<![CDATA[Front Desk Architects and Planners Forum - Structural Elements of Building ]]> https://frontdesk.co.in/forum/ Fri, 29 May 2026 15:50:59 +0000 MyBB <![CDATA[Length or Span to Depth Ratio for beam and slabs]]> https://frontdesk.co.in/forum/showthread.php?tid=3650 Wed, 20 Dec 2023 11:57:25 +0000 Manish Jain]]> https://frontdesk.co.in/forum/showthread.php?tid=3650 For RCC Structure 
According to clause 23.2.1 of IS 456-2000 

The vertical deflection limits may generally be assumed to be satisfied provided that the span to depth ratios are not greater than the values obtained as below:

a) Basic values of span to effective depth ratios for spans up to 10 m:

Cantilever    :    7
Simply supported  :   20
Continuous  :   26

b) For spans above 10 m, the values in (a) may be multiplied by 10/span in metres, except for cantilever in which case deflection calculations should be made.

c) Depending on the area and the stress of steel for tension reinforcement, the values in (a) or (b) shall be modified by multiplying with the modification factor obtained as per Fig. 4.

d) Depending on the area of compression reinforcement, the value of span to depth ratio he further modified by multiplying with the modification factor obtained as per Fig. 5.

For two way slab of shorter span (up to 3.5 m) with mild steel reinforcement, the span to overall depth ratios is given below may generally be assumed to satisfy vertical deflection limits for loading class up to 3 kN/m2 

Span                     span/depth ratio for Mild Steel    span/depth ratio for HYSD bars 
Simply Supported   35                                             28 
Continuous Slab     40                                             32


For Steel Structure : 

It is convenient to remember that serviceable steel section depths are in the range of ½” of depth for each foot of span (L/24). Some people might find it
easier to remember the following simplified rule where the length is expressed in feet and the depth of the member in inches:

Depth of Roof Beams, Roof Joists = 0.5*Length
Depth of Floor Beams, Floor Joists = 0.6*Length
Depth of Composite Beams = 0.55*Length]]> For RCC Structure 
According to clause 23.2.1 of IS 456-2000 

The vertical deflection limits may generally be assumed to be satisfied provided that the span to depth ratios are not greater than the values obtained as below:

a) Basic values of span to effective depth ratios for spans up to 10 m:

Cantilever    :    7
Simply supported  :   20
Continuous  :   26

b) For spans above 10 m, the values in (a) may be multiplied by 10/span in metres, except for cantilever in which case deflection calculations should be made.

c) Depending on the area and the stress of steel for tension reinforcement, the values in (a) or (b) shall be modified by multiplying with the modification factor obtained as per Fig. 4.

d) Depending on the area of compression reinforcement, the value of span to depth ratio he further modified by multiplying with the modification factor obtained as per Fig. 5.

For two way slab of shorter span (up to 3.5 m) with mild steel reinforcement, the span to overall depth ratios is given below may generally be assumed to satisfy vertical deflection limits for loading class up to 3 kN/m2 

Span                     span/depth ratio for Mild Steel    span/depth ratio for HYSD bars 
Simply Supported   35                                             28 
Continuous Slab     40                                             32


For Steel Structure : 

It is convenient to remember that serviceable steel section depths are in the range of ½” of depth for each foot of span (L/24). Some people might find it
easier to remember the following simplified rule where the length is expressed in feet and the depth of the member in inches:

Depth of Roof Beams, Roof Joists = 0.5*Length
Depth of Floor Beams, Floor Joists = 0.6*Length
Depth of Composite Beams = 0.55*Length]]> <![CDATA[Soil liquefaction during earthquake shaking]]> https://frontdesk.co.in/forum/showthread.php?tid=3194 Tue, 19 Oct 2021 11:12:08 +0000 Manish Jain]]> https://frontdesk.co.in/forum/showthread.php?tid=3194 What are the factors that can cause soil liquefaction at a site during earthquake shaking?

The factors that make a site vulnerable to liquefaction during strong earthquake shaking are:
(1) Soil stratum with poorly graded fine-grained cohesionless soil, like sand with most particles of same size,
(2) High ground water table, and
(3) Occurrence of an earthquake of large magnitude in the near vicinity of the site, and that too for a long duration.

When these conditions are met with, the solid soil suddenly becomes liquid soil. This is called soil liquefaction. In the liquid state, the Archimedes Principle comes into force – the weight of the liquid displaced by a body floating in a liquid is the weight of the floating body. This implies that when the said soil below a heavy object placed close to the ground surface or below a light object buried at large depths below ground surface, is shaken by an earthquake of large ground intensity for an extended period of time, the heavy object may sink and the light object is thrown upwards. Therefore, structures built on earth can sink, float, tilt or even collapse when soil underneath them liquefies. If the above factors are likely at a site, a competent Geotechnical Engineer should be consulted. She/He will examine the type of soil and the height of water table, and thereafter do the required calculations. 
If the calculations show that liquefaction is expected at the site, two options are available, namely:
(1) Strengthen the soil underneath, and
(2) Adopt pile foundations, if feasible.

Both of these options can be expensive, and may not give satisfactory results, if the earthquake shaking exceeds the design value. Thus, in general, it
is best to choose sites that do not liquefy during strong earthquake shaking. Further, the said liquefaction hazard affects the potential for landslides.
Also, when undertaking seismic microzonation, both liquefaction potential of a site and landslide hazard at that site should be integrated, when arriving at the land use zoning. This aspect should be addressed by the Municipal Authority.

for more details 
.pdf Simplified_Guidelines_for_earthquake_FD.pdf Size: 3.61 MB  Downloads: 1
]]> What are the factors that can cause soil liquefaction at a site during earthquake shaking?

The factors that make a site vulnerable to liquefaction during strong earthquake shaking are:
(1) Soil stratum with poorly graded fine-grained cohesionless soil, like sand with most particles of same size,
(2) High ground water table, and
(3) Occurrence of an earthquake of large magnitude in the near vicinity of the site, and that too for a long duration.

When these conditions are met with, the solid soil suddenly becomes liquid soil. This is called soil liquefaction. In the liquid state, the Archimedes Principle comes into force – the weight of the liquid displaced by a body floating in a liquid is the weight of the floating body. This implies that when the said soil below a heavy object placed close to the ground surface or below a light object buried at large depths below ground surface, is shaken by an earthquake of large ground intensity for an extended period of time, the heavy object may sink and the light object is thrown upwards. Therefore, structures built on earth can sink, float, tilt or even collapse when soil underneath them liquefies. If the above factors are likely at a site, a competent Geotechnical Engineer should be consulted. She/He will examine the type of soil and the height of water table, and thereafter do the required calculations. 
If the calculations show that liquefaction is expected at the site, two options are available, namely:
(1) Strengthen the soil underneath, and
(2) Adopt pile foundations, if feasible.

Both of these options can be expensive, and may not give satisfactory results, if the earthquake shaking exceeds the design value. Thus, in general, it
is best to choose sites that do not liquefy during strong earthquake shaking. Further, the said liquefaction hazard affects the potential for landslides.
Also, when undertaking seismic microzonation, both liquefaction potential of a site and landslide hazard at that site should be integrated, when arriving at the land use zoning. This aspect should be addressed by the Municipal Authority.

for more details 
.pdf Simplified_Guidelines_for_earthquake_FD.pdf Size: 3.61 MB  Downloads: 1
]]>