Size of beams, slab load distribution and beam layout plan

Part-3: Beam grid

After placing the column position and orientation, the next step is to draw a beam grid and to define the slabs on all floors and submit the plan to the architect for approval.

Size of beam

Before drawing the beam grid plan, let's look at some standard size of beams.

If we use a steel beam then there are some standard sizes because we directly purchase it by the manufacturer and casting steel beam on site is too costly. 

In the case of the RCC beam, we can cast the required size of the beam, although there is some common size of beams that are used in the construction of RCC buildings.

There is no IS code provision for any standard size of the RCC beam. 

The supporting formwork of the beam at the bottom is locally known as "beam bottom" in India. That beam bottom is generally 3 inches thick and 12 or 9 inch wide, made from timber, and used in minor RCC construction project.

Another formwork which used to support by both the side of the beam is called "side" locally in India. the height of the side is varies made from plywood.

We generally change the beam depth instead of beam width if the beam is overloaded, that's why we use a 9 or 12-inch thick beam. 

If we use an 11-inch thick beam, then it could be costly as well as not easy to understand for local labors and contractors.

Following are some commonly used size of beam in inch,

• 9 X 9,
• 9 X 12,
• 9 X 15,
• 9 X 18,
• 9 X 21,
• 9 X 24, etc.,

Beam grid

Draw a beam-grid like below plan in all the floors according to the wall arrangements and column position.

First-floor beam grid

Second-floor beam grid

    

Terrace beam grid

Do the numbering in, start from horizontal to vertical and also specify the length of all beam. 

You can specify the length of beams from center to center. I specify from out to in distance.

I have assumed the size of 9 x 18 inches for now, which will be modified according to load condition.

Specify that the heavy beams will support the light beams. see the first-floor beam grid plan, B-22 is resting on B-4 and B-6. which means the reinforcement of B-4 and B-6 will higher than B-22 and size also can be higher.

There are cantilever beams BC-13, BC-17, and BC-24. BC means cantilever bracket.

All beams are subjected to universal distributed load (UDL) and some beams are subjected to UDL as well as a point load. let's understand this in-depth

Point load

For understanding the loads in the beam, let separate the B-4 from the first floor. see the below image.

Point load

 

B-4 is resting on C-2 and C-3. B-22 is resting on B-4 which means B-22 is create the point load in the B-4.

The length of B-4 is 18 feet and 6 inches and the distance of point load in B-4 is 13 feet 1.5 inch. 

 

The total load on beam depends upon how much the load coming from slabs and how much the load coming on beam from non-structural members like walls, etc., and how much the point loads are acting on that beam.

Slab

After defining the beam grid, the next step is to draw slabs on the plan with proper labeling.

Slab and beam plan for the first floor

Above is a complete plan containing columns, beams, and slab, but there will be so many changes that could occur in sizes after getting the load calculation and analysis report.

Define slab label horizontal to vertical and set the type of slab.

Types of slab

There are two types of slab based on supporting conditions. one-way slab and two-way slab. 

The reinforcement guidelines are different in both one-way and two-way spanning slab.

One-way slab

If the slab is supported only in two opposite edges, it is called a one-way slab.

If the longer span to shorter span ratio (ly/lx) is equal or greater than two, then it is defined as a one-way slab. 

from the above plan, we have S-1, S-2, and S-6 as a one-way slab. 

For example, S-1 has a longer span of 16 feet and 9 inches and a shorter span of 3 feet and the ratio is 5.58 which is greater than 2.

Two-way slab

If the slab is supported in all the four edges and the longer span (ly) to shorter span (lx) ration is less than 2 then, it is called a two-way slab.

The two-way slab can be bend in both the direction. that's why we provide the main reinforcement in both directions in a two-way slab.

Slab load distribution on beam

Now, let's study how the loads are distributed from slab to beam.

Slab load distribution

As shown in the image above, the loads are transferred in a different manner in opposite edges of slabs.

Two-way slabs have two load transfer conditions shape-wise, one is trapezoidal, and the second is triangular. the supporting beams could have a different load value.

For example, let's consider the S-3 slab. S-3 are resting on B-3, B-19, B-15, and B-5. Beams B-3 and B-5 will take only the trapezoidal load of the slab and B-19 and B-15 beams will take only the triangular load of the S-3 slab.

We can see that the trapezoidal load is higher than the triangular loads.

The one-way slab has only a rectangular load as shown in the image. one-way slabs are supporting only two opposite beams.

For example, S-1 is the one-way slab. surrounding beams are BC-14, BC-18, B-1, and B-3. the load will transfer form S-1 is only in B-1 and B-3 only.BC-18 and BC-14 will get zero force from the S-1, also BC-18 and BC-14 will not support the S-1 slab.

The one-way slab has a tendency to bend only in one direction that's why we provide main bars in only one direction. 

Now, we placed the slabs, beams, and columns for our convenience. we need to check whether these arrangements of members are structurally safe or not.

For that, we need to do the calculation and the analysis. our next step will be to design the beam.

The discussion is open in the comment section. 

  

 

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