Kuldeep Singh - Answers

Hi.

First of all, it’s a common problem. This kind of erratic workmanship often happens and there are ways to correct it although none will guarantee perfection as much as properly aligned coloumns. The site supervisor should have not allowed this to happen in the first place but now there’s no choice left to him.

Firstly, it is bending moment development you must be worried about more than shear. If not taken care of the eccentricity would have given rise to additional bending moments especially at the critical locations adding up to bending stresses. But, don’t panic. All building codes provide a factor of safety against these things in colomn design though I don’t know the exact number in the Phillipino code. If it’s an eccentricity of a few inches or less than 5%, then you need not worry much and trust the contractor. He must be providing crank bars in the longitudinal reinforcement after bending the bars which is the most common technique. You can see it in the image below.

It would be very costly and also time consuming to replace the entire colomn. Especially, when the floor slab has been constructed it’s nearly impossible. If the eccentricity is large enough, you should get an expert opinion from an experienced engineer by taking him to site. There are other methods apart from crank bar too.

Also, the adjustment regarding crank bars also requires additional processes like drilling, putting additional shear reinforcement which I can’t exactly tell. You must talk to the contractor or engineers regarding that.

Lastly, don’t go willy-nilly with my answer only. It’s there so you get an idea of what’s going on. I haven’t seen the site so I have only limited information. Best would be to have an engineer look at it at the site.

A redundant truss is nothing but a truss with a degree of redundancy. A truss having redundancy means that there is at least one member in excess to what it needed to be just determinate and unstable.

In other words, any indeterminate truss is a redundant truss. There’s a common misconception that only the trusses having a member carrying zero axial force are redundant truss. But it’s not the case. Even if all members are load carrying, the truss can be a redundant truss if it is indeterminate.

Redundant trusses are the only kind of truss you’ll see though. It’s because of the simple reason that redundant trusses give us a margin for error against local damage. Suppose if the truss were determinate. Of one of the members breaks, the whole truss will collapse. But if the degree of redundancy is one, any one member can be taken out without structural collapse.

Chrome corundum bricks get a Ruby color (close to red) due to the hight content of chromium oxide as compared to alumina and other metals. If chromium oxide is present in a large amount and so are oxides of metals other than aluminium, the color will be a more deeper red. Else, it could be even grey or black depending on the composition.

Water tanks are still designed using the working stress approach because of the serviceability standpoint and unpredictability of load variations.

If we look closer, we’ll find that it’s actually impractical to design water tanks (especially overhead water tanks) using the Limit state method. What limit state method does is it assumes probabilities for magnitudes of loads. These probabilities are generally normally distributed and the maximum load is considered to be even exceeded 5% of the time. That is, the z factor is 1.64 for the maximum load or there is a 5% exceedance probability of the maximum load.

In the above graph, s represents the load the structure is subjected to and R represents the structure’s response. The red area is where the problem is when the load will be exceeded by the value the structure is designed for and the response will actually be insufficient.

The structures designed by the limit state method are those where even if the maximum load is achieved, it won’t last for long since it has a low probability (ends of the normal distribution). Now, can you say that this will always happen in the case of water tanks (especially overhead ones)? No it may not. Sometimes the water tank may be full or nearly full for a week! Maybe there’s a fault in the pipelines and water withdrawal from the tank is deliberately shut to prevent flooding! Maybe there’s a fire hazard or power plant accident and continuous water has to be supplied for a day making the tank run full! Now you might argue that no problem, we always design structures with a factor of safety and it’ll take care for the time being. But, since it’s an overhead water tank, even a small failure has drastically tragic consequences. So, serviceability is extremely important. Even a little thin hairline like crack is completely unwanted.

So, we can’t rely on a factor of safety alone and that too, for how long? Hence the design of water tanks is desirably conservative and deliberately by the working stress method assuming the tank runs full all the time just to be sure that there won’t be any accident or major deficiency or the need to restrict or partially restrict the usage of the water tank till the repair is done. The repair itself is undesirable.