What is Load Combinations

As per IS 1893 (Part 1): 2002 Clause no. 6.3.1.2,
the following load cases have to be considered for
analysis:
1.5 (DL + IL)<>
1.2 (DL + IL ± EL)<>
1.5 (DL ± EL)<>
0.9 DL ± 1.5 EL<>
Earthquake load must be considered for +X, -X,
+Z and –Z directions. Moreover, accidental
eccentricity can be such that it causes clockwise
or anticlockwise moments. Thus, ±EL above
implies 8 cases, and in all, 25 cases as per Table 3
must be considered. It is possible to reduce the
load combinations to 13 instead of 25 by not
using negative torsion considering the symmetry
of the building. Since large amount of data is
difficult to handle manually, all 25-load
combinations are analysed using software.
For design of various building elements (beams or
columns), the design data may be collected from
computer output. Important design forces for
selected beams will be tabulated and shown
diagrammatically where needed. . In load
combinations involving Imposed Loads (IL), IS
1893 (Part 1): 2002 recommends 50% of the
imposed load to be considered for seismic weight
calculations. However, the authors are of the
opinion that the relaxation in the imposed load is
en conservative. This example therefore, considers
100% imposed loads in load combinations.
For above load combinations, analysis is
performed and results of deflections in each
storey and forces in various elements are
obtained.

Compression test for concrete, should test cubes or test cylinders be adopted?

Basically, the results of compression test carried out by using cubes are higher than that by cylinders. In compression test, the failure mode is in the form of tensile splitting induced by uniaxial compression. However, since the concrete samples tend to expand laterally under compression, the friction developed at the concrete-machine interface generates forces which apparently increase the compressive strength of concrete.

 However, when the ratio of height to width of sample increases, the effect of shear on compressive strength becomes smaller. This explains why the results of compression test by cylinders are lower than that of cubes. Reference is made to Longman Scientific and Technical (1987).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

When Things Go Wrong Structural Stress and Fatigue

When a structure is poorly designed or built, it may not be able to withstand all of the forces it has to face. When a structure has to face large

combinations of internal and external forces over a long period of time, the structure might weaken. This may result in structural stress. At first, signs of structural stress may disappear when the internal and external forces are reduced. For example, if you place an abnormally large book on the middle of a bookshelf, the shelf might bend. The

bend in the shelf is a sign of stress. When the book is removed,

the shelf may go back to its original shape. However, if the

shelf cannot withstand the stress, it might crack. Permanent

changes, like the bookshelf cracking, are signs of structural