extend downward from side plates 312, embedding deeply into concrete shear wall 304. This angled configuration enhances the structural anchorage of embed 302 by increasing the bond length within the concrete, improving the ability of the connection to resist shear forces and pullout under seismic and lateral loads. The orientation of dowels 316 also helps distribute forces more effectively, reducing the risk of localized stress concentrations that could lead to cracking or failure in concrete shear wall 304.

[0029] Center plate 314 and side plates 312, both made from ASTM A36 steel (or A50) with a mill scale finish, form the core structural connection between post-tensioned or reinforced concrete slab 306 and concrete shear wall 304. ASTM A36 steel, used for center plate 314 and side plates 312, is a low-carbon structural steel known for strength, weldability, and durability, making it ideal for construction applications. During the hot-rolling process, a mill scale finish naturally forms on its surface, creating a dark gray or bluish oxide layer that provides temporary corrosion resistance. Since these embeds are cast within concrete, they remain protected from environmental exposure, reducing the need for additional corrosion treatments. However, if the steel were to be exposed to air and moisture, mill scale would typically be removed and replaced with a protective coating, such as paint or galvanization, to prevent long-term corrosion.

[0030] Bolt assembly 318 secures center plate 314 between side plates 312, limiting movement along the longitudinal axis while allowing controlled slip once shrinkage forces exceed a threshold slip value. Bolt assembly 318 determines slip load value, which depends on the number and size of high-strength bolts used in the connection. Construction felt slip surface 320 allows for controlled movement, reducing stress between slab 306 and wall 304 while permitting shrinkage displacement without compromising structural integrity.

[0031] Once post-tensioned or reinforced concrete slab 306 is poured, headed concrete anchors 322 secure center plate 314 to slab 306. This final connection ensures slab 306 can move parallel to wall 304 under shrinkage forces but remains locked in place under seismic forces. This installation and configuration allow for controlled slip at a pre-determined force level, addressing shrinkage movements while maintaining seismic and wind force resistance, helping ensure long-term structural stability. The concrete slab is poured over the top of the embed in the same manner as the current temporary slip connections that are currently used. The difference is that once the concrete is poured there is no further action required. Construction may continue without delay, and there is no longer a need to return and weld or grout connections at a future date. There is no special training or changes for construction crews. The new embed simply replaces what they have been using.

[0032] FIG. 4 is a detailed end view of bolt assembly 318 within slab-to-wall connection embed 302 in a post-tensioned concrete podium structure 300. This figure highlights the fastening components that secure center plate 314 and side plate 312 within concrete shear wall 304, helping ensure a strong and properly tensioned connection between post-tensioned or reinforced concrete slab 306 and concrete shear wall 304 while allowing controlled slip.

[0033] Bolt assembly 318 includes a bolt 402 conforming to ASTM F3125 Grade A325 (or Grade A490), which is fully tightened to maintain the connection under seismic and lateral loads. A direct-tension indicating washer ASTM F959 404 is included to verify proper bolt tensioning, while a hardened washer ASTM F436 406 reinforces bolt assembly 318 against compression forces. High-strength nut 408 secures the system, ensuring the plates remain engaged under applied forces. Bolt assembly 318 is preloaded to a calibrated tension force to ensure proper slip resistance. According to AISC specifications, a ¾-inch diameter Grade A 325 bolt has a minimum pretension force of 28,000 lbs. This calibrated pretension force allows the connection to provide controlled slip at a predetermined load level, ensuring that the system accommodates shrinkage forces while maintaining resistance to seismic and wind loads. The preloading of the bolt assembly prevents unintended movement of center plate 314 until the applied shrinkage force exceeds the slip threshold.

[0034] As mentioned previously, construction felt slip surface 320 is placed between slab 306 and concrete shear wall 304, allowing controlled movement under shrinkage forces while ensuring the system remains locked under seismic and wind loads.

[0035] The combination of bolt assembly components, slip interface, and gap preservation using Styrofoam (or other deformable material) as crushable end fill blocks 310 allows the embed system to provide a structurally sound yet flexible connection, balancing shrinkage accommodation with seismic resistance to ensure long-term stability.

[0036] FIG. 5 is an exploded isometric view showing the detailed components of slab-to-wall connection embed 302. This figure highlights the individual elements and their assembly, focusing on how the connection allows controlled movement along longitudinal axis 502 while providing seismic and wind resistance. This exploded view illustrates how each component integrates to form a functional, adjustable connection that allows controlled slip while maintaining seismic, wind, and structural load resistance in the slab-to-wall embed system.

[0037] Center plate 314 features a set of three slotted apertures 504, which allow center plate 314 to move along longitudinal axis 502 until it confronts a shank of bolt 402 (retained in bolt aperture 506 of side plate 312), which acts as a movement limiter. This design enables controlled slip to accommodate shrinkage and temperature movements while maintaining structural integrity under lateral forces.

[0038] Side plates 312 are positioned on either side of center plate 314, forming the primary structural connection within embed 302. Bolt assembly 318, which consists of bolts 402, direct-tension indicating washers 404 (ASTM F959), hardened washers 406 (ASTM F436), and high-strength nuts 408, is used to secure and tension the plates together, ensuring the system functions as intended.

[0039] Additionally, reinforcing bar dowels 316, made from ASTM A706 Grade 60 steel, extend outward from side plates 312 at an angle, providing anchorage within the concrete shear wall. Crushable end fill block 310 is positioned within the assembly to prevent concrete from filling the gap between side plates 312 and center plate 314, ensuring that center plate 314 remains free to slide within slotted apertures 504. In this context and throughout this disclosure, it should be understood that slab-to-wall connection embed 302 is free to slide after the shrinkage force exceeds a threshold slip value. This “free” movement need not completely frictionless but is controlled by bolt assembly 318 and the interaction of the components. In that sense, the term free denotes the allowance of movement in a defined degree of freedom (DOF) (i.e., along longitudinal axis 502) while maintaining lateral and rotational constraints to ensure structural integrity.

[0040] FIG. 6-FIG. 8 illustrate the movement of center plate 314 within slab-to-wall connection embed 302. FIG. 6 shows center plate 314 in its neutral position, aligned within the assembly. FIG. 7 demonstrates center plate 314 displacement to the right, accommodating shrinkage or thermal movement. FIG. 8 shows center plate 314 shifting to the left, indicating movement in the opposite direction. This longitudinal movement occurs within slotted apertures 504, allowing for controlled slip while ensuring the connection remains secure under seismic and lateral loads.

[0041] In practice, bolts 402 and center plate 314 are initially coupled with bolts 402 positioned near the middle of slotted aperture 504 to allow for movement in either direction. This is particularly important because in some cases, the exact movement direction of the slab is not explicitly known at the time of installation. Temperature fluctuations can cause expansion and contraction in both directions, making it essential that the system accommodates bidirectional movement. Additionally, requiring construction crews to install the embeds in a specific orientation to account for movement direction could result in incorrect installations. By centering bolts 402 within the slots during initial placement, the system ensures proper function regardless of potential variability in installation or movement direction.

[0042] FIG. 9 shows a routine 900 for installing slab-to-wall connection embed 302 in a post-tensioned concrete podium structure. As described previously, embed 302 is pre-assembled by tightening bolt assembly 318 that binds center plate 314 between two side plates 312 before embed 302 is deployed for controlled slip along longitudinal axis 502 defined by slotted aperture 504 in center plate 314 (through which bolt 402 extends).

[0043] In block 902, routine 900 positions the assembled embed in a top portion of a concrete shear wall before curing, such that a headed concrete anchor, coupled to the center plate, extends outward from the top portion, and side plates are embedded within the concrete shear wall for securing them therein while leaving the center plate capable of moving longitudinally.

[0044] In block 904, routine 900 pours a post-tensioned concrete slab over the embed, ensuring that the headed concrete anchor secures the center plate to the slab, and the center plate remains free to slide (i.e., when shrinkage forces exceed a threshold slip value) along the longitudinal axis, accommodating shrinkage forces while resisting seismic and wind loads.

[0045] In light of this disclosure, skilled persons will appreciate that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. For instance, a different number of slotted apertures 504 is possible for shorter or longer variants. The scope of the present invention should, therefore, be determined only by claimed inventions and equivalents thereof.

Claims

1. A slab-to-wall connection embed for use in post-tensioned concrete podium structures, comprising:a center plate having a slotted aperture configured to allow longitudinal movement;side plates, each positioned on opposite sides of the center plate and configured to be embedded within a concrete wall;a bolt assembly, including a bolt passing through the slotted aperture in the center plate, in which the bolt assembly is configured to limit movement of the center plate along a longitudinal axis; anda headed concrete anchor securing the center plate to a concrete slab, in which the center plate is configured to slide along the longitudinal axis defined by the slotted aperture, accommodating shrinkage forces while resisting seismic and wind loads.

2. The slab-to-wall connection embed of claim 1, in which the slotted aperture is sized to allow a pre-determined slip movement based on an expected shrinkage displacement of the concrete slab.

3. The slab-to-wall connection embed of claim 1, in which the side plates and center plate are made from ASTM A36 steel with a mill scale finish.

4. The slab-to-wall connection embed of claim 1, in which the bolt assembly further comprises:a direct-tension indicating washer;a hardened washer; anda high-strength nut, in which the bolt assembly is pre-tensioned to ensure controlled slip resistance while maintaining seismic and wind force transfer.

5. The slab-to-wall connection embed of claim 1, further comprising a crushable end fill positioned between the side plates and center plate to prevent concrete from filling an end gap, ensuring that the center plate is configured to slide along the longitudinal axis when shrinkage forces exceed a threshold slip value, thereby accommodating the shrinkage forces while resisting seismic and wind loads.

6. The slab-to-wall connection embed of claim 5, in which the crushable end fill degrades over time or remains flexible after concrete curing to prevent obstruction of center plate movement.

7. The slab-to-wall connection embed of claim 1, further comprising reinforcing bar dowels extending downward at an angle from the side plates, embedding into the concrete wall to improve bond strength and resistance to shear forces.

8. The slab-to-wall connection embed of claim 1, in which the bolt assembly is preloaded to a calibrated tension force, ensuring that the connection provides controlled slip at a predetermined load level.

9. The slab-to-wall connection embed of claim 1, in which the center plate includes multiple slotted apertures for multiple corresponding bolt assemblies.

10. The slab-to-wall connection embed of claim 1, in which the headed concrete anchor includes multiple headed concrete anchors.

11. A method for installing a slab-to-wall connection embed in a post-tensioned concrete podium structure, in which the embed is pre-assembled by tightening a bolt assembly that binds a center plate between two side plates of the embed before it is deployed for controlled slip along a longitudinal axis defined by a slotted aperture in the center plate through which a bolt extends, the method comprising:positioning the assembled embed in a top portion of a concrete wall before curing, such that a headed concrete anchor, coupled to the center plate, extends outward from the top portion, and side plates are embedded within the concrete wall for securing them therein while leaving the center plate capable of moving longitudinally; andpouring a concrete slab over the embed, ensuring that the headed concrete anchor secures the center plate to the slab, and the center plate is configured to slide along the longitudinal axis when shrinkage forces exceed a threshold slip value, thereby accommodating the shrinkage forces while resisting seismic and wind loads.

12. The method of claim 11, further comprising assembling the embed by inserting the center plate between the side plates and performing the tightening.

13. The method of claim 11, further comprising positioning a crushable end fill between side plates and the center plate to prevent concrete from filling an end gap, ensuring the center plate remains free to slide.

14. The method of claim 11, further comprising securing reinforcing bar dowels extending from the side plates into the concrete wall.

15. The method of claim 11, in which the reinforcing bar dowels are pre-welded to the side plates before installation to ensure structural integrity and bonding within the concrete wall.

16. The method of claim 11, in which the slotted apertures in the center plate allow controlled movement along the longitudinal axis while preventing unintended lateral displacement.

17. The method of claim 11, in which the concrete slab is poured immediately after the concrete wall cures.

18. The method of claim 11, further comprising verifying bolt tension using a direct-tension indicating washer, ensuring that the bolt assembly is properly preloaded before installation.

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