Structural design of beam is not a complicated topic. It is fairly easy to design beams once we establish the forces by a structural analysis and after structural analysis results are interpreted. However, there are many simpler points that many times entry level civil design engineers care less about. This blog discusses 7 such points. Some very simple points some a bit more important points that has more structural significance. A civil engineer beginning his career in structural engineering surely should know about these.
1. How to manage effective depth in structural design using ETABS
Entry level Engineers in civil and structural Engineering will have different kind of doubts since they are curious. When they are more determined, they look in to all minor details which they missed in the college education. Here is such a question which is very important to understand. The question is about the effective depth of beams.
The question is ‘’ I wanted to know more clearly the calculation of effective cover in module 1 of the structural design training’’
We at civilera, provides training in structural design of buildings and this is one of the questions asked by a student who is doing his civil engineering project.
This question is also about how to effectively use ETABS for designing the beams. I hope you know that in ETABS you have only one cover definition possible, one each for top and bottom reinforcement. You may have situations where your second layer rebar is curtailed. Then you have 2 effective covers. This also means two effective depth. One at the midspan where you have 2 layers and one away from the midspan where you have only one layer.
You can also have a situation where your beam has multiple spans and your steel in each span is different and the rebar diameter is different. This also create a situation where your effective depth is different in each span of the continuous beam.
Please see the video where I have explained how effectively this can be managed when you do structural analysis and design of beams. The video gives a better understanding of what I have explained here.
NOTE 1: The video has explanation of point 1 above. You have to read the blog for rest of the points
Note 2: Read the topic below on fire proofing which demands more cover and there by reduces the effective depth available.
2. Presence of slab in the model or the Flange action
Most students are unaware that the one-way slab and 2-way concepts are rule of thumbs though it provides very safe designs that will work. However, the reality is that the beam and slab interact, just like the beam and column does and the stiffness plays a role in load distribution. This can be captured in analysis by more accurate modelling. The moment in slab might increase and even the load distribution will be different from that of yield line theory distribution. The moments and shear in the beams might reduce due to the slab action.
As I mentioned for normal regular buildings, triangular and trapezoidal load distribution as per the yield line theory is fine enough. However, it is important to appreciate the fact that there is a better way to model and analyse by bringing in the beam and slab interaction based on stiffness. Based on the conditions like large cut-outs, shape, existence of line loads like that of walls, point load and so on, we need to be able to judge if we need to bring in the slab stiffness in to play.
3. Slenderness limit of beams
Slenderness is mostly associated with columns. However, there is a limit to slenderness of beams as well as per IS 456 Clause 23.3. The clause says that any beam should have a lateral restraint within 60 x b or 250b2/d. Here b is the width of the beam and d, the effective depth as mentioned in this blog initially. Let us consider an example to explain this. Let the beam width be 200 and the effective depth 500. Then the maximum length without restraint cannot exceed 12m. Lateral restraint can be a secondary beam entering this beam in the perpendicular direction or a column. We rarely will have a 12m beam. This may not be critical in many cases. However, the limit needs to be met as per the code if there is any long span beam. However for cantilever beams this restriction to slenderness is 25b or 100b2/d which ever is less.
Another similar and important point to note is that if the beam has 2 directional bending. I.e., normal bending + lateral bending, then it is not possible to design the beam in the same way we design a normal beam. That will be like a column bending in 2 direction. The design will be interactive in nature. What I mean is, we cannot design beams separately as 2 cases for bending in each direction. The reason is that the same steel and concrete is resisting either direction bending. Since the axial load on the beam is lesser, it may be difficult to consider it as a column as well. Then how will we design a beam if it has bending in vertical as well as lateral direction? We will discuss about this in another blog at another time.
4. Cover for Fire proofing
Most civil engineers know about the cover needed for beam reinforcement. The cover is decided by the durability requirement and IS 456 recommends a minimum cover of 20mm in mild conditions. However, this is just a durability requirement. If the fire resistance is required to be met structurally then the cover requirement drastically increases. Steel is very poor in resisting high temperature. National Building Code of India, also called as the NBC recommends a minimum fire rating for important buildings. So some times it becomes mandatory that the fire resistance be achieved structurally and not by a mechanical intervention like sprinklers. If that is the case, the cover needed will be based on Table 16A and Clause 21. Clause 21.3.1 also ask you to ensure adequate sacrificial rebars are provided if the cover is more. Please read both these IS 456 clauses and the table for complete understanding.
Please note that there are minimum width and depth specification as well which affect your structural scheme. For example, if you need 4 hours fire resistance, minimum width of the beam shall be 280mm as per Fig 1 IS 456. Same way minimum dimension of column has to be 450mm.
Now this increase in cover also reduces the effective depth that we discussed in section 1 and there by the steel needed in the beam also will increase. Before deciding the size of the structural members, for important structures like hospitals, we need to keep both this in mind. Else we end up struggling to meet the agreed member sizes which will force you to compromise on cover, sizes and fire resistance.
5. Side face reinforcement
When the beam web depth is more than 750mm, you need side face rebars along both faces. This is a kind of minimum steel that ensures the exposed side faces don’t crack up due to absence of any rebars. Clause 26.5.1.3 of IS 456 speaks about this requirement and please read this for better understanding.
6. Torsion in beams
Torsion in beams is a topic that needs a careful consideration, both for safe and economic design. The main point is about identifying equilibrium torsion and compatibility torsion. Many blindly release torsion with out knowing what is release of torsion. Another set of people blindly ask you to consider torsion and not to release it. The reality is the basic principle of release and designing by IS 456 is similar. I have explained the entire things that you need to know about torsion in beams in another structural blog here.
7. Openings in beams
Openings in beams is something that I generally don’t prefer. However, it is difficult to avoid them especially in Indian conditions. At least ensure there are no core cuts in site.
I have a dedicated blog on sleeves here. Please read that and you will have a good idea about the need of sleeves, why builders like them, what precautions you need to take and the detailing for sleeves.
If you have any questions on these points or if you simply want to join our email list,structural webinar list and groups, please feel free to post in our Forum section or contact us here. Please ensure you write ‘’structural doubt’’ as the subject and then also explain your question so that we understand and get back to you with a structural solution for the doubt.
Summary
While knowing to design a beam in structural software is important, there are many more significant points that needs careful considerations to meet all the structural design requirement. Some of the points if unattended, will create immediate issues while some others create issues in the long term and yet another few thing prove to be serious in case of accidents. I believe our success lies in identifying these and then ensuring correct design without compromising any of the code provisions.