GB2503228A

GB2503228A - Pre-cast column assembly with reinforcement and connection parts

Info

Publication number: GB2503228A
Application number: GB201210828A
Authority: GB
Inventors: Hugh Bowerman; Adrian James Watts; Geoff Davis; Barry Dye
Original assignee: Laing Orourke PLC
Current assignee: Laing Orourke PLC
Priority date: 2012-06-19
Filing date: 2012-06-19
Publication date: 2013-12-25
Also published as: GB201210828D0

Abstract

The assembly comprises upper 4 and lower 2 pre-cast concrete or cementitious column sections containing a rebar 10, the sections being connected through an intermediate concrete slab 6. The lower face of the upper section has a projecting bar 18 that is held by grouting filler within in a void tube 20 in the slab, the void tube being coupled to a rebar in the lower section. A triangular array of swivel sockets receives a swivel thread connector 16 which is coupled to a rebar in the lower column section and projecting from the slab. The swivel connectors may comprise a swivel bolt mounted in a housing such the bolt may slide and rotate but not move axially relative to the housing. The rebars may be held in place by a template at their tops and bottoms. The grout may be cementitious or polymer based.

Description

Assembly of Pre-cast Concrete Columns

Technical Field

[0001] The present invention relates to the assembly of pre-cast concrete columns cast in sections.

Background Art

[0002] Pre-cast columns are used extensively throughout construction. These are typically elongated cylinders or prisms. Internally they comprise of steel bars (rebar) in the long (vertical) direction, said bars acting with the concrete to resist any bending moments applied to the column. Further steel, termed links, wrap around the outside of the longitudinal steel to form a cage. The cage is placed into a mould and surrounded by concrete.

[0003] Columns find common usage in building construction. Column sections are normally aligned vertically one above another. At the joint between vertically adjacent column sections there is normally a concrete slab. The slab transfers load into the column. The vertical component of this load adds to any vertical load already in the column from slabs further up the building. The main purpose of a column is to transfer this vertical load via axial compression down to the foundation.

[0004] Although primarily intended to carry axial compression, columns must also resist bending moments. These arise where slabs connect to a column.

Moments also arise in long slender columns as a result of buckling effects.

The bending is resisted by the longitudinal steel in the column. From a design efficiency perspective, it is desirable for an upper and lower column section to have full moment connection through the slab connection zone.

[0005] A further requirement in columns arises from robustness requirements. In the event that a lower column section is damaged, then the load normally transferred from the slab and down the column must instead be transferred in tension up the upper column and hence redistributed throughout the rest of the structure. This requires a certain minimum of vertical steel to pass continuously from the lower column section, through the slab and into the upper column section.

[0006] The main challenge for pre-cast columns is to provide the continuity of the vertical steel through the slab zone. Existing pre-cast solutions can be divided into two broad categories. These are described below: Grouted dowel bars [0007] In this solution voids are cast into the top of the lower column section.

When the slab is cast these voids are extended fully or partially through the slab. Dowel bars projecting from the base of a to-be-installed upper column section are arranged so as to insert into the voids. Shims are placed under the upper column in order to hold it at the correct vertical elevation. Diagonal props are attached to two approximately orthogonal sides of the column in order to hold the column upright. Once the column is set, the voids are filled with grout. Rebar continuity is achieved by the dowels being of sufficient length to form a full strength lap connection with the vertical rebar already cast in the column. In this system a single, central dowel is typically used, although the system can also be used with multiple dowels. Disadvantages of this system are listed below: * Lapped length of steel is extra to minimum requirements; * Design codes require additional links where vertical steel transfers force through laps; * Where a single central dowel is used, there is a reduction in bending capacity through the slab zone; * Props must be fixed (labour) and act as obstructions; * It can be difficult to prove the efficacy of the grouting operation; * If columns are exposed, they must be made good where the props attach.

Column Shoes [0008] In this solution threaded bars extend from the top of the lower column section, through the slab terminating approximately 5 bar diameters above the slab. A steel base plate is attached to the bottom of the to-be-installed upper column section. The base plate has holes that slot over the projecting threaded bars. The concrete at the base of a column is recessed in order to allow access to the holes. Nuts above and below the base plate can be used to secure and adjust the vertical elevation of the upper column section and its plumb. Columns can be installed quickly, and do not require propping. Threaded bars are typically provided at the corner of the column where they are of maximum benefit for moment transfer.

[0009] A major problem with a base plate is the thickness of the plate. The plate is also dictated by the column cross-section making it a bespoke product.

A range of proprietary systems have been developed to overcome these limitations. Typically each corner of a column is provided with a small sub-plate. Vertically oriented reinforcing bars attached to this sub-plate lap with the vertical steel in the column. Further bars are provided to stop the sub-plate rotating under load eccentricities. The sub-plate with associated bars is termed a column shoe.

[0010] The column shoe offers advantages relative to a grouted connection, but still has a number of disadvantages: * Lapped length of steel is extra to minimum requirements; * Design codes require additional links where vertical steel transfers force through laps. Most proprietary systems require further links to adequately anchor the shoe against the forces generated by eccentric forces; * Where columns are exposed, the nut access recesses in the lower corners need to be grouted up to make the column aesthetically acceptable. Grouting may also be required for fire protection.

Summary of Invention

[0011] The subject of the current invention is a column installation system that overcomes the above problems. The present invention accordingly provides an assembly of upper and lower pre-cast column sections containing rebar, the sections being connected together vertically through a concrete slab, wherein a lower face of the upper column section has at least one dowel type connector comprising a projecting bar that is received in a void tube in the slab which void tube is in turn coupled to a rebar in the lower column section, the projecting bar being held in position in the void tube by grouting; and a triangular array of swivel sockets each adapted to receive a swivel thread connector coupled to a rebar in the lower column section and projecting from the slab.

[0012] The swivel thread connectors provide for the tolerances that inevitably exist even in standardised column sections. This assembly permits rapid and prop-free installation of columns with minimum steel requirements in the column. Whilst a grouting operation is required for the dowel type connector, this uses minimal grout volume. No making good is required to the column.

[0013] The threaded swivel connectors are used to hold the column in position and adjust it to the correct elevation and verticality. The dowel type connectors are used to provide continuity as required in other vertical steel. The combination of these two connector types enables a column to be installed and adjusted without the need for props. A single puddle grout operation completes column installation.

Brief description of drawings

[0014] In order that the invention may be well understood an embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: [0015] Figure 1 is a section through a through a upper column section-slab-lower column section connection made in accordance with the principles of the invention; [0016] Figure 2 shows a vertical section through a swivel thread assembly both before and after assembly; [0017] Figure 3 shows a selection of arrangements for the dowel type connectors and swivel thread assemblies between the column sections; and [0018] Figure 4 shows a plan view of a head of a specially adapted spanner for completing the connection; [0019] Figure 5 is a detail of a gauge tool conveniently provided on the spanner of Figure 4; and [0020] Figure 6 is shows how the gauge tool is used in making the connection.

Description of embodiments

[0021] As illustrated in Figure 1 a lower column section 2 is connected to upper column section 4 through slab 6. Within each column section vertical bars have a female coupler 12 attached at each end so that these couplers are exposed at a lower face of the upper column section and an upper face of the lower column section that each abut the slab 6. The vertical rebars 10 in each column section are held in position using precision manufactured templates top and bottom. Connection between the lower column section 2 and the upper column section 4 is made by two types of connectors, dowel type or mini-dowel connectors 14 and swivel type or swivel thread assemblies 16.

[0022] Each mini-dowel connector 14 comprises three components, a threaded projecting bar 18, a housing or void tube 20 and grout annulus 22.

Threaded bar 18 screws into a female coupler 12 at a base of upper column section 4. The threaded bar material has a yield strength > 1.2 x yield strength of rebar such that the diameter of the thread can be approximately equal to the diameter of the rebar.

[0023] During installation this projecting bar 18 inserts into void tube 20.

Immediately prior to insertion void tube 20 is filled with grout 22. The inside surface of void tube 20 is provided with a profiled surface such that grout 22 keys into it. The void tube may be a sleeve with an internal threaded profile and have a projecting screw 23 that enables it to be connected to a female coupler 12. The annular space between bar 18 and tube 20 is set to be several millimetres larger than the lateral tolerance on the swivel thread connectors 16. Grout 22 has a low viscosity/high wetting characteristic and may be either polymer or cementitious based. It will typically have a long working/gel time (>2 hours at 30°C) such that a column may be installed and set to plumb before it hardens. Grout 10 should ultimately achieve a compressive strength of >8OMPa. High strength grout is used to ensure that the full strength of the rebar 10 can be carried through the connector 14.

[0024] A swivel thread assembly 16 is shown in more detail in Figure 2. A swivel thread connector 30 is connected to the top of the lower column section 2 via a threaded bar 32 screwing into a female coupler 12. The length of threaded bar 32 is selected such that a top surface of a housing 34 carried by the threaded bar is at the desired level relative to the surface of slab 6.

Typically this will be at the same level as the top of void tube 20. The threaded bar material has a yield strength > 1.2 x yield strength of rebar such that the diameter of the thread can be approximately equal to the diameter of the rebar.

[0025] The assembly 16 comprises the housing 34 and a swivel bolt or connector 36. Swivel bolt 36 has four different sections: a head section 38, a shank section 40, a gripping section 42 and a threaded end section 44. The lower head section 38 is circular. It has a 1.2mm radial clearance to the housing 34. In the axial direction it is a non-interference fit in the housing such that the swivel bolt may rotate and slide. Rotation allows the swivel bolt to be screwed into a female coupler 12 which serves as a swivel socket in a lower face of the upper column section 4, without the need to rotate threaded bar 32 or female coupler 12. Sliding the bolt 36 laterally permits a small amount of tolerance between swivel bolt positions during manufacture. The amount of axial free play is less than SOrnicrons.

[0026] The shank section 40 is also circular. It has an outside diameter 2.4mm less than the diameter of the threaded end section 44 giving a radial clearance to the housing of 1.2mm. Section 42 has the same diameter as threaded end section 44 but has six flats machined on to form a hexagonal profile.

[0027] Section 44 is threaded. The thread matches the thread in coupler 12.

[0028] In order to facilitate the making of the connection a cranked open jaw spanner 60 as illustrated in Figures 4 to 6 is employed. The spanner has a head 62, a cranked shaft 64 and two gauges 66, 68 on a specially formed offset shaft end 70. The jaw 72 has a two stage opening with an inner hexagonal seat designed to fit closely the gripping section 42 of the swivel bolt 36. The spanner can be partly retracted from the gripping section to facilitate turning when the spanner is underneath column so that the operator cannot see the hex section 42. The crank has an angle of bend of 20° upwards to facilitate insertion. The gauges 66, 68 are used so that an operator can use the spanner to check whether enough thread is engaged to release the crane and that enough thread is engaged for the service condition.

[0029] The swivel thread connection assembly 16 is made up by rotating swivel bolt 36so that it screws into female coupler 12 which serves as a socket.

The cranked open jaw spanner 60 operating on gripping section 42 is used for this purpose. Where the bottom of the upper column section 4 is desired to finish below the top of slab 6, then a recess 46 is cast into the top of the slab. To permit spanner access, the recess typically extends 50 mm beyond the footprint of the column. Recess depth is typically between 20mm and 25mm.

[0030] The bolt material has yield strength > 1.2 times the yield strength of rebar such that the diameter of the threaded section 44 can be approximately equal to the diameter of the rebar.

[0031] A column connection comprises of three swivel thread assemblies 16 and any number of mini-dowel connections 14. The swivel thread assemblies should be positioned to form a triangle such that the plan centre of gravity of the column falls on or within the triangle. In a square column a swivel thread assembly 16 would be located at three corners with a mini-dowel connection at the fourth corner. Such a column section is installed above a lower column section 2 as follows: 1. Swivel assembly 16 and threaded bar 32 are screwed into the top of lower column section 2 using coupler 12. The top of swivel assembly 16 is set at the correct level. Three such connections are made.

2. Mini-dowel void tube 20 is screwed into the remaining lower column coupler 12.

3. A template is used to locate the top of swivel assemblies 16 and mini-dowel void tube 20. Typically the template would be precision routed from ply, the thickness of the ply corresponding to the desired depth of the recess. Close tolerance holes are required in the template.

Alternatively a laser profiled steel plate can be used.

4. Slab 6 is cast and allowed to harden.

5. The ply template is removed. If necessary, the surface under the template is scabbled to remove laitance.

S

6. Upper column section 4 is lifted by a crane. Threaded bar 18 for the dowel type connector is screwed into one of couplers 12 in a lower face of the upper column section.

7. Immediately prior to the column section being installed, grout 22 is mixed and the appropriate volume placed in mini-dowel housing 20.

8. Upper column section 4 is lowered into position. Threaded bar 18 stabs into housing 20. As the column section is lowered, grout 22 in housing 20 is displaced to form a grout annulus. Grout 22 should ideally flow out the top of housing 20.

9. In the final stage of lowering upper column section 4, the three swivel bolts 36 will try and enter couplers 12. To effect entry, swivel bolt 36 is rotated using the open jaw spanner 60 operating on section 42. The presence of the 20° crank in the spanner shaft allows the spanner head to be received in the recess. Bolts 36 are sequentially rotated until upper column section 4 is at the correct height. The upper gauge 66 is at a distance from an end of the shaft set so that when the spanner is positioned vertically as shown in Figure 6, it is safe to release the crane when it is impossible to get the upper gauge under the column.

10. The verticality of upper column section 4 can be adjusted by rotating swivel bolts 36. Adjustment must be made before grout 22 starts to harden.

11. The final stage of assembly is to inject grout 50 between slab 6 and the underside of upper column section 4. This final stage is not completed until the bolts have been adjusted to a point where it is no longer possible to get the lower gauge 68 under the column.

[0032] Whilst the above description is for a square column, the same installation sequence will work with other shaped columns.

[0033] Figure 3 shows three possible arrangements of connectors within different shaped columns. Fig 3(a) show a circular column with three mini-dowel connectors 14 and three swivel thread connectors 16. Fig 3(b) shows a square column as described above. Note that this is drawn at a slab corner location. The outer corner location would not be accessible from the slab once the upper column section is placed; hence the mini-dowel connector 14 is positioned here. Fig 3(c) show a blade column with three swivel thread connectors 16 and seven mini-dowel connectors 14.

Claims

Claims 1. An assembly of upper and lower pre-cast column sections (4,2) containing rebar (10), the sections being connected together vertically through a concrete slab (6), wherein a lower face of the upper column section has at least one dowel type connector (14) comprising a projecting bar (18) that is received in a void tube (20) in the slab which void tube is in turn coupled to a rebar in the lower column section, the projecting bar being held in position in the void tube by grouting (22); and a triangular array of swivel sockets (12) each adapted to receive a swivel thread connector (36) coupled to a rebar in the lower column section and projecting from the slab.
2. An assembly as claimed in claim 1, wherein the plan centre of gravity of the column falls on or within a triangle defined by the array of swivel sockets (12).
3. An assembly as claimed in any one of the preceding claims, wherein the swivel thread connector is a swivel bolt (36) mounted in a housing (34), the bolt having a bolt head (38) supported such that it may slide and rotate relative to the housing but not move axially.
4. An assembly as claimed in claim 3, wherein the bolt (36) has a gripping section by means of which it can be rotated during assembly.
5. An assembly as claimed in any one of the preceding claims, wherein vertical rebars (10) in each column section are terminated at each face with a connector profile (12) designed to engage with the void tube (20) or serve as a swivel socket.
6. An assembly as claimed in any one of the preceding claims, wherein vertical rebars (10) in each column section are held in position using precision manufactured column templates top and bottom.
7. An assembly as claimed in claim 4, wherein a recess (46) in slab (6) is made using a precision ply template that forms the recess, hole centres in said template matching exactly the hole centres in the column template.
8. An assembly as claimed in any one of the preceding claims, wherein the void tube is a sleeve with an internal thread profile, the two elements held together by a high strength cementitious grout such that the full strength of the re-bar can be carried through the connector.
9. An assembly as claimed in any one of the preceding claims, wherein the projecting bar (18) is a standard rebar.
10. An assembly as claimed in any one of the preceding claims, wherein the grout is polymer based.
11. An assembly of upper and lower pre-cast column sections substantially as herein described with reference to the accompanying drawings.