British Columbia Carpenter Apprenticeship Program

Transcription

1 British Columbia Carpenter Apprenticeship Program Level 2 Line G Competency G-4 Build Suspended Slab Forms and Slab-on-grade Forms

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4 Competency G-4 Build Suspended Slab Forms and Slab-on-grade Forms Contents Objectives... 2 Learning Task 1: Describe Slab-on-grade Construction... 3 Learning Task 1: Self-Test Learning Task 2: Describe Suspended Slab Construction... 8 Learning Task 2: Self-Test Learning Task 3: Build Suspended Slabs Learning Task 3: Self-Test Learning Task 4: Describe Shoring Systems Learning Task 4: Self-Test Learning Task 5: Describe Suspended Slab Forming Products Learning Task 5: Self-Test Learning Task 6: Describe Slab Tables Learning Task 6: Self-Test Learning Task 7: Describe Methods of Relocating Slab Tables Learning Task 7: Self-Test Learning Task 8: Describe Re-Shoring for Live Load Learning Task 8: Self-Test Learning Task 9: Calculate the Volume of Concrete in Commercial Construction Learning Task 9: Self-Test Learning Task 10: Calculate the Volume of Concrete in a Silo Learning Task 10: Self-Test BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 1

5 Competency G-4 Build Suspended Slab Forms and Slab-on-grade Forms The construction of safe solid slabs is an important skill for the carpenter. Slabs may be placed on the ground or suspended on columns, as in high-rise buildings. Simple and complex concrete forming techniques are described in this Competency. Objectives When you have completed the Learning Tasks in this Competency, you will be able to: build slab-on-grade construction describe suspended slab construction build suspended slabs describe shoring systems describe suspended slab forming products describe types of slab tables describe methods of relocating slab tables describe relocating for live load calculate the volume of concrete in commercial construction calculate the volume of concrete in a silo Competencies Written: Build Suspended Slab Forms And Slab-on-grade Forms" You will be tested on your knowledge of terminology and methodology for slab form construction. You will be tested on your knowledge of concrete volumes. Practical: You will be required to construct a suspended slab with girders, beams, walls, columns and pilasters, screeds for a slab, using two methods and a bulkhead with a water-stop. 2 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

6 Competency G-4 Learning Task 1 Learning Task 1 Describe Slab-on-grade Construction Concrete slab-on-grade construction, also known as slab-on-ground, consists of an edge form, that defines the shape of the slab, a well compacted sub-grade, that supports the slab and suitable reinforcing to keep the concrete slab stable and resist cracking. Types of Slabs There are two basic types of slab-on-grade: monolithic and isolated. The monolithic slab is poured with the perimeter footing and any other slab thickenings at the same time. The monolithic slab-on-grade is used in residential and commercial construction. It can be very economical and quick to form and place. One setback is that the footing portion and the floor portion are under different building loads, which may cause stress cracks at these locations. This type of slab has limitations where it can be used and may not be permitted in some jurisdictions. finished grade slab thickening at edge stress point compacted subgrade Figure 1. Monolithic slab-on-grade The other type of slab-on-grade is isolated from the other components of the building and is much more accepted and practiced. The isolated slab-on-grade slab is poured after the footings and walls have been placed. There will be a barrier between the slab and footing and a barrier between the slab and wall. footing foundation wall 1/2 expansion joint consisting of caulking or preformed asphaltimpregnated material rebar floor slab vapour barrier gravel base course Figure 2. Isolated slab-on-grade BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 3

7 Learning Task 1 Competency G-4 Ground Prep Ground prep is crucial to a strong slab-on-grade. The slab is only as strong as the material it is placed upon. If the existing soil (sub-grade) is of suitable bearing, it only needs to be excavated down to grade and leveled before placing the slab. If the subgrade is not smooth enough at this point, a sub-base is laid down. This is usually a material that is more uniform in particle size. If the soil is not suitable, it must be removed and replaced in compacted lifts with proper granular fill specified in the drawings. Special attention must be given while compacting. It must be done in such a way to provide firm, even bearing. Compacting will be done with a plate, roller or ram-type compactor. Figure 3. Plate, Roller and Ram-type compactors Forming Typical forming for an exterior slab-on-grade can be as simple as 2" x 4" to 2" x 12" planks, depending on the thickness of the slab. Wood or metal stakes and braces may be all that is needed to plumb and straighten the forms. For thicker edge forms, using plywood forms supported by studs and plates may be necessary. Strong backs and braces are used to plumb and straighten. 4 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

8 Competency G-4 Learning Task 1 wheels ride on top flange each stake pocket has 2 keys 25 mm diameter steel stake closed stake pocket reinforcing steel plate at both ends of form Figure 4. Wood and metal edge form Strength and Durability Controlling water/cement ratio (W:C ratio) contributes to both strength and durability. Strength In commercial construction, the slab s thickness, strength and design mix are stated in the drawings by the engineer. The slab s finish, depending on the use of the building, may be specified by the architect. The strength of the slab is directly related to the base preparation, the compressive strength of the concrete mix and proper curing. The compressive strength of the concrete is expressed in MPA (megapascals). Other factors are re-inforcing steel and admixtures to the concrete, all of which will be specified by the engineer. In residential construction, most of the above information will be cited in the B.C.B.C. NOTE: See BCBC on the minimum thickness of slabs-on-ground and minimum compressive strengths of slabs within the dwelling and slabs for garages. A mat of re-inforcing steel is placed in the lower portion of the slab protecting it from tensile forces. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 5

9 Learning Task 1 Competency G-4 Durability The durability of a slab-on-grade is based on its compressive strength, admixtures added and treatments after placing. A slab on the main floor of a home may not need any added treatments. It may only receive light traffic during its lifetime. Many industrial buildings require treatments to increase their durability against continuous traffic and heavy loads. For interior slabs, admixtures may be added to the mix to protect from chemicals, oils and greases. Powdered or mechanical hardeners can be applied to the surface of the slab during finishing. This will harden the surface of the floor in a high traffic area. Exterior slabs have to endure very cold weather at times. Air entrainment is specified for all exterior concrete exposed to freeze-thaw cycles. Air entrainment adds microscopic air bubbles to the concrete. It allows it to expand and contract without great risk of damage. Other admixtures will guard against sulphates, salts and acids that may occur naturally or be man-made. Placing the Concrete After edge forms have been installed, they should be oiled along with any accompanying screeds to make them easier to clean. Most slab-on-grade concrete requires a polyethylene barrier to be placed on the sub-base. This will either keep moisture from migrating into the concrete from below, or keep soil gases from entering the building, or both. Wire mesh or re-inforcing steel will be placed on chairs over the poly. After checking the forms for elevation and strength, the slab is ready to pour. Pump trucks are used for large surfaces whenever possible, and cranes with buckets are used for high-rise construction. Tailgating from the concrete truck and wheel barrowing are options for smaller jobs or when pump trucks are not practical. Curbs and Gutters Sometimes curbs or gutters are incorporated with the slab. The methods of forming are to either hand set the forms using planks and plywood, or to have a curbforming machine slip-form them. Now complete Learning Task 1 Self-Test. 6 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

10 Competency G-4 Learning Task 1 Learning Task 1 Self-Test 1. On which kind of surface should a slab-on-grade be placed? 2. What factors affect the durability of slabs-on-grade? 3. What factors affect the strength of slabs-on-grade? 4. What should be done to walls or columns that pass through or beside slabs-on-grade? 5. Why is it important to compact the sub grade? 6. Why are screeds and forms oiled before concrete placement? 7. How long should slab forms remain in place after concrete placement? BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 7

11 Learning Task 2 Competency G-4 Learning Task 2 Describe Suspended Slab Construction Suspended slab construction is the forming of concrete floor systems in any type of multi-storey building. There are many types of floor configurations and several ways of forming them. Types of Suspended Slabs Flat Plate Slab The flat plate slab is the simplest form of a suspended slab. It basically stays the same thickness throughout, and when stripped, it has no distinguishing features in the soffit. It free spans from support to support and relies only on its own strength and re-bar to hold it up. This slab is thick in cross-section and won t span as far as other slabs. Flat Slab The flat slab is similar to the flat plate slab with a couple of added features. The columns that hold this slab up will either have a column capital or drop panel or both to help support the slab. Column capitals and drop panels add extra concrete at the shear areas above the columns. They act to reduce the span of the slab and will allow the columns to be spaced further apart. See Figure 1. Figure 1. Flat slab with column capital and drop panel 8 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

12 Competency G-4 Learning Task 2 One-Way and Two-Way Slabs The term one-way slab basically refers to a slab where there will be some sort of support from column to column, such as a beam where the load-carrying steel in the slab runs perpendicular to the beams, in other words, spans from beam to beam. In the two-way slab, load carrying steel runs in both directions. One-Way Ribbed Slab and Two-Way Ribbed Slab The ribbed slabs come in two varieties. The one-way ribbed slab uses a series of metal pans secured end to end to the slab deck to create a joist pattern in the slab. The ribs or joists are poured monolithic with the slab and allows the slab concrete to be quite thin. This helps to reduce some of the dead loads of the building. When the soffit of the slab is stripped away, the joist shape can be seen running from beam to beam. See Figure 2. A similar process takes place with the two-way ribbed slab except dome pans are used and are installed in a grid pattern. When this slab is stripped, a waffle pattern is seen from below. See Figure 3. nail-down pan tapered end form end cap nail-down pan shore flanges nailed to soffit soffit stringer long pan spans 2 soffits long pan end cap stringer flanges clamp together soffit shore Figure 2. One-way ribbed slab BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 9

13 Learning Task 2 Competency G-4 Figure 3. Two-way ribbed slab. Slab Components Slab Band (Ribbon Beam) Slab bands do a similar job as the drop panels, except they span continuously from column to column, usually in one direction only. They are also rectangular in shape. See Figure 4. Figure 4. Columns and slab band 10 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

14 Competency G-4 Learning Task 2 Girders and Beams As spans get longer and slabs become heavier, more support is necessary to hold up the slab. Now girders (large beams) span from column to column and intermediate beams will span between girders. The orientation of the beams will now dictate the direction of the re-bar in the slab. A beam at the perimeter of a suspended slab is called a spandrel beam. See Figure 5 and 6. beam girder girder beam beam columns Figure 5. Girder and beam spandrel beam intermediate beam Figure 6. Spandrel beam Now complete Learning Task 2 Self-Test. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 11

15 Learning Task 2 Competency G-4 Learning Task 2 Self-Test 1. What is the difference between a flat plate slab and a flat slab? 2. What advantage do column capitals give? 3. What direction does load-carrying steel span? 4. Dome pans are used in which type of slab? 5. What is a spandrel beam? 6. How does a two-way joist system differ from a single joist system? 12 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

16 Competency G-4 Learning Task 3 Learning Task 3 Build Suspended Slabs Before fly tables, most suspended slab forming was done by hand with a bit of help from the crane. Early on, timbers and planks were used for all the forms and falsework. Eventually plywood and metal scaffolding came into use. More recently, strong, lightweight shoring systems are being used. Wood Formwork Beams and Girders After the walls and columns have been stripped, the next step is to prepare for the suspended slab formwork. The whole support system is known as falsework or shoring. From the drawings, you will find the location and dimensions for all the features of the slab and beams. The first thought should be to start with the lowest components: the girders and beams. Start by forming those up. Then building on top of that work, form the components for the soffit of the slab. Beam Bottom Basic beam support starts with either a T shore or double post shore (bent) placed on the floor. Stiffeners or joists will span between shore heads and will support the beam bottom. Narrow beams might only require a T shore to hold them up, while larger beams and girders will need double post shores (see Figure 1). Before continuing, all the shoring for the beam bottoms should be set to elevation. This is done by initially leaving all the shore posts 1" to 1 " low, so that wedging them up level and elevating can be done at this time. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 13

17 Learning Task 3 Competency G-4 head plywood cleat head post plywood cleat cross braces post T-head shore double-post shore Figure 1. T and double post shore Once the shoring is in place and has been braced, chalk lines are snapped on top of the shore heads to show the location of all the beams. NOTE: Always chalk the lines of the concrete, NOT the forms or plates! For narrow beams, solid planks can be used as the beam bottom provided the beam is not architectural. The beam bottom is cut exactly the width of the beam and can be placed directly on the shore heads. Wider beam bottoms will be made from plywood attached to stiffeners that span the shore heads. See Figure 2. 2 plank bottom plywood bottom 2 x 4 stiffeners shore head shore head shore braces shore braces Figure 2. Beam bottoms 14 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

18 Competency G-4 Learning Task 3 Beam Sides Once the beam bottoms have been attached and leveled, the beam sides can be assembled and installed. Plywood can be used for sides of shallow beams which are supported at the bottom by a kicker and at the top by the slab soffit joists. When exact width beam bottoms are used, the beam sides rest directly on the shore head. See Figure 3 and Figure 4. beam or girder side joist ledger chamfer strip kicker secures beam side stiffener blocking to stiffen sides and support ledger T-head shore Figure 3. Plywood beam side Deeper beams will require stronger beam sides so adding support to the plywood is necessary. Units built on the floor, using a bottom plate and studs to attach the plywood, are then passed up to the working platform to be attached to the beam bottom. As shown in Figure 4, after the beam sides have been installed, they must be straightened and braced before the slab forming goes in. Shore heads and joists usually run out far enough past the beam so the bottom of the braces have something to be nailed to. beam or girder sides chamfer strip stiffener bottom T-head shore joist ledger stud kicker Figure 4. Plywood and stud beam side BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 15

19 Learning Task 3 Competency G-4 Spandrel Beam Spandrel beams are beams located at the outside edge of a slab. They act to strengthen the slab at the perimeter where there is no wall support. Since spandrel beams lie at the edge of each floor, the shoring is extended out to provide a guarded platform and a space to brace the beam s edge form. Because this beam is poured monolithic with the slab, it is difficult to strengthen and straighten the top edge of the slab. Special ties called spandrel or deck ties are used with walers to accommodate this problem. The spandrel tie is pointed and bent at a 90-degree angle at one end and is driven into the decking material. See Figure 7. Column Capitals and Drop Panels Column capitals, because of their different shapes, are very time consuming to form with wood, so it is rarely done. Instead, pre-fabricated metal and fiberglass molds are ordered. The extra cost is countered by the fact they can be used over and over again. Capital forms are designed to attach to the column s formwork at the top. Extra shores and bracing will be necessary for large capitals. An alternate to column capitals are drop panels (see Figure 5). Since they are basically square in shape and flat on the bottom, they are much easier to form out of plywood. Using a similar system as for beams, the soffit plywood rests on joists and stingers and finally on a shore head. drop panel form sides slab sheathing column form Figure 5. Drop panel The soffit plywood is made larger than the size of the drop panel to allow for the panel s edge form and bracing. 16 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

20 Competency G-4 Learning Task 3 Soffit Forming The underside of the slab is known as the soffit and the form material, usually plywood, is known as decking. The decking material used to form the soffit will be supported by a series of joists spaced appropriately to hold the weight of the concrete. In most cases, the shoring for the beams will be designed to also hold the ends of the joists that hold up the slab. See Figure 6. guardrail stud wedge beam sides spandrel tie top of concrete floor slab slab sheathing double waler snap tie ledger joist cleat shore walkway kicker walkway brace beam bottom stiffener shore Figure 6. Slab soffit and spandrel beam forming Where the joists meet the beam sides, they will run past the studs and get nailed to them. They also rest on a ledger fastened to the studs. For greater volumes of concrete, blocking is added to help support the ledger. They are spaced so as to transfer weight directly to the shoring system. When spans between beams become too great, a row of shoring will be added at mid-span to help support loads in the middle. Stripping suspended slab forms Stripping slab and beam formwork can be dangerous because all the stripping is done overhead and there are a lot of nails sticking out. Tripping is also a hazard. Therefore the primary concern when stripping is safety. A system should be developed for each situation where the stripping can be done safely and efficiently. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 17

21 Learning Task 3 Competency G-4 Another concern is protecting the new concrete from damage and structural failure during the stripping process. The process begins with making sure the concrete has reached minimum strength for stripping, specified by the engineer. The sequence for stripping may vary, depending on how everything was assembled. However, it is common to begin stripping the beam sides first and leaving the beam bottom undisturbed. This gets rid of most of the material around the perimeter of the slab soffit and frees the soffit joists from the studs of the beam sides. After beam sides and tie ends have been removed, small sections of slab shoring can be dismantled and the first full-size panels can be lowered, making sure large sections don t collapse by mistake. The engineer for the falsework will provide a drawing for the locations of all the re-shoring. Re-shoring is the method of temporarily supporting suspended concrete until it reaches its full strength and construction live loads have been removed. Install re-shore posts one at a time as soon as the soffit becomes exposed. As the panels and infill pieces are removed, the nails must be pulled and the pieces stacked neatly so they can be banded and hoisted up to the next level. Some of the shoring for the beams does not have to be removed. They will stay in place as the re-shores, and only the intermediate ones, will be removed. When built in this way, the only formwork not removed is the beam bottom. It remains in place with the re-shores. Now complete Learning Task 3 Self-Test. 18 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

22 Competency G-4 Learning Task 3 Learning Task 3 Self-Test 1. In what sequence does form stripping usually take place? 2. What is the first step in setting a beam form? 3. What must be done to the forms for deep girders? a. They must be oiled before installation. b. Studs are added to the sides. c. Kickers are exchanged for form ties. d. Intersecting beam pockets must be deepened. 4. Why does a spandrel beam require a wider shore head than other beams or girders of the same width? 5. Where do beam sides usually sit? 6. How are wooden T-shores set to elevation? 7. How are forms for drop panels supported? BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 19

23 Learning Task 4 Competency G-4 Learning Task 4 Describe Shoring Systems The integrity of shoring systems used in suspended concrete construction is vitally important to the safety of the workers and to the quality of the finished concrete structure. The shoring system is usually referred to as the falsework. The falsework supports the formwork for the concrete structure; it also supports loads from construction materials stored on the formwork. Falsework The falsework is a structure itself, supporting vertical and horizontal loads. The loading varies and is often not uniform; for example, if a lift of reinforcing steel is allowed to rest on the slab deck, it will impart a huge load and the falsework will have to accommodate it. Similarly, if the contents of a concrete bucket are accidentally dumped in one location, a large point load is created. The falsework requires a foundation, vertical and horizontal structural components, horizontal bracing in both directions and diagonal bracing. In the past, falsework was primarily built from wood, using 4x4" uprights, 4x4" or 4x6" horizontal members and 1x6" horizontal and diagonal bracing. Wood is a very versatile material, allowing almost any type of formwork to be supported. Today most of the falsework is metal, either steel or aluminum. Metal is significantly stronger and because much of the shoring is pre-fabricated, the metal shoring is extremely quick to set up. Falsework or Formwork? The division between the falsework and the formwork is not always clear. Slab formwork in multi-story buildings is usually formed with a special system called a slab table. The tables are usually aluminum shoring with the slab forms built on top. The whole system is crane lifted from one floor to the next as a unit consisting of the falsework and formwork. By definition, the falsework supports the formwork and the formwork moulds the fresh concrete. Whether the aluma-joists that support the plywood are part of the formwork or part of the falsework is not important; what is important is that this configuration must work together to allow safe and accurate construction. 20 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

24 Competency G-4 Learning Task 4 Single Vertical Shores The simplest shore is a single 4 4 that is cut to fit and used as a vertical support. Although most of the falsework is now built with metal shoring, these simple wooden shores are still used. They are particularly useful when elevations are varied and as re-shores to support the concrete as it cures. The 4x4 wooden shore should be cut about 1 to 1 inches short of its intended supporting length, it is cut short to allow for a pair of wedges to be used to adjust the shore length. Once the wedges are put into place and adjusted, they are nailed together and nailed to the shore itself to prevent slipping. By cutting the wooden re-shore short, there is no chance of stressing the green concrete by pounding a tight re-shore into place. Slenderness Ratio Depending upon the amount of load, each single shore may require one or two horizontal braces along their height. These braces will keep the shore from bowing out of plumb as the load is applied. To see this bowing action, take a long thin piece of wood and place one end on the floor and push down on the top. If the wood is thin and long, it will easily bow at the center. Now have someone support the stick at its mid point while you push down again. It will take significantly more force to bow the stick and the bowing will create an S shape in the stick. Try it with two points of support. If overloaded, the shores in falsework will do the same thing. The slenderness ratio of vertical shores for formwork should not exceed 1:50 where the 1 is proportional to the minimum cross sectional dimension and the 50 is the unsupported height of the shore. Example: The task is to shore up a vertical height of 18 feet and the shores are rough cut 4x4. Convert the feet into inches and divide by 50 (slenderness ratio). If the answer is more than 4", then the posts need to be laterally braced at their mid-point = 216" 216 / 50 = 4.32" In this case, 4.32" is greater than the smallest dimension of the post, 4", so the posts must be laterally supported. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 21

25 Learning Task 4 Competency G-4 SHORE POST ROTATE TO RAISE AND LOWER SHORE POST Figure 1. Adjustable shore jack Adjustable Shores There are many types of adjustable single shores. Simple wooden shores can be fitted with post shore jacks that allow them to be adjusted by turning the screw jack at the bottom (Figure 1). FORCE Ellis Clamps Two wooden shores can be joined and made adjustable with a pair of Ellis clamps (Figure 2). An Ellis clamp is a patented metal clamp designed to transfer the load from one 4 4 wooden shore to another 4 4 wooden shore. As the force is exerted onto the top of the shore the right hand side of the clamp is forced down. As the right hand side tries to move down the clamp tightens up. This type of clamp is easily adjusted by using an Ellis Shore Jack. The jack is used to move the upper shore up and to take off the pressure so that the shores can be released. Once the correct elevation for the upper shore is set the clamps are fastened in place using duplex nails. Figure 2. Ellis shore clamps Steel Shores Single telescoping steel shores are often used as part of the falsework. The shore head is used to accommodate a stringer and each shore would be braced at its mid point in both directions to resist bowing (Figure 3). 22 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

26 Competency G-4 Learning Task 4 If the steel shores were used as re-shores, the shore head would be a flat plate, similar to the bottom base plate. If horizontal bracing is required it will have to be adequately fastened to the shore at mid height. Some shores have a plate welded to the shore to accommodate the bracing. Scaffold Shoring Most of the falsework built today uses heavyduty scaffold shoring. The strength of metal shoring and the speed at which it can be assembled makes it the most economical system for falsework. Scaffold shoring is a patented system requiring all of the parts for the system to come from the same manufacturer. The engineer must approve the design of the falsework, including the scaffold shoring. The same engineer will inspect the installation and must certify that it is safe prior to its use. Shore Head Rotate to raise and lower shore head Figure 3. Adjustable steel shore Although similar to scaffolding used for accessing a worksite, scaffold shoring is designed to support heavier loads. For example, standard metal scaffolding supports approximately 50 pounds per square foot of scaffold surface, or 250 pounds per scaffold leg. In contrast, scaffold type shoring supports concrete formwork that can have loadings of 10,000 pounds or more per scaffold leg. Figure 4 shows two tiers of scaffold type metal shoring. The shoring usually sits on a mudsill and is used to support stringers that support the formwork. STRINGER SCREW JACK MUDSILL Figure 4. Metal scaffold shoring BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 23

27 Learning Task 4 Competency G-4 Flying Forms Larger construction projects use prefabricated formwork units that are lifted with a crane. These units are usually referred to as flying forms because they are cranelifted, or flown, into place. Two types are very common: gang forms used for forming walls, and slab tables used for forming suspended slabs. Both types are usually made from a combination of wood and metal except for the forms from EFCO, which are completely steel. The sheathing is usually plywood and often the supports for the sheathing are also wood, but the main structural components are metal. A very common material that is being used extensively is the Aluma-beam. Alumabeam is a manufacturer s brand name but has become the standard name for an aluminum extrusion that is in the shape of an I. The I-beam includes a wooden 2 2 or composite material in the top and a groove in the bottom that will accept the head of a bolt to fasten the beam to the scaffold type shoring (Figure 5). SLOT FOR HEX HEAD BOLT Figure 5. Aluma beam 24 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

28 Competency G-4 Learning Task 4 Slab Table The implementation of the slab table has revolutionized suspended slab construction. Modern tables consist of an aluminum truss shoring, stringers and/or joists and screw jacks for leveling and a plywood deck. To attach the slings, access holes are cut in the deck. SLING ATTACHMENT ACCESS POINTS Figure 6. Prefabricated slab table Now complete Learning Task 4 Self-Test. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 25

29 Learning Task 4 Competency G-4 Learning Task 4 Self-Test 1. What is the slenderness ratio? 2. Describe three ways single shores are made adjustable. 3. Why are re-shores not fitted tightly to the concrete that they are supporting? 4. Describe a slab table. 5. What are flying forms? 6. How are aluma-beams attached to the falsework? 7. What is a mudsill used for? 8. Using a slenderness ratio of 1:50, what is the maximum unsupported height of a rough 4 4 wooden shore? 26 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

30 Competency G-4 Learning Task 5 Learning Task 5 Describe Suspended Slab Forming Products Pans, Domes and Voids Pans and domes are used to create concrete one-way and two-way joist systems in suspended concrete slabs. The pans are used to create a void in the underside of the slab, thus creating the joists. In some designs, the engineer specifies the use of void forms that are placed in the middle of the slab; in this case, the slab is smooth on both the bottom and top with open spaces or voids in the middle. Voids are used to reduce the overall weight of the slab itself. The slab will be made more efficient, in cost and strength, by reducing the amount of concrete used to build it. Forming voids in a concrete slab is not an easy job. If the formwork for the voids is too complicated, the cost savings created by using less concrete will be lost. Using prefabricated pan, dome, and void forming systems will help to reduce the formwork costs. Another reason for forming voids into the underside of a suspended concrete slab is for appearance. The shapes formed by the exposed voids can be very attractive. Concrete left exposed to view after the building is completed is referred to as architectural concrete. Architectural concrete work requires significantly more attention to detail as the off the form concrete surface is the finished surface and formwork defects, honeycomb or segregation will all show up. Slab Types There are two basic types of suspended concrete slabs that require the use of pans and domes in the formwork: the ribbed slab (one-way) and the waffle slab (twoway). Ribbed Concrete Slab The ribbed slab shown in Figure 1 is also referred to as a one-way joist slab. The joists are supporting the slab and spanning from one point of support to another. Long pans are used for this slab. The dimensions and the rebar sizes shown in Figure 2 are approximately what you will see on the structural drawings but the engineer will design the slab thickness, joist sizes and reinforcing for each job individually. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 27

31 Learning Task 5 Competency G-4 RIBBED CONCRETE SLAB PANS USED TO FORM THE JOISTS CONCRETE JOISTS Figure 1. Ribbed slab mm mm Figure 2. Cross section of concrete joists Like wooden floor joists, a beam or a wall must support concrete joists. Where the joists end and the beam or wall begins, the pan forms will need to be closed off. Special end pan forms are used to make the connection to the beam or wall. SQUARE END PANS TAPERED END PANS Figure 3. End pans for forming joists Figure 3 shows two types of end pans. The tapered end pan is used to form joists with a fish tail or widening at the point of support. The wider end to the joist is used to make a less abrupt transfer of the load carried by the joists to the beam that supports it. 28 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

32 Competency G-4 Learning Task 5 Waffle Concrete Slab The waffle slab is also referred to as a two-way ribbed slab, a coffered slab, or a twoway joist slab. The sloped sides of the joists form a tapered rectangular void in the underside of the slab giving it a desirable appearance of a breakfast waffle. These voids are usually formed using a prefabricated dome. The waffle slab produces the greatest reduction of weight and is the most efficient material-wise, but also the most complicated to build. See Figure 4. CONCRETE JOISTS Figure 4. The underside of a waffle slab Pan and Dome Types Pans are used for forming one-way concrete joist systems, and domes are used for forming two-way joist systems. In both cases, there is a choice of a having a nailing flange at the bottom or having a straight leg. Figure 5 shows the difference. FLANGED STRAIGHT LEG Figure 5. Joist forming pans BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 29

33 Learning Task 5 Competency G-4 The flanged pans are easy to install but when stripping, the shoring for the concrete joist must be removed to allow the pan to be removed. If the pans must be reused soon after the placing of the concrete then straight leg pans would be more appropriate because the pans can be removed without disturbing the support of the joist. Fixed Pans Fixed pans are always flanged. The depth of the joist is set by the height of the leg of the pan form. The fixed pans may be laid on stringers or directly onto a plywood slab deck. In either case, they will have to stay in place until the strength of the concrete is sufficient to allow the supports to be removed. Adjustable Pans Adjustable pans may be flanged or straight legged. Figure 6 shows an example of the two styles. The examples shown are used to demonstrate the differences between flanged and straight-legged. Actual formwork design must be specific for every workplace situation. FLANGED STRAIGHT LEG Figure 6. Joist forming with adjustable pans When adjusting the depth of the joist using straight-legged pans, the depth can be set by securing the pans in place at a specific location on either side of the supporting stringer. This method also allows for the pan removal without disturbing the support of the newly placed concrete. 30 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

34 Competency G-4 Learning Task 5 Domes Dome pans are used to form the voids for a two-way joist, or waffle slab. These pans are usually flanged and are placed on stringers or a flat plywood slab deck with the flanges butted tight. These flanged pans would have to be left in place until the concrete has gained enough strength to support itself during stripping. Some dome pans are fitted with an air nipple to allow compressed air to be injected into the space between the pan and the concrete to facilitate the stripping of the pans (Figure 7). AIR NIPPLE SLOPED SIDES Figure 7. Dome pan for forming a waffle slab Materials Pan forms can be made from a variety of materials. The choice of material will depend upon the quality of the finished concrete surface, the number of re-uses and the forming system used. Steel Steel is the most common material for pans used to form ribbed slabs. Steel is very durable and allows many re-uses. The steel pans are interlocking and join by overlapping; this allows them to be adjustable to fit various lengths. The overlapping at each joint does show up in the finished concrete surface. For this reason, steel pans are not used to form architectural concrete. Fibreglass Architectural concrete often requires unique and decorative waffle ceilings, using irregular and sometimes curved joists. These unusual pan and dome shapes are constructed using fibreglass-reinforced plastic (FRP). When building FRP pans, a positive model of the finished pan or dome is formed and then a mould is made from it. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 31

35 Learning Task 5 Competency G-4 Void Forms Void forms are used in slabs and beams to reduce the dead load or weight of the building. The void forms are usually left in place. The materials that are used for forming these voids include corrugated steel pipe, styrofoam and cardboard. There are inflatable rubber void forms that are deflated after the concrete has been placed but in most cases the forms are left in place. Void forms must be tied down to resist the uplift caused by the fresh concrete. These tie downs must be engineered to resist the very heavy uplifting forces of the concrete. TIE DOWNS Figure 8. Void form tie downs As the concrete is being placed around void forms, it must be vibrated well to ensure that the concrete flows under the void forms and that there are no pockets of air trapped under the forms. Stay-in-Place Forms Suspended concrete slab formwork is often built using structural members that are left as structural components in the finished slab. Hollow core pre-cast and pre-stressed concrete slabs span from beam to beam with a non-structural slab placed over them. The pre-cast slab panels are pre-stressed and positively cambered, giving the finished suspended slab excellent spanning capability. Structural open web steel joists (OWSJ) are similarly spanned across openings from beam to beam or beam to wall. Once the joists are in place, plywood panels are fitted in-between the joists leaving the top chord of the joist in the concrete slab. 32 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

36 Competency G-4 Learning Task 5 CORED CONCRETE SLAB OPEN WEB STEEL JOISTS (OWSJ) Figure 9. Stay-in-place formwork Proprietary Slab Forms There are many companies that have designed and patented their own systems for different types of formwork, including suspended falsework. Their goals are similar: to supply a durable, cost-efficient system that is safe, easy to install and a quick turn around cycle. The following describes just two systems designed for large area slabs. Peri The Peri 80 Sky table system is capable of forming very large areas of suspended slab with only one crane pick. Some of the table sizes being used can measure more than 4 metres high by 5 metres wide and 20 metres long. Peri uses several systems and many different components. The 80 Skytable is made up of metal shores, truss-type stringers and metal joists under a plywood deck. See Figure 10. Figure 10. Peri SKYTABLEs BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 33

37 Learning Task 5 Competency G-4 Doka Another falsework system is Doka. Their system is safe, fast, easy to adapt and boasts short repositioning times. The components are steel shore posts, swivelhead, (shown in Figure 11), steel primary beam and wood I joists and deck. One advantage using Doka is that the system comes with its own repositioning equipment, so most of the forming can be done without the use of the crane. Figure 11. Dokamatic swivel-head 40 Now complete Learning Task 5 Self-Test. 34 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

38 Competency G-4 Learning Task 5 Learning Task 5 Self-Test 1. What are void forms used for? 2. What type of slab formwork uses dome form pans? 3. How is a ribbed slab formed? 4. What is an OWSJ? 5. When would FRP be used in constructing forms? 6. Sketch how to adjust the depth of concrete joists when forming a ribbed slab. 7. Why would a waffle slab be used? 8. What would be a typical spacing for concrete joists? BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 35

39 Learning Task 6 Competency G-4 Learning Task 6 Describe Slab Tables Flying forms are defined as forms that are crane lifted into position. The slab form tables for high-rise construction are a very common type of flying form. The form tables must be strong enough to withstand not only the loads imposed by the fresh concrete, but also by the stress of being moved from floor to floor. For taller buildings, the individual tables will be used 50 to 150 times. Slab Table Construction A registered professional engineer does the design of the construction of slab tables. The construction of each table should follow the engineer s design. Prior to lifting any slab tables they must be inspected by the engineer. Typically, slab tables are made with aluminum components and a plywood deck (Figure 1). The main structural component is the aluminum trusses, there may be two, three or four trusses depending upon the width of the table. REMOVABLE COVERS OVER THE SLING ATTACHMENT LOCATIONS ALUMINUM TRUSS FRAME ADJUSTABLE SUPPORT LEGS Figure 1. A typical slab table Aluma beams are bolted at right angles to the top chord of the truss frame. The truss frame includes screw jacks to allow adjustment for height. The screw jacks fold up and out of the way during lifting of the tables. 36 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

40 Competency G-4 Learning Task 6 During the stripping of the form, the bottom chord of the truss frame is set on rollers to allow it to be moved out of the building. Adjustable Support Legs Slab tables use adjustable leg supports to allow the table to be positioned to the exact elevation required. Separate jacks are used to lower the slab table during stripping. The adjustable legs are only used to fine adjust the table to elevation. Joists and Decking The joists support the plywood and are screwed, clamped or bolted to the truss frame. The joists can be wood or metal but in either case the base of the joist should be wide enough to make the joist inherently stable. Aluminum I joists, called aluma joists, are used with the aluminum truss frames. The I joists have wide flanges for stability. There is a bolting slot in the bottom and a wooden nailing strip in the top (Figure 2). The decking is 19 mm form plywood, nailed or screwed to the joists. Resin impregnated form plywood is used to withstand the many re-uses required in highrise construction. Figure 2. Deck supports BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 37

41 Learning Task 6 Competency G-4 lnfill Panels and Backing Rod Where columns, stairwells and elevator shafts intrude into the floor plan, the slab tables leave gaps between them and the walls. Infill panels are used to fill these gaps. The slab tables are set and leveled and then plywood filler panels are used to build the missing slab form. If the floor layout is typical from floor to floor, the infill panels are saved during the stripping process and used again on the next floor. It is not good practice to fit the infill panels too tightly. Fill the gaps with foam backing rod to create a tight joint. This prevents leakage as shown in Figure 3. BACKER ROD DECK SURFACE INFILL PANEL TRUSS FRAME Figure 3. Infill panels and backer rod Deck Covers The crane s slings attach directly to the truss frame. Plywood covers are made to fit the holes in the deck where the slings pass through. The deck covers are attached to the deck when the slab table is flown to the next floor. Now complete Learning Task 6 Self-Test. 38 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

42 Competency G-4 Learning Task 6 Learning Task 6 Self-Test 1. How are slab tables constructed? 2. How are the adjustable legs used? 3. How are odd spaces formed? 4. Who is responsible for the design of the formwork? BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 39

43 Learning Task 7 Competency G-4 Learning Task 7 Describe Methods of Relocating Slab Tables Moving slab tables safely requires a very well rehearsed series of steps. The entire process is laid out in the erection drawings. The procedures must be followed carefully. Any changes to the system will require the approval of the engineer prior to being implemented. Rollers The rollers are used to allow the slab table to be rolled out of the building. The rollers rest on the floor slab under the frame and use steel rollers. Six rollers are needed to safely move the slab table. Floor Jacks The floor jacks are used to lower the slab table onto the rollers. The floor jacks are part of the formwork system and must be identified in the erection drawings. Usually the floor jacks are specifically designed to work with the type of truss frames being used. A minimum of four floor jacks is needed to lower the formwork safely. Each form has four to six attachment points for crane cables. Location of these points is critical for a balanced lift, and they must be strong enough to keep the frame undamaged. Cable Jacks Cable jacks are used to move the slab tables horizontally out of and into the building. The jacks may be omitted if the tables can be moved by hand. Cable jacks are basically a come-along that has been adapted to be used for moving slab tables. Two cable jacks are needed to move larger slab tables. 40 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

44 Competency G-4 Learning Task 7 Stripping and Moving the Slab Tables The following is a list of the steps used in moving slab tables: 1. Watch the wind speed carefully. Do not lift formwork if gusts exceed 55km/h (35 mph). If the wind is gusting to over 100km/h (60 mph) evacuate the job site. 2. All workers near the unguarded edge of the building must be wearing appropriate fall protection. 3. Screw the support jacks down just enough to allow the weight of the deck forms to cause them to pull away from the overhead concrete. Then screw them down far enough for the truss frame to rest on the floor jacks. The floor jacks transfer the load to the rollers as shown in Figures 1 and Position the rollers towards the outside of the building from the support jacks. JACKS LOWERED ROLLERS RE-SHORING UNDER POINT LOADS Figure 1. Jacks lowered to allow form to come loose from the slab ROLLERS Figure 2. Slab table lowered onto the rollers and the support jacks stowed BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 41

45 Learning Task 7 Competency G-4 5. Use cable jacks to move the slab table so that it is cantilevered past the edge of the slab just enough to allow the first and second pairs of slings to be attached to the lifting points. ONLY TWO PAIRS OF SLINGS ATTACHED Figure 3. First two pairs of slings attached 6. Attach the first two pairs of slings to the lifting points. Take care to have the slings arranged without twists or overlaps as shown in Figure 3. ALL THREE PAIRS OF SLINGS SUPPORTING THE SLAB TABLE Figure 4. All three pairs of slings attached 7. Slide the slab table out of the building while the crane picks up the slack on the slings. Only slide the table out far enough to attach the third pair of slings to the lifting points as shown in Figure BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

46 Competency G-4 Learning Task 7 8. Slide the slab table completely out of the building while the crane takes up the slack on the slings. Do not allow the table to leave the support of the building until the crane has tension on the last two slings. 9. Move the slab table to the same grid line location on the next floor up. The engineer must approve any changes to the lifting procedure. Install the re-shoring for the bay vacated by the slab table as soon as the table is moved out. Efficiency The following will make the work more efficient: 1. Use chalk marks on the slab table deck to show the locations of the re-shoring. The chalk marks will show up on the underside of the concrete slab. 2. Mark the support jacks with the approximate height needed for the correct elevation of the deck. 3. Mark the sling ends with the appropriate lifting point. 4. Colour code the first, second and third pair of slings. Now complete Learning Task 7 Self-Test. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 43