A Critique Of 1/360

 

 

Consider the following discussion concerning the inability to accurately calculate and predict how much a reinforced concrete beam will sag or deflect when placed under a given load.           

Although the research engineer in the laboratory is able to carry out carefully controlled loading tests in which measured instantaneous elastic deflections are within 20 or 30 percent of those predicted by empirical equations for deflection, the practicing engineer must expect deviations greater than 30 percent between predicted and measured deflections, constructed under actual field conditions. Deflections are minimized when beams are carefully constructed out of high-strength, low slump concretes that are well compacted and effectively cured. In the field the engineer has a certain limited control over construction methods and procedures by means of the plans and specifications covering the design of concrete mix and details of placing steel and concrete; however what the designer specifies what the construction crews produce can differ widely. Water content may be increased at the job site, incomplete compaction may leave voids and honeycombing, and reinforcing bars may be improperly positioned. By reducing the quality of the concrete, these and other construction procedures can produce members that will undergo larger than expected deflections.

Consider what the American Concrete Institute (ACI) says about the relation between the magnitude of actual deflection and the calculated deflection in concrete beams.

It should be emphasized that the magnitude of actual deflection in concrete structural elements, particularly in buildings, which are the emphasis and the intent of this Report, can only be estimated within a range of 20 – 40 percent accuracy. This is because of the large variability in the properties of the constituent materials of these elements and the quality control exercised in their construction. Therefore, for practical considerations, the computed deflection values… ought to be interpreted within this variability.        

This statement is from ACI 435R-95 Control of Deflection in Concrete Structures. Reported by ACI Committee 435. American Concrete Institute Box 19150 Redford Station, Detroit Michigan 48219  

Why are these calculations poor predictors of performance for concrete beams?

There are several things to keep in mind. The purpose of any design method is not to predict future performance. It is to design to

 

Why a critique of 1/360?

You will sometimes hear a reference to 1/360 coupled with a statement that the foundation is bending excessively or is “out of tolerance.” What I will argue is that the misuse of 1/360 show you  ,e

 

What do I mean by 1/360?

To ,answer this question Many beams are designed to limit deflection to some specific value.

As & After the concrete is placed

Once the concrete is placed there is no easy way to continue to agitate the mix to make sure that the concrete is more or less consolidated and homogeneous.

Ideally, the finishers can and should use a vibrator, especially in the perimeter grade beams and the stiffening beams. I have occasionally seen this done for the perimeter grade beams, but not very often. What I have seen is a finisher use a sledgehammer to tap the perimeter as he walks around the foundation.

I have never seen this done for the interior stiffening beams.    

The reason that the concrete filling the stiffening beam trenches and the perimeter grade beams need to be consolidated is to assure there are minimal voids and sand pockets in the concrete mix. Irregularities such as sand pockets and void areas can result in grade beams and stiffening beams that are not nearly as stiff as the design intended.  

 

Print Friendly, PDF & Email