Steel Detailing | Precast Detailing | Estimation

Author: Murugan K

  • Load Bearing & Non-load bearing concepts in precast

    What is load bearing precast element?

    Load bearing precast element is anything that receives load from a precast or structural members and transfers the load to another structural member or precast. These precast elements can also be used along as façade, boundary walls, or for architectural design purposes.

    What is non-load bearing precast element?

    Non-load bearing precast element is anything that may receive load but must not transfer loads to any other structural members or precast. Normally these kind of walls are used only for partition, façade, boundary walls, or for architectural design purposes

    What is Partially-load bearing precast element?

    Partially-load bearing precast element is anything that receives load from a precast or structural members and transfers the load to another structural member or precast. But only a portion of precast receives and transfers load and the remaining portions will act as non-load bearing ones. (Single precast element which acts both as load bearing & non-load bearing)

    Data’s, panel types  & other details for

    Load-bearing element  Non-bearing element  Partially-load bearing element
    Grade, can be higher upto 80MPaGrade enough be 40MPa (UNO)Grade, can be higher up to 80MPa
    Gap between two elements are filled with grouting material of required gradeGap between two elements are filled with Soft joint caulking only. no grouting.Gap between two elements are filled with grouting material of required grade, extent of filling is for load bearing portion. Remaining portion will be soft joint caulking only
    Dowels to be black finish (UNO)Dowels to be Galvanized finishDowels to be Black or Galvanized finish, as per load bearing extend (UNO)
    Dowel length, size & spacing as per Engineer’s requirementDowel length, size & spacing as minimum as possible after discussing with Manufacturer or EngineerDowel length, size & spacing as per Engineer’s requirement  for LBP portion Dowel length, size & spacing as minimum as possible for NLBP portion
    Dowel & Grouting materials will transfer load to adjoining elementDowel will be used to position the precast element & no grouting material present to transfer load. So loads will not be transferred.Combination of LB & NLB criteria
    No ring seal around the dowelHigh density foam ring seal around the dowelNo ring seal around the black dowel High density foam ring seal around the Galvanized dowel
    Grouting material will cover the dowel, so oxidation in dowel is negligibleNo Grouting material, oxidation in dowel is possible. So high density foam ring seal is used and dowels are galvanized to prevent oxidation in dowelCombination of LB & NLB criteria
    No additional tubes or caps neededPlastic tube will be placed over upper half of dowel. Compressible cap will be placed over the top of the dowel bar.Combination of LB & NLB criteria
    Connection with slab mustConnection with slab may not needed with respect to engineer requirementCombination of LB & NLB criteria

  • TEMPORARY CONNECTION WITH DETAILS

    Precast erection procedure having the set of sequences. When the panel is installed on the slab, props are fixed to stable the panel, then panel was unhooked from crane then the temporary stich plate or stich angles are connected to the next panel through cast in ferrule, which is casted on the precast panel.

    The temporary connection is additional support for the precast panel for until grouting the grout tube & slab were connected to the particular panel. The panels are easily leveled up with respect to the connected panel. Minimize the no of brace connections. Easy to maintain the panel gap.

    Based on the position of panels & face of the connection it is classified as below,

    1. In-line stich plate connection.
    2. Internal Stich angle connection.
    3. External stich angle connection.
    4. On edge stich angle connection.

    1. In-line stich plate:

    When the panel are next to each other the temporary connections are made by the straight stich plate with ferrule as mentioned below picture.

    2.Internal Stich angle:

    When the panel is perpendicular each other and inside corner is access to perform the connection, the internal angle stich angle is being used.

    3. External stich angle:

    When the panel is perpendicular each other and outside corner alone is access to perform the connection, the external angle stich angle is being used.

    4.On edge stich angle connection.

    Some cases, same level panels one is erected on top of slab and another one is located on ceiling level is supported by on-edge ferrule mentioned as below picture.

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  • CAST-IN PLATE WITH DETAILS

    A steel plate which is casted in to concrete for connection purpose is named cast-in-plate. Connections may be required to carry shear and axial force between the precast elements for applied load conditions.

    Cast-in plates are used to tie/ fix,

    1. Concrete or precast panels to each other.
    2. Precast to insitu slab.
    3. Steel beams to the Precast panels/slabs.

    All purpose of connection except steel beam to precast connections required pair of cast in plate connected by a welded plate . But the steel beam to precast connection required only a cast-in plate with cleat plate is enough . All the plates and connections details are need to approved by Structural Engineer.

    1 Panel to Panel connection:

    Normally the lift shaft panels & stair panels have to connect withstand maximum shear and axial loads. The vertical panel joints are made by the cast-in plate connection. Minimum two no of connections per level is needed (need to get structural engineer advise).

    The cast in plate casted with pocket to cover on later stage. Both perpendicular & in line panels are also can be able to connect. We can able to connect insitu wall to precast panel also by this method. Refer blow picture for better understanding.

    2. Precast to Insitu slab Connection:

    For Particular requirement the precast should be connected with slab. One cast in plate casted in to the slab and another one is casted in to the precast and both the plates are connected through weld plate by weld on site. Refer below picture for better understanding.

    3. Steel beams to the Precast Connection:

    Now a days the construction system are versatile various types of constructions systems are used to complete the same building. Steel building with precast walls are also preferred as a one type of construction system.

    Where the steel beams are meet the precast panels the cast-in plate is used to connect it. Here instead of weld plate cleat plate is used to connect between the steel member to precast wall. The cast in plate on the precast walls not cast with the pocket it is flush with surface.

  • TABLE PRECAST FINISHES

    Table precast finishes is the replication process. The surface of the manufacturing table affecting the precast surface. Sometimes the table is covered by form liners, bricks, signage letters & pattern to make the finishes.

    a) Normal table finish:

    Non-exposed surface of the precast is made by this type of table. Same table is used for multiple time for casting the precast. Surface of the table is drilled and nailed for component fixing. Components fixing damages not fixed at every time of repetition, which result poor surface finish.

    b) Good table finish:

    The damaged table surface is being fixed by welding and grinding before every time of casting. Which means each and every time the surface of the precast panel surface will be made well & good.

    c) Form Liner finish:

    custom façade design with architectural concrete, elastic form liners are used to form the finish by placing it on the table before pouring the concreate for the particular design of concrete facades.

    d) Retarder finish:

    It is a type of acid wash, spraying a retarding agent on the surface of the fresh concrete and leaved long time and washed to make aggregate exposed.

    e) Patterned finish:

    As per the Architecture or client requirement various shape of profile needs to make on precast surface by placing the readymade patterns on the table to form.

    f) Brick snap finish:

    Bricks are arranged on the table for required shape before pouring the concrete and demold the panel which has the bricks attached surface of the precast panel.

  • NON-TABLE PRECAST FINISHES

    The finishes are performed without help of table faces. These type of finishes are listed as below…

    a) Float finish:

    Float is a tool used to finish a concrete surface by floating on it. After the concrete is poured on the table is levelled by using screed, then float tool is used. Float may be the small or large handle tool sometimes its powered by engine.

    b) Trowel finish:

    Trowel also a tool used to finish a concrete surface by pushed and pulled across the concrete surface. Trowels are performed in both manual and mechanical types.

    A manual trowel consists of a flat steel blade with an attached handle on top. A Mechanical trowel is used for large commercial projects which have rotational of large fans with the blades directly against the concrete surface.

    c) Polished finish:

    Polished Finish is a multi-step process where a Precast concrete is polished with bonded abrasives or chemicals in order to grind a precast concrete surface. It is then refined with each grind in order to achieve a specified level of appearance.

    d) Broom finish:

    Broom finish of the concrete is rough surface finished made by forcing or dragging a broom on the troweled surface while the concrete is still fresh.

    e) Sand Blast Finish:

    A sandblasted precast panel has a rough finish which is achieved when a precast panel surface is subjected to a high-pressure blast of siliceous sand through an air gun.

  • LIST OF PRECAST FINISHES

    Precast panels are ready made concrete element, which is the part of the building as structural as well as façade panels, which required the surface finish as per the customer or Architect requirements. Various type of finishes are used on the both side of panel and edges according to where the panels are positioned on the building.

    Due to the process of precast manufacturing, the finishes are made by replication of table on the one side, and the other side our traditional or conventional methods are preformed after pouring the concrete. As per concern of precast finishes, its classified as

    1. Non-Table finishes.
    • Float finish.
    • Trowel Finish
    • Polished Finish.
    • Broom Finish.
    • Sand blast finish. etc..

    2. Table Finishes.

    • Normal Table Finish.
    • Good Table Finish.
    • Form liner Finish.
    • Retarder Finish.
    • Patterned Finish.
    • Bricks Snap Finish. etc.

  • POST FIXED PRECAST PANEL

    Every multi story buildings typically need the loading bay or hoist for each and every level during construction. A loading bay or loading dock is an area of a building where goods are loaded and unloaded to it from vehicle or site. On hoist or loading area of façade walls is being unbuilt up to end of construction. Final stage of construction only it needs to be built. Refer below picture for different types of loading bays are highlighted.

    If the rest of the building is constructed by the precast, the hoist area also needs to be covered or fixed by precast panel. This type of covering panels are named post fix panel. Which means the panel will fix on the existing structure after completion of required task.

    These post fix panels have different type of erection procedure, connection details compared to the all-other precast panels.

    Propping Method for Post Fix Precast:

                The temporary bracket connection between panel and to the slab need to be done at the top of the panel. The rest of the panel hanging below slab where braces are connected to plumb the panel as well as support. That the brace is anchored below level slab. The bottom of post fix panels is connected with dowel & grout tube arrangement to the below level precast panel. Refer the propping snap.

    Permanent Connection of post fix panel:

                The grout tube with dowel connection between the panels are only resist the horizontal movement, the additional permanent connection is made between panel and the slab through cast in plate and weld on site at both top and bottom the slab. The structural connections details are need structural engineer approvals. Refer below picture for typical connection details.

  • DEMOULDING SLOPE

    Demoulding of precast member without damage to either the components like lifter or mould is critical to successful replication process for the particular complicated design. During mould design, the designers concentrate to make minimum draft (nearly 10 ) on mould to wherever the possible  to minimize demoulding force and resultant stress on lifters and prevent on weaker part of the precast member.

    For Example,

    CONTRIBUTORS DURING DEMOULDING WITHOUT SLOPE

    The above picture shows influencing factors for demoulding force. In this case the resultant DEMOULDING forces like vacuum & friction (Area of contact, Coefficient of friction & Normal contact Pressure) along with self-weight of precast will increase demoulding force. If the demoulding force exceed the lifter capacity limit leads to fail the lifters. So, we can’t able to lift this precast member. To eliminate this type of failure. We need to provide slope where contact pressure or interface adhesion develop.

    CONTRIBUTORS DURING DEMOULDING WITH SLOPE

    The above picture shows the effect of demoulding slope. Where the 10 slope didn’t affect too much the original shape of precast, but considerably minimize the demoulding force. The slope reduces the frictional force & provide passage to air enter where vacuum force develops.

  • CONCRETE WITH SPECIFICATION DIFFERENCE (N & S)

    CONCRETE:

     (AS 1379 Specification and supply of concrete) A mixture of Cement, aggregates and water with or without the addition of chemical admixtures or other materials.      

    Cement: (AS 3972 Portland or blended cement) A hydraulic binder composed of Portland or blended cement used alone or in combination with one or more supplementary cementitious materials.

    Concrete is defined as follows,

    • Plastic concrete:

    Concrete in the state between completion of mixing and initial set as defined in AS 1012.18 Methods of determining setting time of fresh concrete, mortar and grout by penetration resistance.

    • Hardened concrete:

    Concrete after initial set, as represented by test specimens that have been subjected to a specified process and duration of curing.

    • Normal- Class Concrete:

    Concrete that is specified primarily by a standard compressive strength grade up to 50 MPa and otherwise in accordance with Clause 1.5.3.

    • Special- Class Concrete:

    Concrete that is specified to have certain properties or characteristics different from, or additional to, those of normal-class concrete and otherwise in accordance with Clause 1.5.4.

    SPECIFICATION OF CONCRETE:

    Concrete shall be specified,

    (a) as either

    (1) Normal-class(N), or

    (2) Special-class(S), or

    (b) By strength grade or other readily verifiable parameter by which compliance with the specification can be assessed.

    NOTE: Standard strength grades should be specified wherever possible.

    • NORMAL-CLASS CONCRETE:

    Normal-class concrete shall be specified only by the parameters given in Clause 1.5.3.2(Basic parameter), and shall have the following attributes:

    • A mass per unit volume in the range 2100 kg/m3 to 2800 kg/m3 when determined in accordance with (AS 1012.12.1 Determination of mass per unit volume of hardened concrete) in the saturated, surface-dry condition.
    • Acid-soluble chloride and sulfate contents within the limits given in Clause 2.7, when determined in accordance with Clause 5.5.2.
    • A shrinkage strain not exceeding 1000 × 10−6, when determined in accordance with Clause 5.6 after 56 days drying.

    NOTE: This maximum value of 1000 × 10−6 is consistent with the use for design purposes of a median basic shrinkage strain value of 850 × 10−6.

    • A mean compressive strength at 7 days, assessed in accordance with Clause 5.7, of not less than the values of Grade designation for N20-9MPa, N25-12MPa, N32-16MPa, N40-20MPa & N50-25MPa.
    • A cement complying with (AS 3972 Portland or blended cement) alone or in combination with one or more supplementary cementitious materials.
    • No lightweight aggregate as defined in AS 2758.1 Aggregates and rock for engineering purposes Concrete aggregates.

    Basic parameters of normal-class concrete:

    The following basic parameters shall be specified by the customer:

    • A standard strength grade selected from 20MPa,25MPa,32MPa,40MPa, 50MPa,65MPa,80MPa or 100MPa and designated as one of N20, N25, N32, N40 or N50.
    • The slump at the point of acceptance, selected as one of 20 mm, 30 mm, 40 mm, 50 mm, 60 mm, 70 mm, 80 mm, 90 mm, 100 mm, 110 mm or 120 mm.

    NOTES:

    1. The customer should carefully consider that the specified slump of concrete suits the placement method.
    2. For residential slabs and footings, if the slump is not specified by the customer, the specified slump should be considered to be 100 mm.
    3. The maximum nominal size of aggregate, selected as one of 10 mm, 14 mm or 20 mm. Unless otherwise specified, the default value shall be taken as 20 mm.
    4. The intended method of placement, including relevant details of equipment.
    5. If project assessment is required to be carried out by the supplier (see Note).

    NOTE: If unspecified, it will be assumed that project assessment is not required.

    • If required, a level of air entrainment up to a maximum of 5.0%.

    (2) SPECIAL- CLASS CONCRETE:

    Concrete other than normal-class concrete shall be specified by the customer as specialclass and, if applicable, by strength-grade. The parameters and attributes that should be specified for special-class concrete should be as set out listed below with reference to Appendix B and Table B1 on AS1379.

    Special-class concrete commonly has the same basic parameters as normal-class concrete with some additions and(or) exceptions. Parameters or attributes that are different from, or additional to, those of normal-class concrete should be included in specification below. If the requirements of specification for any concrete are inconsistent with those for normal-class concrete then the requirements of specification take precedence for that concrete.

    Where any parameter other than strength grade requires the specification of a special-class concrete, or the proportions of the mix are specified, the concrete should be identified by an appropriate code agreed to between the supplier and customer that identifies that particular mix.

    Basic parameter for specification of special-class concrete:

    • It is recommended to select from standard strength grades of S20, S25, S32, S40, S50, S65, S80 and S100.
    • Where concrete is specified as special-class and a strength grade is applicable, the strength grade is designated by the prefix:

    S, for compressive strength grades;

    SF, for flexural strength grades; or

    ST, for indirect-tensile strength grades.

    Where concrete is special-class and any property other than strength grade is Specified as the principal criterion, or the proportions of the mix are specified, it is designated by an appropriate alphanumeric code, agreed between the supplier and the customer, to indicate the criterion.

    • Special-class concrete should be subject to project assessment.
    • Certain concrete exposure classifications may require special provisions for aggregate durability (AS 2758.1 Aggregate & Rock for engineering Purposes.)
    • Any departures from the parameters or composition, or both, of normal-class concrete and any other criteria or limitations shall be specified by the customer in consultation with the supplier.

    NOTE: A summary list of several such parameters, some or all of which may be specified for the production of special-class concrete for a project, is given in Appendix B on AS 1379.

    • Other requirements additional to these parameters may be specified.