Monday, April 12, 2021

Environmentally Lighting Design Strategies

 Several approaches to energy - effective and energy - efficient lighting have been proposed by noted lighting experts. Nancy Clanton suggests six aspects of lighting to focus on to achieve a low - power density (i.e., watts used per square foot):

1.     Quality daylight

2.     Ambient, task, and accent lighting

3.     Light - colored surfaces

4.     Energy - efficient lighting equipment

5.     Combination of automatic and manual lighting controls

6.     Light surfaces, not volumes

The Advanced Building Systems Integration Consortium, Center for Building Performance and Diagnostics (ABSIC/CBPD), provides seven guidelines for high performance lighting (Center for Building Performance and Diagnostics n.d.):

1.     Daylight-dominant lighting

2.     Task lighting and ambient lighting

3.     Indirect - direct lighting

4.     High-performance luminaires

5.     Plug-and-play fi xtures

6.     Dynamic zoning and advanced controls

7.     System integration

The ABSCIC/CBPD also has twelve major decisions for other interior systems.

A set of strategies to achieve energy - effi cient lighting has been presented by Minnesota Department of Public Service in Commercial Building Lighting Standards: Educational Project (1993). The focus is on six aspects of lighting (needs, hardware, daylighting, control, maintenance, and operations scheduling) and defi nes action items for each:

1.     Lighting needs (tasks and illumination requirements)

·        Identify visual tasks and locations.

·        Group task with same illuminance requirements.

·        Properly locate luminaires to provide light to tasks.

·        Consider light colors for walls, floors, ceilings, and furniture.

2.     Lighting hardware (lamps and luminaires)

·        Install lamps with higher efficacy .

·        Investigate the use of reduced wattage lamps in existing luminaires when illuminance levels are greater than recommended .

·        Consider reduced - wattage fl uorescent lamps in existing luminaires.

·        Consider replacing existing low - wattage incandescent lamps with fewer high - wattage incandescent lamps or compact fluorescent lamps.

·        Assess luminaire effectiveness for lighting [distribution and efficiency].

·        Consider energy - effi cient, electronic ballasts.

·        Consider using heat - removal luminaires.

3.     Daylighting

·        Use daylighting when it is appropriate.

·        Coordinate the plan organization to maximize the use of daylighting.

·        Assess which daylighting tasks are critical and noncritical (indirect, reflected, or filtered daylight vs. direct sunlight).

·        Maximize the effectiveness of fenestration and shading controls.

·        Consider the use of light colors (see above).

·        Increase the distribution of light deep into the space by using light shelves and light - colored room surfaces.

·        Integrate electric lighting with daylighting design.

4.     Light controls

·        Install switching to adjust illumination levels to activity requirements.

·        Consider occupancy sensors to turn lights on and off as room occupancy varies.

·        Consider the use of dimming systems to adjust illumination levels.

·        Consider the use of time clocks to adjust lighting with occupancy schedule.

5.     Lighting maintenance

·        Evaluate the present lighting - maintenance program.

·        Clean luminaires and replace lamps on regular maintenance schedule.

·        Replace outdated or damaged luminaires.

6.     Operating schedules

·        Analyze lighting use during working and building - cleaning periods.

·        Light building for occupied periods only and as security requires.

·        Try to schedule routine building cleaning during occupied hours.

·        Restrict night parking to specific lots.

Environmentally responsible lighting design that is both energy effective and energy efficient becomes easier and more achievable with better products and processes.

The contribution of ERLD to environmentally responsible interior design can be substantial.

Sunday, February 28, 2021

Lighting Controls in the Environmentally Responsible Design

Lighting controls provide one of the easiest ways to increase the energy efficiency of the lighting system. Conservation of energy through the use of controls occurs by integrating daylighting with electric lighting, turning off unneeded lights, and reducing peak - demand electricity usage. Additional energy savings occur when light levels are controlled as part of lamp - lumen adjustment and adaptation compensation. Control strategies need to integrate with the building design, HVAC system, and building - use patterns for full effectiveness.

Three major objectives for using light control are to: (1) reduce light energy use and cost, (2) improve the function and aesthetics of the space for the occupants, and (3) aid in code compliance. Achieving these objectives is easier with advances in lighting - system controls technology that provide options for flexibility and function.

Controls can range from simple switches and dimmers to more advanced sensors and timers and even more complex building - automation systems. Integration of controls in new construction is assumed, but retrofi tting is a viable energy - saving option for existing spaces. The energy savings from lighting controls is estimated to be up to 50 percent in existing buildings and 35 percent in new building. Savings of this magnitude are dependent on the control system being used. Thus, a thorough analysis of the use of the space and the expectations of the occupants is an important component of the control - design process. To be effective, the system must accommodate the occupants ’ use patterns, their commitment to energy savings, and their ability to cope with the control system. The Lighting Controls Association (n.d.) Web site has considerable information available on selecting and using controls, including a room -by-room analysis.

A distinction is made between lighting - control strategies and lighting - control devices in Lighting Controls: Patterns for Design. The planned approach to the control of light is a strategy, while a device is the equipment used to achieve the desired control. Different devices may be used to achieve a lighting - control strategy. Lighting strategies include seven approaches:

1. Occupancy response is appropriate where lights are turned on when needed and turned off when occupancy or a task does not need the light. Control devices for this strategy include:7

  • Manual off/on switch : The simplest lighting control device, requiring user control, has limited adaptability.
  • Occupancy sensor: Detects motion in a space and turns lights off when the space is unoccupied. This automatic control device is useful where the occupancy pattern is unpredictable. There are several variations including:

1.     -Passive infrared (PIR) : Requires a direct “ line of sight ” with a 15 to 20 foot effective distance and 40 foot maximum.

2.     Active ultrasonic (ULT): Good in irregular spaces but can pick up false signals.

3.     PIR/ULT dual technology: Useful in large spaces. Options include a manual on/off override that provides maximum savings when used with PIR or ULT. Systems with only auto - on/auto - off are most appropriate in common-use spaces, such as restrooms and corridors. Use of sensors with HID lamps should be restricted to a high - low option due to the long restrike time if these lamps are turned completely off.

2. Prescheduled or timed response has lights turned on and off on a predetermined, regular schedule or automatically turns lights off after a preestablished time period. Exact light levels are set for time periods that may be varied within a day, week, month, and/or seasonal cycle. Control devices include:

  • Timer : Inexpensive and easiest control device to install; useful in short - occupancy spaces.
  • Time clock : Range from a simple mechanical to complex, programmable electronic units covering 356 days. Variations include:

1.      24 - hour or 7 - day time clocks

2.      Astronomical time clock that adjusts on/off times to sunrise/sunset times 

While rapid - start fluorescent lamp life may be decreased with frequent switching, the calendar replacement time for lamps may not be affected since the lamps are off more time. The energy saved by turning off lamps for periods of time far offsets (by more than three times) the cost of relamping, that is, replacing the lamps (Rundquist et al. 1996).

3. Tuning involves adjusting the light level to match users ’ needs or desires, which may vary with daylight availability, personal preferences, and energy awareness. Two variations of this strategy are task tuning, which involves individualized light control in a work space, and manual dimming, which involves light - level control of a large space. Since light levels that are turned down 25 percent are not noticed by occupants, the potential for a direct energy savings is obvious. Avoid systems that have abrupt changes in light levels, as this annoys occupants. Two approaches to tuning are:

  • Continuous dimming : Reduces both light output and energy consumption. Automatic dimming integral with photoelectric sensors should be used in daylighting control systems.

1.      Linear fluorescent and some compact fl uorescent lamps can be dimmed, typically down to 1 to 5 percent of the lamp output, if matched with dimming ballasts. Using electronic ballasts and dimming controls, the energy saved and light - level reduction are almost proportional (Wilson 2004). Lamp life may be decreased with extensive dimming. Some dimming ballasts can be “ addressed ” : their dimming level is preset and achieved automatically by the push of a button on the control.

2.      Incandescent lamp dimming saves energy and extends the lamp life, while warming the color of the light. A 20 percent reduction in energy results in about a 50 percent reduction in light output, so the savings from dimming is less than with fl uorescent lamps. Low - voltage lamps need the dimmer matched to the transformer. Low - voltage lamps using a dimmer need to be burned at full power periodically to activate the halogen cycle.

3.      HID dimming is more problematic, requiring both a special ballast and a dimming device. An abrupt change in light level results in a color shift for metal halide lamps. Due to advances in fl uorescent light technology and controllability, dimmable HID lighting is not recommended for indoor applications (Wilson 2003).

  • Stepped or multilevel switching : Turning off a portion of the lamps in a multilamp luminaire or banks of lights in a space, based on activity needs or daylight availability. This is a cost - effective means of reducing the light in increments. This reduction is accomplished by switching individual ballasts and lamps or by reducing the power load to the ballast. Multilevel ballasts can be used with HID to change the light output in steps. Stepped switching can be a low - cost method of providing some adjustment in light levels in a space.

4. Daylighting requires a response that adjusts the use of electric light to the amount of daylight available in interior spaces. As noted earlier, daylight will vary due to weather conditions, time of day, and season. Control is gained by the detection of daylight illuminance at a sensor that is integrated with the switching and dimming systems described above. Photoelectric control systems include:

  • Photo switch (or photocell): Generally unacceptable in interior spaces due to the abrupt change in light level with on/off switching. In locations with uniform daylight conditions and in spaces where daylight levels are well above the target levels, these devices may be acceptable and can save more energy than using photosensors. In these situations, they can be used most successfully with stepped switching (see above) to turn off some lights in a multilamp fi xture, if adequate daylight is present.
  • Photosensor : Uses continuous sensing of available light to control fluorescent electronic - dimming ballasts and adjusts the electric - light levels, based on a predetermined light level. The system needs to be custom designed, is complex, requires detailed commissioning, and is difficult to maintain, but it is effective in saving energy. Most photosensors have important adjustments for time delay, response speed, and response sensitivity. Small individual sensors are being integrated into individual luminaires, promising even more refi ned control in the future. Two types of photosensor systems are used:

1.     Interior closed loop: The sensor looks into the space it controls, registering the sum of daylight and electric light in its sensing range. Using direct feedback, a single sensor can control only a small number of luminaires in a relatively small space. In addition to use in daylighting - control strategies, this sensor is used for tuning, lumen maintenance, and adaptation - compensation strategies.

2.     Interior open loop: The sensor looks out the window or skylight from a location remote from the space being illuminated. With remote sensing, a single sensor can control multiple luminaires in large areas.

Special considerations and cautions related to photosensor applications include:

  • Sensors respond to refl ectance changes, including a person wearing white clothing.
  • Some sensors respond to spectral cues and should be matched to the visual - response curve (which peaks around 550 nanometers).
  • Closed - loop sensors need to be placed so that they read the task surfaces, rather than open areas.
  • Both sensors and photocells need to be shielded from direct light of the fixtures they are controlling as well as direct sunlight and bright skies.

6. Adaptation compensation responds to the human visual system and the adjustment of the eye to changes in light levels. Lower interior light levels at night are more comfortable and safer, since the eye does not have to adapt as much, especially when going from light to dark. This strategy is useful for saving energy at night, but it should only be used in areas that operate outside of daylight hours and where no critical visual tasks occur during these hours. Devices appropriate for this strategy include stepped switching and continuous dimming with photocells (see above.)

7. Demand limiting or load shedding is an economic response to energy availability and unit cost. During electrical emergency – alert periods, the automatic dimming of light levels and turning off unnecessary lights can help avoid brownouts. With advance recording systems, the total electricity demands of a building can be monitored and reduced when kilowatt costs temporarily rise during peak demand periods of a day.

8. Lumen maintenance aims to maintain an even light level over the life of a group of lamps. A reduced - light (or dimmed) level is used when lumen output of new lamps is higher than needed. As the lamps age and their lumen output decreases, the power is gradually increased to full range. With advances in lamp technology, there is currently minimal light depreciation for most lamp sources. Thus, the use of this strategy is generally not economically viable.

Lighting - control systems link together devices that determine the need for light (sensors) and the supply of power to the luminaires. Logical devices that integrate information signals from various devices may be part of the system. The simplest system combines devices in a local area or single space. Combining these local systems with a central computer and master control station expands the control to a whole building system. Combining the light - control strategies discussed above into an integrated system results in the maximum energy savings.

Other electric systems may also be controlled by this integrated building - automation approach. So - called smart buildings integrate other electrical systems (e.g., motorized shades, fans, air - conditioning and heating, and possibly security and alarm) with the lighting system. These total building systems are called energy - management systems (EMS) or building - automation systems (BAS). They have the ability to sense environmental conditions: time, amount of light, temperature, and air quality. Additionally, they can sense human intervention: occupancy or motion. These systems involve central or building protocol with a computer network – control system to communicate information between components in the system, which provides the potential of substantial energy savings (Center for Building Performance and Diagnostics n.d).

The central system information carrier can be a low - voltage or relay system (operatesa relay inserted in a luminaire power circuit), which uses small wire and consumes little electricity. One such system that communicates through low - voltage wire is DALI (digital addressable lighting interface); it communicates to luminaires and individual - control devices, and from lamps or ballasts. This international standard for ballast control is reducing the problem of incompatibility between controls and ballasts.

Another information - carrier system option is a power - line or carrier - current system, which uses the building wire system to send high - frequency signals, thus offering a low - cost system with great fl exibility. But this option is subject to interference and malfunction without the installation of special additional electrical components. The third and newest system uses radio frequency for communication. Wireless Mesh (ZigBee) and WiFi are two such systems.

Location options for the processors needed for the controls are: (1) local, next to device controlled; (2) central, utilizing a computer network – control system; and (3) distributed, decisions locally programmed but run centrally (unit failure affects only unit, not whole system).

The decision process for selecting a control system is based on occupant’s needs, type of space use and functional needs, energy rates, and an electricity - use profile. Daylighting potential is also a factor. For straightforward projects, a decision model is available. For more complex situations, an economic - payback analysis is appropriate.

Commissioning is another key step in achieving the energy efficiency and effectiveness of an advanced control system. This is the process of tuning, calibrating, and adjusting the devices to be sure they work appropriately for the installation requirements and at peak performance levels. The time and expense of this critical task is being managed by computerized monitoring components. Commissioning is required in a project seeking LEED certifi cation.

Computer programs are available to assist designers in developing daylight/electric - light layouts, including quantifying relative energy savings, modeling annual daylighting characteristics in a particular location, and sensor placement. One recent tool, Sensor Placement + Optimization Tool, or SPOT, was developed as part of California ’ s Public Interest Energy Research (PIER) Program Lighting Research Project (LRP). While initially developed for classroom daylighting, it is useful for other types of spaces.

A more comprehensive discussion of the increasingly complex array of control devices and systems is beyond the scope this discussion. Detailed information can be found in four sources: Lighting Controls: Patterns for Design; IESNA Lighting Handbook; Advanced Lighting Guidelines; and the Lighting Controls Association Web site ( ). In making a decision about using any lighting - control strategy, an analysis of the life - cycle cost of the controls (unit, installation, electricity, and maintenance) needs to be weighed against the energy savings from using the system. Within the context of ERLD, these decisions will affect both the energy efficiency and effectiveness of interior lighting.

Wednesday, January 6, 2021

Environmentally Responsible Electric Lighting Design

A systems approach to electric lighting is needed to achieve maximum energy - effective and energy - efficient design. This is the energized system that includes luminaires and controls. Luminaire is the technical names for the complete lighting unit that consists of the housing, lamps, ballasts, and transformers as well as light - controlling elements such as reflectors, shielding devices, and diffusing media.

In making any lighting decision, illumination needs must be established and trade - offs between electric lighting options need to be assessed. The cost - value benefit analysis for each option includes energy cost, lighting - system costs, operating costs, and lighting - quality issues. Lighting - quality issues cover a range, including employee productivity and absenteeism, security and safety, business image and environmental “mood,” and accommodation of spatial changes.


Efficiency of the electric lighting system is dependent on characteristics of each individual component as well as how the components work together to produce electric illumination. There are different measures of efficiency related to different components of the system.

  • Lamp efficacy is the technical term to describe how efficient a lamp converts electricity of visible light. This is stated as lumens per watt (LPW).
  • Luminaire efficiency is the ratio between the light output from the fi xture and the light output generated by the lamps it houses. This is stated as a percentage.
  • Coefficient of utilization (CU) is concerned with the amount of light that reaches the work surface relative to the amount of light produced by the lamp. This measure of efficiency is affected not only by the characteristics of the luminaire but also by the size and shape of the room, as well as the reflectance of the ceiling, walls, and fl oor. Standardized procedures are used by the manufacturer to establish a CU table of values for each luminaire.
  • Visual comfort probability (VCP) is a value that indicates how much glare a luminaire is likely to produce. The room dimensions infl uence this rating. A VCP of 80 or higher is considered necessary for highly computerized offices. This number means that 80 percent of the users located in the least desirable spot in the space would not be bothered by direct glare from an even pattern of luminaires mounted on or in the ceiling.
  • Ballast factor (BF) is the relative light output produced by a lamp and ballast system relative to the manufacturer ’s rated light output of the lamp itself. A high ballast factor means fewer lamps and ballasts are needed to achieve a specific level of illumination. A low BF ballast would permit lowering the light level in an overilluminated space without replacing or rearranging existing luminaires.


Effective lighting is dependent on perception of the appearance of the light as well as the general appearance of the space, especially related to color. Two additional ratings are used to identify the color appearance of the light source and its effect on surfaces in a space:
  • Color temperature (CT) or correlated color temperature (CCT): CT is used for lamps with fi laments, including standard incandescent and halogen sources. CCT is used for nonfi lament light sources, including fl uorescent and metal - halide lamps. This measure, stated in degrees Kelvin (K), is based on the color change of a test wire as it is heated. The color of the wire goes from yellow to orange/red to white to blue as it increases in temperature. Sunny daylight at noon is about 5,500K, an overcast sky is about 7,000K, a 100 - watt incandescent lamp is about 2,800K.
  • Color rendering index (CRI) provides an estimate of how “ natural ” or expected a standard set of colors appear when seen under a specifi c lamp relative to their color appearance under the standard test source with the same CCT. This latter appearance is rated as 100 CRI. Current energy codes defi ne a rating of 70 CRI as the minimum value for lamps used in most interior environments.
These two color ratings are linked to energy effi ciency in recent research findings. As CCT increases, the blue content of the light increases. A 5,000K light source has been found to provide more contrast and better resolution of details. With this bluer light, it is possible to design with lower foot - candle levels to achieve a perception of the same brightness in a space.

Light sources with a higher CRI have been linked to energy efficiency. The IESNA Lighting Handbook reports that “ lamps with color rendering indexes of 70, 85, and 100 require about 10%, 25%, and 40% lower illuminance levels than lamps with a CRI of 60, respectively, to achieve impressions of equivalent brightness ” (Rea 2000). Thus, the higher the CRI number of the light source, the brighter a space should appear with the same energy use.

Sunday, September 27, 2020

Environmentally Responsible Daylighting Design

 Daylight, part of the nonenergized system, is an important component of environmentally responsible lighting design. Architectural daylighting design decisions can help or hinder the potential for effective use of daylight and achieving visual comfort inside buildings. While a detailed development of daylighting design is beyond the scope of a discussion of environmentally responsible interior design (ERID), the integration of daylighting with electric lighting is important. “ Daylighting, ” a section from the Whole Building Design Guide , provides a substantial discussion of all aspects of daylighting (Daylighting n.d.). An understanding of the issues, benefits, and guidelines related to daylighting is important to developing ERID.

Windows provide a psychologically important connection to the outdoors. Access to a view and interaction with daylight provide valuable environmental information: a dynamic measure of time passage, information about immediate weather conditions, and a sense of place. Having a view to the outside reduces eye strain, allowing the muscles contracted from extended near - focus to relax. Research evidence from education, corporate, retail, and health-care settings is confi rming the positive health and performance impacts of daylighting.

A major benefit of daylighting is the reduction in both fossil fuel and electric energy use. Fossil fuel is used for heating and cooling, while electric energy is used for lighting. Daylighting can provide savings of 35 to 65 percent in electric use for lighting and 20 to 60 percent overall energy savings. Coupled with energy - efficient and effective electric lighting, the savings will be even greater. Energy budgets can be substantially reduced to well below American Society of Heating, Refrigerating and Air - Conditioning Engineers, Inc. (ASHRAE) Standard 90.1 requirements.

Daylighting does have limitation and liabilities that must be addressed for its successful use in interior environments. Using daylight requires recognition of the variability of this light source due to geographic location, time of day, time of year, and weather conditions that affect the sky; this affects the color of the light as well as the brightness and distribution.

The problem is not so much daylight but, rather, sunlight. A direct beam of sunlight is an extremely high source of light as well as heat. Thus, direct sunlight as a light source needs to be minimized, while maximizing the use of diffused daylight. Excessive heat gain and ultraviolet damage to materials can be two negative results of daylighting that need to be balanced with the advantages gained from daylighting in a particular building. Glazing material can be selected to reduce both of these problems. Tinted glass though can block biologically important wave lengths and/or distort the color spectrum of daylight. Glazing should have high visible - light transmission (VLT) qualities and a low solar - heat gain coefficient (SHGC) for maximum daylighting benefits.

Glare is the biggest daylighting problem; it reduces the ability to see details. A school study of daylighting effects reported that glare from windows reduced test scores 15 to 25 percent. High contrast is created along with glare, either directly between the glazed surface and adjacent interior surface or indirectly where light falls on a surface. Daylight needs to be balance with light - colored interior surfaces to reduce the potential for these strong contrasts. Guidelines note that the window should not be more than 100 to 300 times as bright as the objects in the room. Furthermore, direct sunlight on task areas or reflected glare on television and computer screens can be avoided through orientation and shading. “ Daylight factor ” is a calculation used to compute the amount of daylight outside compared to a point inside.

One daylighting strategy involves “ harvesting ” light. The idea is to minimize direct sunlight penetration while maximizing daylight use. Integrating high ceilings and bringing in daylight from two directions help to increase daylight utilization. Sidelighting uses vertical glazing. For daylighting use, the glazing is located high on the wall or overhead, while vision or view glazing is positioned within a seven - foot distance from the floor.

Light shelves are a frequent addition to high lights or clerestories to increase the penetration of daylight further into the interior space. Continuous horizontal windows are better than individual windows or vertical ones. A light shelf facilitates deeper daylight penetration into a space, but it contributes even more to a uniform distribution of the daylight. A light shelf can also be effective in blocking direct sun at certain times of the year and day.

Light shelves

Toplighting is any daylighting delivered from the ceiling plane. This location for daylighting provides the potential for even distribution of daylight throughout a larger space and integrates well with electric lighting. Wall washing is possible with toplighting. Care in the design and placement of skylights is needed to avoid the problems of glare, excessive heat gain, and harsh contrast from direct sunlight. If toplighting is designed using deep wells and/or diffuse materials, these problems will be reduced. Sawtooth ceilings, light monitors, and north - facing clerestory windows, which were all popular in industrial settings a century ago, are effective ways to harvest daylight.

The following are Strategies and principles for effective daylight in interior spaces:
  • Collaborate early with design team members to maximize building features that support daylighting.
  • Provide soft, uniform light throughout the space.
  • Provide thermal barriers for the windows to reduce heat gain or loss during unoccupied times.
  • Use HVAC (heating, ventilating, and air - conditioning) to compensate for the additional radiation during daylighting hours.
  • Provide glare - control and heat - gain shading systems.
  • Orient a worker ’ s sight line away from windows, preferably with daylight coming from the side of a person. Rear lighting may produce shadows on the work material.
  • Integrate automatic controls with a manual override for the shading system.
  • Provide control mehanisms that adjust electric illumination when adequate daylighting is available. These include: on/off system, continuous dimming, and step switching or step dimming for individual ballasts and lamps .
  • Use a closed - loop photosensor that reads electric light and daylight in preference to an open - loop sensor that reads only the daylight.
  • Use an advanced lighting system with electronic ballasts to supplement daylight to maximize energy savings.
In developing an integrated daylight and electric light strategy, light levels from daylight need to be higher in a space than comparable light levels from electric light. The footcandle perception of the two is not equal. The IESNA suggests a rule of thumb: add 1 lumen of electric lighting for the loss of 2 – 3 lumens of daylight. While maximizing daylight use is an environmentally responsible strategy, electric lighting is a necessary supplement. Understanding and integrating daylight with electric light is important for achieving energy - effective and efficient electric lighting.

Monday, December 2, 2019


There are mainly two types of R.C.C. footings:

1. One way reinforced footings.
2. Two way reinforced footings.

1. One Way Reinforced Footing: These footings are for the walls. In these footings main reinforcements are in the transverse direction of wall. In longitudinal directions there will be only nominal reinforcement.

2. Two Way Reinforced Footings: For columns two way reinforced footings are provided.
The following types of the footings are common:

(i) Isolated Column Footings: If separate footings are provided for each column, it is called isolated column footing. Figure 1 shows a typical isolated column footing. The size of footing is based on the area required to distribute the load of the columns safely over the soil . These footings are provided over a 100 to 150 mm bed concrete. Required reinforcements and thickness of footing are found by the design engineers. Thickness may be uniform or varying.

(ii) Combined Footings: Common footings may be provided for two columns. This type of footing is necessary when a column is very close to the boundary of the property and hence there is no scope to project footing much beyond the column face. Figure 2 shows a typical combined footing. The footing is to be designed for transferring loads from both columns safely to the soil. The two columns may or may not be connected by a strap beam.

(iii) Continuous Footings: If a footing is common to more than two columns in a row, it is called continuous footing. This type of footing is necessary, if the columns in a row are closer or if SBC of soil is low. Figure 3 shows this type of footing.

(iv) Mat Footing/Raft Footing: If the load on the column is quite high (Multistorey columns) or when the SBC of soil is low, the sizes of isolated columns may work out to be to such an extent that they overlap each other. In such situation a common footing may be provided to several columns as shown in Fig. 4 Such footings are known as raft footings. If the beams are provided in both directions over the footing slab for connecting columns, the raft foundations may be called as grid foundation also. The added advantage of such footing is, settlement is uniform and hence unnecessary stresses are not produced.

Wednesday, November 27, 2019


This type of foundations are commonly used for walls and masonry columns. These foundations are built after opening the trenches to required depth. Such footings are economical up to a maximum depth of 3 m. As these foundations are suitable depth, they are grouped under shallow foundations.

Figure 1 shows a conventional spread footing for a wall and Fig. 2 shows it for a masonry column.


Before building these footing trenches are opened to required depth and the soil is rammed well. Then a plain concrete of mix 1 : 4 : 8 is provided. Its thickness varies from 150 to 200 mm. Over this bed, stone masonry footing is built. It is built in courses each course projecting 50 to 75 mm from the top course and height of each course being 150 to 200 mm. In case of wall footing the projections are only one direction while in case of columns, they are in both directions. The projection of bed concrete from the lowest course of foundation masonry is usually 150 mm.

Friday, September 6, 2019

Dimensions of Foundation

Guidelines for minimum dimensions are given below:

(a) Depth of Foundation: For all types of foundations minimum depth required is calculated
using Rankine’s Formula:

However in any case it is not less than 0.9 m. Finding safe bearing of the soil is an expert’s job,
and it is found after conducting tests in field or in Laboratories. However general values for common
soils are listed in Table 1.

(b) Width of Foundation: Width of wall foundations or size of column footing is determined by
first calculating the expected load and then dividing that with SBC. Thus,

Friday, February 9, 2018

Plank floor

Plank floor

An alternative to the inverted T-beam is the pre-cast plank floor. These are reinforced lightweight concrete planks which sit side by side, supported as before by the internal leaf of blockwork. The planks are built into the blockwork at the sides as well as the ends and therefore restraint straps are not necessary in this instance.

Plank floor

Monday, September 25, 2017

Trust Me, I'm Engineer

Monday, September 11, 2017

Combined Rectangular Footing.

Fig. 3.14 shows a combined rectangular footing for two columns A and B carrying loads


W1 and W2, and spaced l centre to centre. If W' is the weight of the footing and qs is the safe bearing capacity, the footing area is given by

Suitable values of length L and breadth B of the footing are chosen, so that B x L = A. The longitudinal projections a1 and a2 should be so chosen that the C.G. of footing coincides with the C.G. of the two loads.

From the above, the projection a1 and a2 can be determined.

The net upward pressure p0 is given by

This net pressure intensity is used for structural analysis and design combined footing. A combined foundation may be either of reinforced cement concrete (R.C.C.)  or of steel grillage type.

(i) Combined rectangular footing of R.C.C. A rectangular footing of R.C.C consists of a reinforced concrete slab which is designed for both longitudinal  bending as well as transverse bending. If the distance between the columns. Typical details of a R.C. footing, without longitudinal beam, are shown in Fig. 3.15 Fig. 3.16 Shows typical details of rectangular footing, having longitudinal beam. The longitudinal beam may be provided either below the footing slab, or it may project above the slab.

(ii) Combined steel grillage rectangular footing. Such a footing is provided to support two steel stanchions.

The upper tier of steel joists receives the loads from the two columns and transfers the load to the lower tier. Fig. 3.17 shows typical details.

Monday, April 11, 2016

Structures: Folded Plate

The effect of folding on folded plates can be visualized with a sheet of paper. A flat paper deforms even under its own weight. Folding the paper adds strength and stiffness; yet under heavy load the folds may buckle. To secure the folds at both ends increases stability against buckling.

1. Flat paper deforms under its own weight
2. Folding paper increases strength and stiffness
3. Paper buckling under heavy load
4. Secured ends help resist buckling
Structures: Folded Plate

Friday, December 11, 2015

Vierendeels Configurations

Vierendeels may have various configurations, including one-way and two-way spans. One-way girders may be simply supported or continuous over more than two supports. They may be planar or prismatic with triangular or square profile for improved lateral load resistance.  Some highway pedestrian bridges are of the latter type.  A triangular cross-section has added stability, inherent in triangular geometry.  It could be integrated with bands of skylights on top of girders.

When supports are provided on all sides, Vierendeel frames of two-way or three-way spans are possible options.  They require less depth, can carry more load, have less deflection, and resist lateral load as well as gravity load.  The two-way option is well suited for orthogonal plans; the three-way option adapts better to plans based on triangles, hexagons, or free-form variations thereof.

Moment resistant space frames for multi-story or high-rise buildings may be considered a special case of the Vierendeel concept.

1  One-way planar Vierendeel girder
2  One-way prismatic Vierendeel girder of triangular cross-section
3  One-way prismatic Vierendeel girder of square cross-section
4  Two-way Vierendeel space frame
5  Three-way Vierendeel space frame
6  Multi-story Vierendeel space frame

Vierendeels Configurations

Monday, November 30, 2015

Joist, Beam, Girder

Joists, beams, and girders can be arranged in  three different configurations: joists supported by columns or walls1; joists supported by beams that are supported by columns2; and joists supported by beams, that  are supported by girders, that are supported by columns3.  The relationship between joist, beam, and girder can be either flush or layered framing.  Flush framing, with top of joists, beams, and girders flush with each other, requires less structural depth but may require additional depth for mechanical systems.  Layered framing allows the integration of mechanical systems. With main ducts running between beams and secondary ducts between joists.  Further, flush framing for steel requires more complex joining, with joists welded or bolted into the side of beams to support gravity load. Layered framing with joists on top of beams with simple connection to prevent displacement only

2  Single layer framing: joists supported directly by walls
3  Double layer framing: joists supported by beams and beams by columns
4  Triple layer framing: joists supported by beams, beams by girders, and girders by columns
5  Flush framing: top of joists and beams line up May require additional depth for mechanical ducts
6  Layered framing: joists rest on top of beams Simpler and less costly framing May have main ducts between beams, secondary ducts between joists

A Joists
B Beam
C Girders
D Wall
E Column
F Pilaster
G Concrete slab on corrugated steel deck

Joist, Beam, Girder

Thursday, October 15, 2015

Gerber Beam

The Gerber beam is named after its inventor, Gerber, a German engineering professor at Munich. The Gerber beam has hinges at inflection points to reduce bending moments, takes advantage of continuity, and allows settlements without secondary stresses.  The Gerber beam was developed in response to failures, caused by unequal foundation settlements in 19th century railroad bridges.

1.  Simple beams over three spans
2.  Reduced bending moment in continuous beam
3.  Failure of continuous beam due to unequal foundation settlement, causing the span to double and the moment to increase four times
4.  Gerber beam with hinges at inflection points minimizes bending moments and avoids failure due to unequal settlement
Gerber Beam

Monday, September 7, 2015


Optimizing long-span girders can save scares resources.  The following are a few conceptual options to optimize girders.  Optimization for a real project requires careful evaluation of alternate options, considering  interdisciplinary aspects along with purely structural ones.

1  Moment diagram, stepped to reflect required resistance along girder
2  Steel girder with plates welded on top of flanges for increased resistance
3  Steel girder with plates welded below flanges for increased resistance
4  Reinforced concrete girder with reinforcing bars staggered as required
5  Girder of parabolic shape, following the bending moment distribution
1  Girder of tapered shape, approximating bending moment distribution


Tuesday, June 16, 2015

Structures: Bending, Effect of Overhang

Bending moments can be greatly reduced, using the effect of overhangs.  This can be describe on the example of a beam but applies also to other bending members of horizontal, span subject to gravity load as well.  For a beam subject to uniform load with two overhangs, a ratio of overhangs to mid-span of 1:2.8 (or about 1/3) is optimal, with equal positive and negative bending moments.  This implies an efficient use of material because if the beam has a constant size – which is most common – the beam is used to full capacity on both, overhang and span.  Compared to the same beam with supports at both ends, the bending moment in a beam with two overhangs is about one sixth !  To a lesser degree, a single overhang has a similar effect. Thus, taking advantage of overhangs in a design may result in great savings and economy of resources.

1. Simple beam with end supports and uniform load
2. Cantilevers of about 1/3 the span equalize positive and negative bending moments and reduces them to about one sixth, compared to a beam of equal length and load with but with simple end support.

Structures: Bending, Effect of Overhang

Thursday, April 23, 2015

Portal Method For Rough Moment Frame Design

The Portal Method for rough moment frame design is based on these assumptions:

•  Lateral forces resisted by frame action
•  Inflection points at mid-height of columns
•  Inflection points at mid-span of beams
•  Column shear is based on tributary area
•  Overturn is resisted by exterior columns only

1.    Single moment frame (portal)
2.    Multistory moment frame
3.  Column shear is total shear V distributed proportional to tributary area:
4.   Column moment = column shear x height to inflection point
5.  Exterior columns resist most overturn, the portal method assumes they resist all
6.  Overturn moments per level are the sum of forces above the level times lever arm of each force to the column inflection point at the respective level:
7.  Beam shear = column axial force below beam minus column axial force above beam Level 1 beam shear:
Portal Method For Rough Moment Frame Design

Monday, March 16, 2015


Global moments help to analyze not only a beam but also truss, cable or arch. They all resist global moments by a couple F times lever arm d:
The force F is expressed as T (tension) and C (compression) for beam or truss, and H (horizontal reaction) for suspension cable or arch, forces are always defined by the global moment and lever arm of resisting couple.  For uniform load and simple support, the maximum moment M and maximum shear V are computed as:
For other load or support conditions use appropriate formulas


Beams resist the global moment by a force couple, with lever arm of 2/3 the beam depth d; resisted by top compression C and bottom tension T.


Trusses resist the global moment by a force couple and truss depth d as lever arm; with compression C in top chord and tension T in bottom chord.  Global shear is resisted by vertical and / or diagonal web bars. Maximum moment at mid-span causes maximum chord forces.  Maximum support shear causes maximum web bar forces.


Suspension cables resist the global moment by horizontal reaction with sag f as lever arm.  The horizontal reaction H, vertical reaction R, and maximum cable tension T form an equilibrium vector triangle; hence the maximum cable tension is:


Arches resist the global moment like a cable, but in compression instead of tension:
However, unlike cables, arches don’t adjust  their form for changing loads; hence, they assume bending under non-uniform load as product of funicular force and lever arm between funicular line and arch form (bending stress is substituted by conservative axial stress for approximate schematic design).