Can I erect a 9-inch wall over a 4-inch reinforced concrete slab?

I think you can design slab for taking load of 9" brick wall. First check that slab can take one way acting shear generated due to load. considering 9" wall weigh 0.54 t/sqm /m height and 3.0 m height of wall will have load of 0.54*3=1.62 t/m length. As load is transferred through two edges ,each edge will have shear force of 0.81 t/m. hence depending upon thickness of slab and % of steel the shear stress will be t_v if this is less than t_c then load transfer is o.k. Considering the line load on the slab identify effective width for calculation of B.M. per unit length and then design the slab.

However as in any other case since slab is supporting wall, as rightly pointed deflection will have to be checked so that additional deflection is not greater than span/350 or 20 mm whichever is lower.

Where is a gantry girder most used?

1.  The purpose of gantry Girder is to pick and move heavy machinery components within industrial building.
2. A girder is commonly used many times in the building of bridges.
3. Gantry girders are used in workshops/factories for movement of cranes which lifts material and shifts from one location to other.
4. Overhead travelling cranes are used in factories and workshops to lift heavy materials, equipment's, etc. and to carry them from one place to the other.

Why are newly constructed or ongoing construction buildings covered in green cloths?

The green cover provided around the building under construction prevents:

1. People who are passing by become cautious of a construction site around, can take care of safety measures required.
2. any dust or debris spreading out to the surrounding.
3. The accidental fall of debris / pieces to fall on the people who may be standing underneath or might be passing from the nearby walk ways.
4. from direct sunlight . It also maintain ventilation