Cable Stayed Bridge

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The cable-stayed bridge is related to the cantilever bridge. The cables are in tension, and the deck is in compression. The spans can be constructed as cantilevers until they are joined at the centre. A big difference between cantilever bridges and cable-stayed bridges is that the former usually have a suspended span, and the latter do not.

A cable stayed-bridge lacks the great rigidity of a trussed cantilever, and the continuous beam compensates for this to some extent. Indeed, while a long cable-stayed span is under construction, there may be great concern about possible oscillations, until the cantilevers are joined. For the Pont de Normandie, there was even thought of using active correctors if things threatened to get out of hand. In fact, the construction went smoothly.

The cables are of high tensile steel. In a few examples these are encased in concrete. Towers are often made in concrete, though steel is also used.

Advantages of cable-stayed bridges

The two halves may be cantilevered out from each side. There is no need for anchorages to sustain strong horizontal forces, because the spans are self-anchoring. They can be cheaper than suspension bridges for a given span. Many asymmetrical designs are possible.

Disadvantages of cable-stayed bridges

In the longer sizes, the cantilevered halves are very susceptible to wind induced oscillation during construction. The cables require careful treatment to protect them from corrosion.

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Suspension Bridge

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A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 19th century. Simple suspension bridges, which lack vertical suspenders, have a long history in many mountainous parts of the world.

This type of bridge has cables suspended between towers, plus vertical suspender cables that carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc upward for additional clearance. Like other suspension bridge types, this type often is constructed without falsework.

The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground. The roadway is supported by vertical suspender cables or rods, called hangers. In some circumstances, the towers may sit on a bluff or canyon edge where the road may proceed directly to the main span, otherwise the bridge will usually have two smaller spans, running between either pair of pillars and the highway, which may be supported by suspender cables or may use a truss bridge to make this connection. In the latter case there will be very little arc in the outboard main cables.

The main forces in a suspension bridge of any type are tension in the cables and compression in the pillars. Since almost all the force on the pillars is vertically downwards and they are also stabilized by the main cables, the pillars can be made quite slender.

In a suspended deck bridge, cables suspended via towers hold up the road deck. The weight is transferred by the cables to the towers, which in turn transfer the weight to the ground.

Comparison of a catenary and a parabola with the same span and sag

The catenary represents the profile of a simple suspension bridge, or the cable of a suspended-deck suspension bridge on which its deck and hangers have negligible mass compared to its cable. The parabola represents the profile of the cable of a suspended-deck suspension bridge on which its cable and hangers have negligible mass compared to its deck. The profile of the cable of a real suspension bridge with the same span and sag lies between the two curves.

Assuming a negligible weight as compared to the weight of the deck and vehicles being supported, the main cables of a suspension bridge will form a parabola (very similar to a catenary, the form the unloaded cables take before the deck is added). One can see the shape from the constant increase of the gradient of the cable with linear (deck) distance, this increase in gradient at each connection with the deck providing a net upward support force. Combined with the relatively simple constraints placed upon the actual deck, this makes the suspension bridge much simpler to design and analyze than a cable-stayed bridge, where the deck is in compression.

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Basic Bridge Terms

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An important first step in understanding the principles and processes of bridge construction is learning basic bridge terminology. Although bridges vary widely in material and design, there are many components that are common to all bridges. In general, these components may be classified either as parts of a bridge superstructure or as parts of a bridge substructure.

SUPERSTRUCTURE

The superstructure consists of the components that actually span the obstacle the bridge is intended to cross and includes the following:

• Bridge deck
• Structural members
• Parapets (bridge railings), handrails, sidewalk, lighting and some drainage features

The deck is the roadway portion of a bridge, including shoulders. Most bridge decks are constructed as reinforced concrete slabs, but timber decks are occasionally used in rural areas and open-grid steel decks are used in some movable bridge designs (bascule bridge). As polymers and fiber technologies improve, Fiber Reinforced Polymer (FRP) decks may be used. Bridge decks are required to conform to the grade of the approach roadway so that there is no bump or dip as a vehicle crosses onto or off of the bridge. The most common causes of premature deck failure are:

• Insufficient concrete strength from an improper mix design, too much water, improper amounts of air entraining admixtures, segregation, or improper curing
• Improper concrete placement, such as failure to consolidate the mix as the concrete is placed, pouring the concrete so slowly that the concrete begins the initial set, or not maintaining a placement rate.
• Insufficient concrete cover due to improper screed settings or incorrect installation of the deck forms and/or reinforcement

A bridge deck is usually supported by structural members. The most common types are:

• Steel I-beams and girders
• Precast, prestressed, reinforced concrete bulb T beams
• Precast, prestressed, reinforced concrete I beams
• Precast, prestressed, concrete box beams
• Reinforced concrete slabs

Secondary members called diaphragms are used as cross-braces between the main structural members and are also part of the superstructure. Parapets (bridge railings), handrails, sidewalks, lighting, and drainage features have little to do with the structural strength of a bridge, but are important aesthetic and safety items. The materials and workmanship that go into the construction of these features require the same inspection effort as any other phase of the work.

SUBSTRUCTURE

The substructure consists of all of the parts that support the superstructure. The main components are abutments or end-bents, piers or interior bents, footings, and piling. Abutments support the extreme ends of the bridge and confine the approach embankment, allowing the embankment to be built up to grade with the planned bridge deck.

When a bridge is too long to be supported by abutments alone, piers or interior bents are built to provide intermediate support. Although the terms may be used interchangeably, a pier generally is built as a solid wall, while bents are usually built with columns.

The top part of abutments, piers, and bents is called the cap. The structural members rest on raised, pedestal-like areas on top of the cap called the bridge seats. The devices that are used to connect the structural members to the bridge seats are called shoes or bearings. Abutments, bents, and piers are typically built on spread footings. Spread footings are large blocks of reinforced concrete that provide a solid base for the substructure and anchor the substructure against lateral movements.

Footings also serve to transmit loads borne by the substructure to the underlying foundation material. When the soils beneath a footing are not capable of supporting the weight of the structure above the soil, bearing failure occurs. The foundation shifts or sinks under the load, causing structure movement and damage.

In areas where bearing failure is likely, footings are built on foundation piling . These load-bearing members are driven deep into the ground at footing locations to stabilize the footing foundation. Piling transmits loads from the substructure units down to underlying layers of soil or rock.

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Soil Improvement – Liquefaction

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The main goal of most soil improvement techniques used for reducing liquefaction hazards is to avoid large increases in pore water pressure during earthquake shaking. This can be achieved by densification of the soil and/or improvement of its drainage capacity.

Vibroflotation

Vibroflotation involves the use of a vibrating probe that can penetrate granular soil to depths of over 100 feet. The vibrations of the probe cause the grain structure to collapse thereby densifying the soil surrounding the probe. To treat an area of potentially liquefiable soil, the vibroflot is raised and lowered in a grid pattern. Vibro Replacement is a combination of vibroflotation with a gravel backfill resulting in stone columns, which not only increases the amount of densificton, but provides a degree of reinforcement and a potentially effective means of drainage.

Vibroflotation

Dynamic Compaction

Densifiction by dynamic compaction is performed by dropping a heavy weight of steel or concrete in a grid pattern from heights of 30 to 100 ft. It provides an economical way of improving soil for mitigation of liquefaction hazards. Local liquefaction can be initiated beneath the drop point making it easier for the sand grains to densify. When the excess porewater pressure from the dynamic loading dissipates, additional densification occurs. As illustrated in the photograph, however, the process is somewhat invasive; the surface of the soil may require shallow compaction with possible addition of granular fill following dynamic compaction.

Dynamic Compaction

Stone Column

As described above, stone columns are columns of gravel constructed in the ground. Stone columns can be constructed by the vibroflotation method. They can also be installed in other ways, for example, with help of a steel casing and a drop hammer as in the Franki Method. In this approach the steel casing is driven in to the soil and gravel is filled in from the top and tamped with a drop hammer as the steel casing is successively withdrawn.

Stone Column

Compaction Piles

Installing compaction piles is a very effective way of improving soil. Compaction piles are usually made of prestressed concrete or timber. Installation of compaction piles both densifies and reinforces the soil. The piles are generally installed in a grid pattern and are generally driven to depth of up to 60 ft.

Compaction Grouting

Compaction grouting is a technique whereby a slow-flowing water/sand/cement mix is injected under pressure into a granular soil. The grout forms a bulb that displaces and hence densifies, the surrounding soil. Compaction grouting is a good option if the foundation of an existing building requires improvement, since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.

Compaction Grouting

Drainage Techniques

Liquefaction hazards can be reduced by increasing the drainage ability of the soil. If the porewater within the soil can drain freely, the build-up of excess pore water pressure will be reduced. Drainage techniques include installation of drains of gravel, sand or synthetic materials. Synthetic wick drains can be installed at various angles, in contrast to gravel or sand drains that are usually installed vertically. Drainage techniques are often used in combination with other types of soil improvement techniques for more effective liquefaction hazard reduction.

Verification of Improvements

A number of methods can be used to verify the effectiveness of soil improvement. In-situ techniques are popular because of the limitations of many laboratory techniques. Usually, in-situ test are performed to evaluate the liquefaction potential of a soil deposit before the improvement was attempted. With the knowledge of the existing ground characteristics, one can then specify a necessary level of improvement in terms of insitu test parameters. Performing in-situ tests after improvement has been completed allows one to decide if the degree of improvement was satisfactory. In some cases, the extent of the improvement is not reflected in in-situ test results until some time after the improvement has been completed

Image Credit: http://menardbachy.com.au

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What is Liquefaction?

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Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.

Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other.

Earthquake shaking often triggers this increase in water pressure, but construction related activities such as blasting can also cause an increase in water pressure.

When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges is reduced as seen in the photo above of the overturned apartment complex buildings in Niigata in 1964.

Liquefied soil also exerts higher pressure on retaining walls,which can cause them to tilt or slide. This movement can cause settlement of the retained soil and destruction of structures on the ground surface as shown below.

Increased water pressure can also trigger landslides and cause the collapse of dams. Lower San Fernando dam (below) suffered an underwater slide during the San Fernando earthquake, 1971. Fortunately, the dam barely avoided collapse, thereby preventing a potential disaster of flooding of the heavily populated areas below the dam.

Courtesy: http://www.ce.washington.edu

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What Is a Girder Bridge?

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Girder is a term used in construction to refer to a supporting, horizontal beam that can be made from a variety of construction materials such as stainless steel, concrete, or a combination of these materials. A girder bridge is a basic, common type of bridge where the bridge deck is built on top of such supporting beams, that have in turn been placed on piers and abutments that support the span of the bridge. The types of beams used for girder bridges are usually either I-beam girders, so called because their shape is reminiscent of a capital Roman letter I, or box girder beams that are made of steel or concrete and shaped like an open box. Girder bridges are most commonly used for straight bridges that are 33-650 feet (10-200 m) long, such as light rail bridges, pedestrian overpasses, or highway fly-overs. The longest girder bridge in the world is 2,300 feet (700 m) long and located in Brazil.

There are four types of girder bridges, classified depending on the construction material and type of girders used. A rolled steel girder bridge is built using I-beams made from prefabricated steel, while a plate girder bridge is constructed by welding flat pieces of steel together on-site to make the I-beams. Concrete girder bridges are constructed using concrete I-beam girders that can be made from various kinds of reinforced concrete, including pre-stressed concrete and post-tensioned concrete. A box girder bridge can be made from either steel or concrete, and uses box girders to support the bridge deck.

 

 

Whether I-beam girders or box girders are used to construct a girder bridge depends on various factors. It is easier and cheaper to build and maintain a girder bridge using I-beam girders. However, these girders do not always offer sufficient structural strength and stability if the bridge is very long or the bridge span is curved, because they are sensitive to the twisting forces, or torque, such a span is subject to. Box girders are preferred for such bridges. There have been concerns raised of corrosion of box girders, especially if rain water seeps into the open space inside the girders.

 

Girder bridges belong to a category of bridges called beam bridges. This category of bridges includes girder bridges, truss bridges and trestle bridges. Beam bridges can be constructed by using a wide variety of materials including stone, timber, steel, iron, and concrete. An example of a basic type of beam bridge is a log or slab of stone laid across a creek.

Credit: Written By: M. Haskins

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Sloped vs Stepped Footings

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First published in Concrete International Magazine, March 2009

Generally, it’s most economical to place wall footings at a constant elevation. If the site or finished grade slopes along the length of the wall, however, the footing may end up a considerable distance below finished grade. This is clearly not economical, as it requires extra excavation and material. Two other options are therefore preferred (Fig. 1):

• Slope the footing with the site so its depth below the finished grade is nearly constant along its length; or
• Step the footing so its depth below finished grade is not excessive at any point along its length.

SLOPED FOOTING ISSUES

The sloped footing option may seem appealing because of the simple geometry and apparent ease in formwork construction. It does, however, create the following construction issues (Fig. 2):

• Vertical wall bars above the footing will have different lengths, creating major challenges in the fabrication plant and on the job site. Two of these—managing the inventory and placing the bars in their correct locations— can be eased by detailing the bars with variable lap splice lengths. This will, however, increase the quantity of vertical reinforcement;

• Horizontal reinforcing bars in the lower portion of the wall will also have different lengths because they are interrupted by the sloped footing. If constant length horizontal bars are used at the wall base, they can be fanned out, but this will create a variable vertical spacing of the reinforcing bars;

• Sloped footings will require trapezoidal formwork. This will require modifications to standard rectangular formwork;

• A sloped footing could be unstable, particularly on a very steep slope; and

• Concrete placement and finishing could be difficult, and a stiff concrete mixture might be required to prevent the concrete from flowing downhill, which may lead to segregation. Alternatively, the top of the form may have to be closed.

Because of these challenges, most engineers and contractors prefer to use stepped footings instead of sloped footings.

CONSIDERATIONS FOR STEPPED FOOTINGS

As with any aspect of a design, cost should be considered before a system is selected. If the slope of the finished grade is less than 2 ft (0.6 m) for a 20 to 30 ft (6 to 9 m) long wall, a lower but constant bottom bearing elevation may be more economical than a stepped footing. For a very long wall, however, even a 1 ft (0.3 m) variation in the site elevation may make a stepped footing more economical. Communication with the contractor during the design phase regarding the number and length of steps can be very helpful.

It’s generally more cost effective to minimize the number of steps. For example, it may be more economical to design for a 6 ft (1.8 m) change in elevation using three 2 ft (0.6 m) steps or two 3 ft (0.9 m) steps rather than six 1 ft (0.3 m) steps. This minimizes the number of wall sections to be detailed and formed. Before deciding on the footing step locations, however, consider the horizontal distance between them. Distances should preferably be multiples of available or standard form lengths.

Before completing a design, it’s a good idea to communicate with area formwork contractors. The horizontal runs should be dimensioned in 2 or 4 ft (0.6 or 1.2 m) increments to conform to standard plywood or form system dimensions. Unless the site slopes drastically, try to keep a minimum horizontal run of 10 ft (3 m) for each step, if possible.

Keep the detailing simple. Avoid using Z-shaped bars (Fig. 3). Their geometry may make it necessary to slant the riser out of plane to meet cover requirements for the treads.

It’s also prudent to evaluate other footing options. For example, the individual spread footings or piers supporting grade beams shown in Fig. 4 may be more economical than a continuous spread footing option. Because the wall can span between footings or piers, similar configurations can be constructed without the grade beam.

Situations can vary along the wall length, so it’s prudent to show specific details rather than generic details. This will expedite placing drawing preparation and perhaps minimize requests for information (RFIs).

CLOSURE

The use of sloped or stepped footings depends on site conditions, finished grade elevations, finished wall slope, and various reinforcing bar placement and construction issues. Regardless of the footing system selected, the engineer is required to follow the design requirements of Section 15.9 in ACI 318-08.[1] Section 15.9.1 requires that the angle of slope or depth and location of steps be such that the design requirements are satisfied at every section. Additionally, Section 15.9.2 requires footings designed as a unit to be constructed to ensure they act as a unit.

Acknowledgments

Thanks to Joint ACI-CRSI Committee 315 member Javed Malik, Jacobs Carter Burgess Engineering, Houston, TX, for providing the information in this article.

References

1. ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2008, 465 pp.

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Shearwalls & Boundary Elements

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By Javed B. Malik
First published in Concrete International Magazine, December 2007

Critical intersections require critical review

Boundary elements are the heavily-reinforced, critical zones of shearwalls normally located close to the edges of the wall or next to large wall openings (Fig. 1).

In the depth of the floor system, where floor beams and link beams must frame into the wall, boundary elements can become very congested. As Fig. 2 shows, the horizontal reinforcement for two floor beams as well as the horizontal and diagonal reinforcement for the link beam must be threaded between:

• Vertical reinforcement for the boundary element and the wall;
• Horizontal reinforcement for the wall; and
• Ties needed to confine the boundary element vertical reinforcement.

Some of this reinforcement may be located in several layers and some of the bars may be hooked, making the congestion even more severe.

If the walls are sized without proper consideration of the potential conflicts, two problems typically occur. First, it becomes very difficult to fit all the bars in the allotted space—bars may interfere with each other and may not fit. Even if there is enough room for all of the bars to fit in their final positions, however, congestion may make it difficult, if not impossible, to assemble the reinforcing bar cages. Hooked bars can be particularly challenging to place in their final position when having to thread them through a congested area of reinforcement.

The second problem is that concrete placement is very difficult around congested areas. Not only is it difficult to get the concrete into these areas, it is also difficult to insert the vibrators properly. If not properly vibrated, voids can be created in the wall at the most critical locations. If undetected, these voids may jeopardize the structural performance of the wall.

Suggestions for constructibility

A short time spent during the schematic design and construction document phases can save a lot of time and trouble during construction. The following suggestions will help ensure a constructible design. When considering these suggestions, designers should keep in mind that they are made strictly from a constructibility point of view. Their impact on structural performance should be carefully evaluated by the designer. Reference 1 deals with some of these issues in detail.

The starting point for the designer should be to draw a sketch of the critical areas to scale, study the clearances, and make sure that all of the reinforcement can fit. With modern software, critical joints can be drawn and studied in three dimensions.

An obvious way to reduce congestion is to increase the thickness of the wall, creating more room for concrete and reinforcing bars. This may not always be possible however, due to architectural constraints and loss of precious lease space. An alternative is to thicken only the boundary elements. Making the boundary elements only a few inches thicker than the wall can greatly reduce congestion in the joint by moving the boundary element vertical reinforcement outside of the link beam horizontal reinforcement, as shown in Fig. 3. If the boundary element is thickened, the link beam can also be easily widened to provide additional room for link beam reinforcement. Because it may pose problems with space planning, the option of thickened boundary elements should be carefully studied, particularly where they encroach into elevator shafts.

Similar to increasing the thickness of a boundary element, increasing the length of boundary elements can help spread the vertical bars apart, leaving more room for floor beam reinforcement perpendicular to the wall. This, however, may also increase the quantity of the vertical reinforcing steel required for the design.

Splices for both vertical and horizontal bars should be moved outside of the joint, if possible. As shown in Fig. 4, this reduces the number of bars taking up space in the highly congested area of the joint.

Similarly, terminating the longitudinal link beam reinforcement with a straight development length instead of a hook can reduce congestion but also makes the bars much easier to install. When hooks are required at each end of a bar in a floor beam perpendicular to a wall, splicing the bar in the middle of the beam allows much easier installation than placing a single bar with hooks on both ends in a congested joint (Fig. 5). The savings in placement time can easily offset the increased steel quantity. Another option may be to use headed bars instead of hooks.[2]

Diagonal bars for link beams can be especially difficult to properly coordinate with other reinforcement. It’s important to keep in mind that each bar is located in a separate layer and to be certain that the wall or the boundary element is wide enough to accommodate all these bar layers. Another location that can be difficult for installing diagonal bars is where they extend into the wall or boundary element. Often, this location is also where longitudinal bars from floor beams perpendicular to the wall enter the joint. This may require moving the floor beam bars to clear the diagonal bars from the link beam.

Another issue to look for is when the floor beams and link beams are of the same depth. The floor beam bars will have to be raised or lowered to clear the longitudinal bars from the link beam. This will change the height of the beam stirrups.

Similarly, if the link beams and the shearwall are the same width, the link beam horizontal bars will be located inside of the wall vertical bars. This will increase the clear cover for the link beams and make the stirrups narrower. This needs to be brought to the attention of the steel detailer by a section cut through the floor beams. If not addressed properly, the detailer would probably deduct 3 in. (75 mm) from the overall width and depth of the beam to get the stirrup dimension.

Some other suggestions for keeping these joints constructible include placing the horizontal wall bars and boundary element ties in the same plane and using mechanical splices. Placing the horizontal bars and the ties in the same plane reduces the number of reinforcement planes and increases clearances. Vertical wall bars will thus be located inside the horizontal bars. Mechanical splices can be especially helpful in alleviating congestion at splices located in joints, but relocating the splice to another location is often an even better choice.

As a final note, remember that the actual bar diameter for calculating clearances is larger than the nominal diameter due to the deformations. Similarly, the curvature of column ties, beam stirrups, and hooks should be taken into account because these also reduce clearances.

Acknowledgments

The author is thankful to the members of ACI Committee 315-B, Details of Concrete Reinforcement—Constructibility, for their valuable suggestions and contributions.

References

Wyllie, L.A. Jr., and La Plante, R.W., “The Designer’s Responsibility for Rebar Design,” Structural Bulletin Series 1, Concrete Reinforcing Steel Institute, Schaumburg, IL, Aug. 2003, 16 pp.

Mobeen, S.S.; Elwi, A.E.; and Ghali, A., “Double-Headed Studs in Shearwalls,” Concrete International, V. 27, No. 3, Mar. 2005, pp. 59-63.

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Design to Minimum Dimensions

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By Javed B. Malik
First published in Concrete International Magazine, July 2007

Focusing on member size can defeat the purpose
Structural engineers generally strive to optimize the cost of structures, often by minimizing the sizes of structural members. An emphasis on minimizing the size of concrete members, however, can lead to unintended consequences that may defeat the global goal of minimizing the construction cost for the overall project. In short, it’s important to step back to consider how the individual components interact. Although this may seem rather basic, I’ve observed that problems occur often enough to warrant a reminder, especially for younger engineers and detailers.

ISSUES
Concrete members sized purely on the basis of applied loads may not be large enough to accommodate the required amount of reinforcing steel with the proper spacing between bars. Conflicts can be created by the reinforcement for the member in question, reinforcing bars from adjacent members, and embedded anchor bolts or headed studs. These conflicts can potentially lead to honeycombs and voids in the concrete, inadequate cover, and inadequate embedment. Designing individual members to minimum dimensions can also create a large number of similar, but not identical, members. This can significantly impact cost by limiting reuse of the formwork and reducing the efficiencies of workers and inspectors.

EXAMPLES
The following are some common examples where designing to minimum overall dimension can create problems. Addressing these and similar issues during the design phase saves time, reduces requests for information as well as change orders, and avoids headaches for both the contractor and the engineer.

Piers and pier caps
Sizing lightly loaded piers considering only the applied loads and allowable soil bearing capacity can result in relatively small piers. For piers supporting steel columns, this can create a conflict such as shown in Fig. 1(a), where the anchor bolts or bearing plates will not fit inside the steel cage for the pier. Obviously, this can be resolved by increasing the pier diameter as shown in Fig. 1(b), or a wider pier cap at top of piers can be installed to accommodate the anchor bolts as shown in Fig. 1(c). To minimize the number of pier sizes installed at a site, it’s preferable to change the shaft diameters in increments of at least 6 in. (150 mm) and the bell or under-ream diameters in increments of at least 12 in. (300 mm).

Spread footings
To minimize the number of different footing types, the length or width should be changed in minimum increments of 12 in. (300 mm). Before finalizing the footing thickness, the depth required to develop the column dowels or embed anchor bolts for steel columns should be checked because it may control the footing thickness (Fig. 2). An alternative to thickening the entire footing is to locally thicken it at the column location. For practical reasons, a minimum thickness of 12 in. (300 mm) is suggested.

Grade beams
To eliminate formwork, the sides of grade beams are often placed against earth, requiring a clear concrete cover of at least 3 in. (75 mm). To accommodate bend diameters at the corners of stirrups in grade beams, it’s good practice to use a minimum grade beam width of 12 or 15 in. (300 or 380 mm) as shown in Fig. 3. If the sides of the grade beams are formed, clear cover on the stirrups can be reduced to 1-1/2 in. (40 mm), and the grade beam can be made narrower. In these cases, a note should be added on the drawings requiring the contractor to increase the beam width by 1-1/2 in. (40 mm) on each side if the decision is made to eliminate forms.

Columns
It’s good practice to standardize column sizes on a job as much as possible. Ideally, all interior columns should be of one size and exterior columns of another size, if necessary. This will simplify the formwork and steel placement. It’s generally economical to keep the same column sizes for as many floors as possible and use higher strength concrete and more longitudinal reinforcement on the lower floors.

Beams
Beam dimensions, especially depth, should also be standardized on a job. It’s generally economical to use the same depth for all beams at a floor except for heavily loaded girders or spandrel beams. As shown in Fig. 4, making the beams slightly wider or narrower than the columns can help prevent interference between beam bars and vertical column bars. Although beams that are wider than the columns may be preferred to simplify formwork, the designer must also check the beam-column joint for any special reinforcement required in special moment frames for seismic applications.

Walls
Designing to the minimum thickness for walls can produce several problems. Walls are not only reinforced with vertical and horizontal steel, but sometimes have ties enclosing the vertical steel such as at boundary elements. In addition, bars from slabs, floor beams, and link beams terminate in the walls. As shown in Fig. 5, link beam bars placed in several planes can further complicate the placement and congestion of the reinforcement. If these issues are not carefully considered during design, the wall can become heavily congested at locations where several elements intersect and make it very difficult to place the bars and consolidate the concrete properly.

Tilt-up wall panels
For tilt-up walls, panel thickness is often set at about 1/48th the vertical span of the wall.[1] It’s important to note, however, the effect of architectural reveals on the net wall thickness. This is needed not only for design, but also for detailing. For example, to ensure that the wall is thick enough for embedment plates with headed studs, designers must verify that sum of the plate thickness, the stud length, and the cover on the end of the studs doesn’t exceed the net wall thickness (Fig. 6).

Using double mats of reinforcing can significantly increase the moment capacity as well as the cracked moment of inertia (and thus, the axial capacity of slender wall elements). For panels thinner than 6 in. (150 mm), however, double mats of reinforcement are not preferred as they will be located nearly on top of each other. Finally, note that a standard hook may not fit well in a thin wall panel, so it may be necessary to place the hook in the plane of the wall or use welded-bar mats.

STEPPING BACK
Although the size of structural members must be appropriate for the applied loads and material properties, this should only be considered the starting point. By simply taking a step back and looking at how various elements interface with one another, the issues discussed in this article and other, similar issues can often be easily found and corrected. Making this a continuous process during design and detailing can help avoid having to redesign elements when conflicts are found, and it can lead to a better understanding of how the structural elements interact as a whole.

Acknowledgments
The author is thankful to the members of ACI Committee 315-B, Details of Concrete Reinforcement—Constructibility, for their valuable suggestions and contributions.

References
“Tilt-Up Construction and Engineering Manual,” 6th Edition, Tilt-Up Concrete Association, Mount Vernon, IA, Aug. 2006, p. 9-2.

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Rebar and Waterstops

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By Javed B. Malik
First published in Concrete International Magazine, May 2006

Constructible solutions to a common problem

Waterstops are commonly used at cold joints in concrete structures, such as water tanks, water treatment plants, and below-grade structures, to prevent the seepage of fluids through the joint. Although they come in several forms and shapes, the two most commonly used types are adhesive and mechanical waterstops as shown in Fig. 1 and 2, respectively. Adhesive waterstops can be hydrophilic or hydrophobic. Hydrophilic waterstops prevent the seepage of fluids by swelling when they come in contact with moisture, hydrophobic waterstops act as internal joint sealants, and mechanical waterstops rely on embedment into the concrete on both sides of a joint to form a diaphragm that seals off liquids.

Because they are typically smaller than mechanical waterstops and don’t have to be embedded on both sides of a joint, adhesive waterstops can generally be installed without conflicting with the reinforcing bars. Mechanical waterstops, however, can often conflict with reinforcement when their size and location are not properly taken into account during design and detailing.

CONFLICTS

The most common conflict between mechanical waterstops and reinforcement occurs at the joint between a slab or mat and a wall, as shown in Fig. 3. The waterstop is generally embedded into the slab 3 in. (75 mm) or more, producing the potential for interference with the top layer of reinforcing bars in the slab. If this conflict isn’t addressed on the contract drawings, the steel detailer will specify the height of the reinforcing bar supports based on the cover requirements shown on the drawings, and the iron worker will place the bars accordingly. Because the waterstop is typically the last item installed before the concrete is placed, the workers will either curl the waterstop so it lies above the steel, or cut notches in the waterstop so it clears the bars. Neither of these remedies is acceptable practice.

Potential conflicts become even more pronounced when the contract documents call for shear keys at wall-to-slab joints (Fig. 2). Not only does the shear key effectively increase the embedment of the waterstop in the slab, the concrete contractor must split the form for the shear key and install the shear key between the resulting form components. After the concrete hardens, it’s difficult to remove the form pieces without damaging the waterstop. It’s therefore a good idea to consider the use of alternate means for shear transfer at the cold joint, such as roughening the surface of the slab.

COMMON SOLUTIONS

The three most common solutions to the interference are:

• Form a “starter” wall that raises the waterstop clear of the top slab bars;
• Deflect the top slab bars so they pass below the waterstop; and
• Lower the top slab bars to clear the waterstop.

The first option, shown in Fig. 4, is to form and place a small portion of the wall (called a starter wall) monolithically with the bottom slab, thus raising the waterstop sufficiently clear of the top steel in the slab. This may be the ideal solution from a designer’s viewpoint. Many builders consider it problematic, but others like having the starter wall to tighten the wall forms against.

The second option, shown in Fig. 5, is to deflect the top slab bars below the waterstop. For smaller-diameter bars, this can be accomplished by pushing the bars down at the waterstop location, but larger bars need to be bent by the fabricator in the shop. Because the moment capacity of the slab is reduced due to a smaller lever arm, this option works best if the negative moment in the slab is small. The end of the deflected top bar will be very low in the slab if it’s deflected at a steep angle or over a long distance. Therefore, this solution becomes less practical the farther the wall is from the slab edge, and it’s not practical at interior walls.

The third option, shown in Fig. 6, is to lower the top mat of steel to clear the waterstop. If the depth of the slab is not increased, this solution may require additional top bars in the slab because of the reduced effective steel depth. However, if the reinforcing quantity is controlled by creep and shrinkage or temperature requirements, the reduced moment capacity may not be a concern. For either the second or third option, a shear key will increase the required correction.

WHICH OPTION WORKS BEST

From a constructibility point of view, forming a starter wall is the best option when the slab has a large amount of top reinforcing bars, or when the wall is an interior wall. Forming the starter wall incurs a certain amount of cost, but can be offset by the savings due to better constructibility. If creep and shrinkage criteria control the steel quantity, or the bars are of a relatively large diameter and closely spaced, lowering the top layer of reinforcement in the slab to clear the waterstop may be a good option. For slabs with smaller-diameter bars, deflecting the top bars to clear the waterstop should only be considered if the reduced moment capacity of the slab is not a concern. No matter which option is selected, it’s best if the design engineer addresses the condition before the construction phase and indicates the preferred method on the contract documents.

Acknowledgments

Thanks to ACI member Dick Birley for his contributions to this article.

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