Reinforced Concrete Beam Detailing According to ACI Code

Clearly, the detailing of the reinforced concrete members is the key to good design and execution of work at the site.That is why poor detailing of reinforcement makes the structure undergo cracking, excessive deflection, or even collapse.

Reinforcements resist tensile forces. They may also be required in the compression zones to increase the compression capacity, enhance ductility, to reduce long-term deflections, or increase the flexural capacity for beams.

Added to that, they prevent cracking of concrete due to shrinkage and temperature stresses.

Finally, different aspects of reinforced concrete beam detailing based on ACI Code is explained in the following sections.

Types of reinforcement in a beam

• Main bars (bottom steels)- deal with tension force
• Top bars (anchor bars)-hold stirrups in their position
• Cut off bars- deal with tension forces
• Stirrups with different spacing-tackle vertical and diagonal shear.

Hooked anchors

The function of hooked anchors is the provision of additional anchorage when there is inadequate straight length available to develop a bar.

Commonly, standard hooks described in ACI Code Section 7.1 utilized apart from the case where it is specifically specified.

The ACI standard hooks are as follows:

ACI standard hooks for primary reinforcement

• 180-degree bend plus 4db extension, but not less than 65 mm at free end of bar.
• 90-degree bend plus 12db extension at free end of bar.

Fig.1: Standard hook, 180degree

Fig.2:Standard hook, 90degree

ACI standard hooks for stirrups

• 16 bar and smaller, 90-degree bend plus 6db extension at free end of bar.
• 19, No. 22, and No. 25 bar, 90-degree bend plus 12db extension at free end of bar
• 25 bar and smaller, 135-degree bend plus 6db extension at free end of bar.

Fig.3:Stirrup hooks

Fig.4:seismic stirrup, 135degree

Minimum bend diameters

Standard bends in reinforcing bars are described in terms of the inside diameter of bend because this is easier to measure than the radius of bend.

The primary factors which control the minimum bend diameter are feasibility of bending without breakage and avoidance of crushing the concrete inside the bend.

• Diameter of bend measured on the inside of the bar, other than for stirrups in sizes No. 10 through No. 16, shall not be less than the values in Table 1.
• Inside diameter of bend for stirrups shall not be less than 4db for No. 16 bar and smaller. For bars larger than No. 16, diameter of bend shall be in accordance with Table 1.
• Inside diameter of bend in welded wire reinforcement for stirrups shall not be less than 4db for deformed wire larger than MD40 and 2db for all other wires. Bends with inside diameter of less than 8db shall not be less than 4db from nearest welded intersection.

Table 1 minimum inside diameter of hooks based on the size of the bars

Bar size	Minimum diameter
No.10 to No.25	6db
No.29, No.32, and No.36	8db
No.43 and No.57	10db
db is the diameter of the bar

Fig.5:Inside diameter of hooked bar

Stirrups

In beam detailing, the detailer or designer shall provide sizes, spacing, location, and types of all stirrups which include open and closed stirrups. stirrups may be vertical or inclined.

Fig.5:Various stirrup configurations

Moreover, where the design requires closed stirrup for shear, the closure may consist of overlapped, standard 90o end hooks of one or two-piece stirrups, or properly spliced pairs of U-stirrups.

Furthermore, when the design requires closed ties for torsion, the closure may consist of overlapped, standard 135o hooks of one- or two-piece ties enclosing a longitudinal bar.

Lastly, there are different techniques for anchorage of stirrups, but the most common is to use one of the standard hooks as shown in figure 3 and figure 4.

Development length (ld)

The development length (ld) is the shortest length of bar in which the bar stress can increase from zero to the yield strength (fy). It is different in tension and compression, and basic equation for each case are as follow:

Development length in tension

Use the following expression to calculate development length in tension, but no less than 30.48cm.

Where:

cb: factor that represents the smallest of the side cover. the cover over the bar or wire (in both cases measured to the center of the bar or wire)

Ktr: factor which denotes the contribution of confining reinforcement across potential splitting planes.

: the traditional reinforcement location factor to reflect the adverse effects of the top reinforcement casting position.

: coating factor reflecting the effects of epoxy coating. The product of  should not exceed 1.7.

: reinforcement size factor that reflects the more favorable performance of smaller diameter reinforcement.

: factor reflecting the lower tensile strength of lightweight concrete and the resulting reduction of the splitting resistance, which increases the development length in lightweight concrete.

(cb+ Ktr)/db equal to 2.5 if the result of (cb + Ktr)/db is less than 2.5.

db: bar diameter

Atr: total cross-sectional area of all transverse reinforcement within the spacing s.

n: number of bars or wires being developed along the plane of splitting

Factor in development length equation	Cases	Values
	Horizontal reinforcement so placed that more than 30cm of fresh concrete cast in the member below the development length or splice	1.3
	Other reinforcement	1
	Epoxy-coated bars or wires with cover less than 3db or clear spacing less than 6db	1.5
	All other epoxy-coated bars or wires	1.2
	Uncoated and galvanized reinforcement	1
	No. 19 and smaller bars and deformed wires	0.8
	No. 22 and larger bars	1
	For lightweight-aggregate concrete utilization	0.75
	when the splitting tensile strength is specified, it is equal to fc’^0.5/1.8fct but not more	1
	for normal-weight concrete	1

Development length in compression

development length can be calculated using the greatest of the following formula, but it shall not be smaller than 20.32cm.

ldc=(0.02fy / )db  Equation 3

ldc=( (0.0003fy)db  Equation 4

where

ldc: development length in compression bars

fy: yield strength of bars

db: bar diameter

Development of bundled bars

Individual bar development length within a bundle, in tension or compression, shall be that for the individual bar, increased 20 percent for three-bar bundle, and 33 percent for four-bar bundle.

Development Length ldh for standard hooks in tension

The development length, ldh, for standard hooks in tension is given as:

Where:

= 1.2 for epoxy-coated reinforcement

= 1.3 for lightweight aggregate concrete.

For other cases,  and   are equal to 1.0.

Fig.5:development length in end hooks

Bar cut off in beams and development length in flexural reinforcement

Bar cut off in beams

Commonly, reinforcements are provided close to the bottom of beams so as to counter act tensile forces on the beam.

Moreover, these reinforcements computed based on maximum moments at mid spans and at face of supports.  these moments reduce in beam regions other than mid span and support faces.

Therefore, it is possible to cut off bars in zones where they are no longer required. hence, cut off bars provide economic advantages.

However, bar cut off shall be kept as minimum as possible to decline design and construction complexities.

Furthermore, it is important to extend the cut off bars beyond cut off point by development length (ld) to ensure adequate bond between bar and concrete. Development length will be discussed in the section below.

Lastly, the location of points where bars are no longer needed is a function of the flexural tensions that results from the bending moments and the effects of shear on these tensile forces.

Fig.6:Cut off bars

Development length in flexural reinforcement in beams

In beams, development length is provided at points of critical stress. The critical stress points in beams are Points of maximum positive and negative moments are critical sections, from which adequate anchorage ld must be provided.

Moreover, critical points are also at points within the span where adjacent reinforcement cut off; continuing bars must have adequate anchorage ld from the theoretical cut-off points of terminated bars.

Added to that, cut off bars shall extend beyond the theoretical cut-off points to resist flexure for a distance equal to d or 12db.

Development of positive moment reinforcement

For simple members, minimum one-third of positive moment reinforcement and one-fourth of positive moment reinforcement in continuous members must extend along the element face into the support.

These reinforcements in beams shall extend into the support at least 152.3mm.

Development of negative moment reinforcement

Negative moment reinforcement in a continuous, restrained, or cantilever member, or in any member of a rigid frame, shall be anchored in or through the supporting member by embedment length, hooks, or mechanical anchorage.

Additionally, at least one-third the total tension reinforcement provided for negative moment at a support shall have an embedment length beyond the point of inflection not less than d, 12db, or clear span /16, whichever is greater.

Fig.6:Anchorage of negative moment reinforcement

Fig.7:Typical reinforcement details of non perimeter beams with open stirrups

Fig.8:Typical reinforcement details of non perimeter beams with closed stirrups

Fig.9:Typical reinforcement details of perimeter beams

Source:- theconstructor.org

Residential Buildings – Types and Site Selection

Residential Buildings – Types and Site Selection for Residential Building

What is a Residential Building?

A residential building is defined as the building which provides more than half of its floor area for dwelling purposes. In other words, residential building provides sleeping accommodation with or without cooking or dining or both facilities.

Types of Residential Buildings

Residential buildings are divided into following types

• Individual houses or private dwellings
• Lodging or rooming houses
• Dormitories
• Apartments
• Hotels

Individual houses or Private dwellings

Individual houses or private dwellings are generally owned by members of a single family only. If more than one family residing in that building then it is called as multiple family private dwelling.

Lodging or Rooming Houses

Lodging or rooming houses are multiple or group of buildings which come under one management. In this case, Accommodation is provided for separately for different individuals on temporary or permanent basis.

Dormitories

Dormitories are another type of residential buildings, in which sleeping accommodation is provided together for different individuals. School hostels, military barracks come under this category.

Apartments

Apartments or flats are big buildings which consists separate dwellings for different families. Apartment will resides minimum three or more families living independently of each other.

Hotels

Hotels are just like lodging houses and also managed by single management but they provide accommodation primarily on temporary basis. inns, motels etc come under this category.

Site Selection for Residential Buildings

Selection of site for any building is a very important and experts job and should be done very very carefully by an experienced engineer. The requirements of site for buildings with different occupancies are different.Following are some of the important factors which should be considered while selecting site for any residence.

1. The site should be in fully developed area or in the area which has potential of development.
2. There should be good transport facilities such as railway, bus service, for going to office, college, market, etc.
3. Civic services such as water supply, drainage sewers, electric lines, telephone lines, etc. should be very near to the selected site so as to obtain their services with no extra cost.
4. Soil at site should not be of made up type as far as possible. The buildings constructed over such soils normally undergo differential settlement and sometimes become the cause of collapse. Cracks in buildings in such conditions, are quite common
5. The selected site should be large enough; both to ensure the building abundant light and air to prevent any over dominance by the neighboring buildings.
6. The ground water table at the site should not be very high.
7. Nearness of schools, hospitals, market, etc. are considered good for residential site but these facilities do not carry any significance in the selection site for other public buildings.
8. Good foundation soil should be available at responsible depth. This aspect saves quite a bit in the cost of the building.
9. The site should command a good view of landscape such a hill, river, lake, etc.
10. Residential house site should be located away from the busy commercial roads.
11. Residential site should not be located near workshops, factories, because such locations are subjected to continuous noise.
12. Orientation of the site also has some bearing on its selection. Site should be such in our country that early morning sun and late evening sun is accepted in the building in summer and maximum sun light is available in most of winter.

 Source:- theconstructor.org

Performance Levels of Buildings Against Earthquakes

Performance level of structures against earthquakes describes limiting damage condition that assumed to be satisfactory for a given building and a given ground motion.

Moreover, building damages, danger to life safety of occupants in the building due to the damage, and post-earthquake serviceability of the building describe and control the limiting damage condition.

Added to that, building performance level against earthquakes is a combination of the performance of both structural and nonstructural components.

Lastly, performance levels of building structures against earthquake will be presented in the following sections.

Fig.1:Performance levels of buildings against earthquakes

Performance levels of buildings against earthquakes are as follows:

• Operational performance level
• Immediate occupancy performance level
• Life safety performance level
• Collapse prevention performance level

Operational performance level

• This performance level associates with functionality of the structure. Generally, all systems important to normal operation are operational.
• Damage to the building is limited, so the overall damage is very light and hence immediate occupancy is not questionable.
• The structure does not experience permanent drift.
• The building retains original strength and stiffness considerably.
• facades, partitions, and ceilings as well as structural elements suffer minor cracking only.
• Nonstructural component damage is negligible.
• The structure requires minor repairing which can be done without important disruption to occupants.
• Finally, power and other utilities are available; possibly from standby sources.

Fig.2:operational building performance level, very light overall damage

Immediate occupancy performance level

• The structure experience light damages
• There is no permanent drift.
• The building retains original strength and stiffness substantially.
• Minor cracking of facades, partitions, and ceilings as well as structural elements.
• Elevators can be restarted.
• Fire protection operable.
• The building space and systems are anticipated to be fairly usable. however, equipment and contents are generally secure but may not operate due to mechanical failure or lack of utilities.
• Concrete frame experience minor hairline cracking, limited yielding at few locations, and no crashing (strain of concrete less than 0.003)
• Steel moment frames experience minor local yielding at few locations. No buckling, fracture, and observable distortion of members.
• Lastly, braces of Braced steel frame structure suffer minor yielding or distortion

Fig.3:Immediate occupancy building performance level, moderate overall damage

Life safety performance level

• This level intended to obtain a damage condition that presents a substantially low probability of danger to life safety. Whether the danger is due to structure damage or fallen of nonstructural components of a building.
• The building experiences moderate overall damage
• All stories of a structure retain some residual strength and stiffness left in.
• Gravity-loadbearing elements function.
• There will be no out of plane failure of walls or tipping of parapets.
• However, the structure undergoes some permanent drift.
• partitions suffer damage.
• Building may be beyond economical repair.
• Falling hazards mitigated but many architectural, mechanical, and electrical systems are damaged.
• Concrete frame beams damage extensively, shear cracking and cover spall off occur in ductile columns, and minor cracking develops in nonductile columns.
• Hinges create in steel moment frames. In addition to local buckling of some beams, serious joint distortion, and fracture of isolated moment connection. However, shear connection would remain sound and few elements might suffer partial fracture.
• Lastly, in braced frames, majority of bracing buckle or yield but do not fail entirely, and several connections may fail as well.

Fig.4:life safety occupancy building performance level, moderate overall damage

Collapse prevention performance level

• This level of building performance mainly relates to the vertical load carrying system and the structure need to stable under vertical loads only.
• Generally, the building damage is severe
• The structure retains little residual stiffness and strength.
• however, load bearing columns and walls function.
• The building suffers Large permanent drifts.
• Some exits blocked.
• Infills and unbraced parapets failed or at incipient failure.
• Building is near collapse.
• Nonstructural components damage extensively.
• In concrete frames, hinges and extensive cracking develop in ductile elements, nonductile columns experience splice failure and limited cracking, and short columns damage seriously.
• Beams and columns distort heavily in steel frames. Added to that, several moment connections fracture but shear connections remain intact.
• Finally, braces yield and buckle extensively in braced frames, and even many of them along with their connections could fail.

Methods of Rainwater Harvesting

Methods of Rainwater Harvesting -Components, Transport and Storage

Methods of Rainwater Harvesting

Broadly there are two ways of harvesting rainwater

1. Surface runoff harvesting
2. Roof top rainwater harvesting

Rainwater harvesting is the collection and storage of rainwater for reuse on-site, rather than allowing it to run off. These stored waters are used for various purposes such as gardening, irrigation etc. Various methods of rainwater harvesting are described in this section.

1. Surface runoff harvesting

In urban area rainwater flows away as surface runoff. This runoff could be caught and used for recharging aquifers by adopting appropriate methods.

2. Rooftop rainwater harvesting

It is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the catchments, and the rainwater is collected from the roof of the house/building. It can either be stored in a tank or diverted to artificial recharge system. This method is less expensive and very effective and if implemented properly helps in augmenting the groundwater level of the area.

Rooftop Rainwater Harvesting System

Components of the Rooftop Rainwater Harvesting

The illustrative design of the basic components of roof top rainwater harvesting system is given in the typical schematic diagram shown in Fig 1.

Fig 1: Components of Rainwater Harvesting

The system mainly constitutes of following sub components:

• Catchments
• Transportation
• First flush
• Filter

Catchments

The surface that receives rainfall directly is the catchment of rainwater harvesting system. It may be terrace, courtyard, or paved or unpaved open ground. The terrace may be flat RCC/stone roof or sloping roof. Therefore the catchment is the area, which actually contributes rainwater to the harvesting system.

Transportation

Rainwater from rooftop should be carried through down take water pipes or drains to storage/harvesting system. Water pipes should be UV resistant (ISI HDPE/PVC pipes) of required capacity. Water from sloping roofs could be caught through gutters and down take pipe. At terraces, mouth of the each drain should have wire mesh to restrict floating material.

First Flush

First flush is a device used to flush off the water received in first shower. The first shower of rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the probable contaminants of the atmosphere and the catchment roof. It will also help in cleaning of silt and other material deposited on roof during dry seasons Provisions of first rain separator should be made at outlet of each drainpipe.

Filter

There is always some skepticism regarding Roof Top Rainwater Harvesting since doubts are raised that rainwater may contaminate groundwater. There is remote possibility of this fear coming true if proper filter mechanism is not adopted.

Secondly all care must be taken to see that underground sewer drains are not punctured and no leakage is taking place in close vicinity.

Filters are used for treatment of water to effectively remove turbidity, colour and microorganisms. After first flushing of rainfall, water should pass through filters. A gravel, sand and ‘netlon’ mesh filter is designed and placed on top of the storage tank. This filter is very important in keeping the rainwater in the storage tank clean. It removes silt, dust, leaves and other organic matter from entering the storage tank.

The filter media should be cleaned daily after every rainfall event. Clogged filters prevent rainwater from easily entering the storage tank and the filter may overflow. The sand or gravel media should be taken out and washed before it is replaced in the filter.

A typical photograph of filter is shown in Fig 2.

Fig 2: Photograph of Typical Filter in Rainwater Harvesting

There are different types of filters in practice, but basic function is to purify water. Different types of filters are described in this section.

Sand Gravel Filter

These are commonly used filters, constructed by brick masonry and filleted by pebbles, gravel, and sand as shown in the figure. Each layer should be separated by wire mesh. A typical figure of Sand Gravel Filter is shown in Fig 3.

Fig 3: Sand Gravel Filter

Charcoal Filter

Charcoal filter can be made in-situ or in a drum. Pebbles, gravel, sand and charcoal as shown in the figure should fill the drum or chamber. Each layer should be separated by wire mesh. Thin layer of charcoal is used to absorb odor if any. A schematic diagram of Charcoal filter is indicated in Fig 4.

Fig 4: Charcoal Filter

PVC –Pipe filter

This filter can be made by PVC pipe of 1 to 1.20 m length; Diameter of pipe depends on the area of roof. Six inches dia. pipe is enough for a 1500 Sq. Ft. roof and 8 inches dia. pipe should be used for roofs more than 1500 Sq. Ft. Pipe is divided into three compartments by wire mesh.

Each component should be filled with gravel and sand alternatively as shown in the figure. A layer of charcoal could also be inserted between two layers. Both ends of filter should have reduce of required size to connect inlet and outlet. This filter could be placed horizontally or vertically in the system. A schematic pipe filter is shown in Fig 5.

Fig 5: PVC-Pipe filter

Sponge Filter

It is a simple filter made from PVC drum having a layer of sponge in the middle of drum. It is the easiest and cheapest form filter, suitable for residential units. A typical figure of sponge filter is shown in Fig 6.

Fig 6: Sponge Filter

Methods of Rooftop Rainwater Harvesting

Various methods of using roof top rainwater harvesting are illustrated in this section.

a) Storage of Direct Use

In this method rainwater collected from the roof of the building is diverted to a storage tank. The storage tank has to be designed according to the water requirements, rainfall and catchment availability.

Each drainpipe should have mesh filter at mouth and first flush device followed by filtration system before connecting to the storage tank. It is advisable that each tank should have excess water over flow system.

Excess water could be diverted to recharge system. Water from storage tank can be used for secondary purposes such as washing and gardening etc. This is the most cost effective way of rainwater harvesting.

The main advantage of collecting and using the rainwater during rainy season is not only to save water from conventional sources, but also to save energy incurred on transportation and distribution of water at the doorstep. This also conserves groundwater, if it is being extracted to meet the demand when rains are on. A typical fig of storage tank is shown in Fig 7.

Fig 7: A storage tank on a platform painted white

b) Recharging groundwater aquifers

Groundwater aquifers can be recharged by various kinds of structures to ensure percolation of rainwater in the ground instead of draining away from the surface. Commonly used recharging methods are:-

a) Recharging of bore wells

b) Recharging of dug wells.

c) Recharge pits

d) Recharge Trenches

e) Soakaways or Recharge Shafts

f) Percolation Tanks

c) Recharging of bore wells

Rainwater collected from rooftop of the building is diverted through drainpipes to settlement or filtration tank. After settlement filtered water is diverted to bore wells to recharge deep aquifers. Abandoned bore wells can also be used for recharge.

Optimum capacity of settlement tank/filtration tank can be designed on the basis of area of catchment, intensity of rainfall and recharge rate. While recharging, entry of floating matter and silt should be restricted because it may clog the recharge structure.

First one or two shower should be flushed out through rain separator to avoid contamination. A schematic diagram of filtration tank recharging to bore well is indicated in Fig 8 .

Fig 8 :Filtration tank recharging to bore well

d) Recharge pits

Recharge pits are small pits of any shape rectangular, square or circular, contracted with brick or stone masonry wall with weep hole at regular intervals. Top of pit can be covered with perforated covers. Bottom of pit should be filled with filter media.

The capacity of the pit can be designed on the basis of catchment area, rainfall intensity and recharge rate of soil. Usually the dimensions of the pit may be of 1 to 2 m width and 2 to 3 m deep depending on the depth of pervious strata.

These pits are suitable for recharging of shallow aquifers, and small houses. A schematic diagram of recharge pit is shown in Fig 9.

Fig 9: Recharge pit

e) Soakway or Recharge shafts

Soak away or recharge shafts are provided where upper layer of soil is alluvial or less pervious. These are bored hole of 30 cm dia. up to 10 to 15 m deep, depending on depth of  pervious layer. Bore should be lined with slotted/perforated PVC/MS pipe to prevent collapse of the vertical sides.

At the top of soak away required size sump is constructed to retain runoff before the filters through soak away. Sump should be filled with filter media. A schematic diagram of recharge shaft is shown in Fig 10.

Fig 10 : Schematic Diagram of Recharge shaft

f) Recharging of dug wells

Dug well can be used as recharge structure. Rainwater from the rooftop is diverted to dug wells after passing it through filtration bed. Cleaning and desalting of dug well should be done regularly to enhance the recharge rate. The filtration method suggested for bore well recharging could be used. A schematic diagram of recharging into dug well is indicated in Fig 11 shown below.

Fig 11: Schematic diagram of recharging to dug well

g)Recharge trenches

Recharge trench in provided where upper impervious layer of  soil is shallow. It is a trench excavated on the ground and refilled with porous media like pebbles, boulder or brickbats. it is usually made for harvesting the surface runoff.
Bore wells can also be provided inside the trench as recharge shafts to enhance percolation. The length of the trench is decided as per the amount of runoff expected.

This method is suitable for small houses, playgrounds, parks and roadside drains. The recharge trench can be of size 0.50 to 1.0 m wide and 1.0 to 1.5 m deep. A schematic diagram of recharging to trenches is shown in Fig below 12.

Fig 12: Recharging to trenches

h) Percolation tank

Percolation tanks are artificially created surface water bodies, submerging a land area with adequate permeability to facilitate sufficient percolation to recharge the groundwater. These can be built in big campuses where land is available and topography is suitable.

Surface runoff and roof top water can be diverted to this tank. Water accumulating in the tank percolates in the solid to augment the groundwater. The stored water can be used directly for gardening and raw use. Percolation tanks should be built in gardens, open spaces and roadside greenbelts of urban area.

Source: theconstructor.org