Expert: Arshad Khan - 5/3/2011

Question
QUESTION: Dir sir. I'm a just graduated civil engineer. I want to rebuild my family's home in KHARTOUM-SUDAN by the load bearing wall system. I've demolished the walls that were built from local bricks with a clay mortar till retching the grad beam (30*30cm with 4 bars 12mm) and stopped their; to rebuilt the walls on above the G.B. with cement mortar because we're willing to build a second floor above this. so I'm asking you sir about: that if I can construct another G.B. above the first one so I can rise the home's level and give it more strength to hold the building on.
Because I'm confused and don't want to lose more money.
The soil is firm, the depth of the foundation (Rocks) is 80cm below the ground level.

ANSWER: Hi Shehabaldin,

If the existing grade beam is still in good condition, and it is resting on firm rock at 80 cms depth, then you do not need to build another grade beam above that- all you need to do is build up the loadbearing blockwork to whatever level you need to go.

It will be better if you build a capping/ring beam at the top of your walls, which should connect with your roof slab or roof trusses, and with any columns at the corners. This will make the building more robust.

---------- FOLLOW-UP ----------

QUESTION:    How can I know that the G.B is in a good condition?
   What is a firm rock?
   What is a load bearing block work?
   What is a capping/ring beam?
   Should it be connected rigidly or semi rigidly?
   Do you mean by the roof trusses, the jack arch? Or else roof?
   With any columns at the corners!? explain, please.

Answer
Hi Shehabaldin,

1. You expose it and inspect the concrete for any deterioration and expose the rebar to see if it is corroded.

2. Firm rock, in your situation, would mean continuous rock for at least 1m depth below the grade beam and capable of bearing between 200 to 300 kN/m2 without much settlement.

3. Blockwork designed to carry loads from floors or roofs- should be at least 7 N/mm2 crushing strength at 28 days age.

4. An RC beam at the top of the blockwork that can also act as lintols over windows and doors.

5. Beam would just be cast on the top of the wall, possibly with some wall ties.

6. This can be any type of roof- could be timber beams or steel beams, or a hipped/sloping roof with tiles, or an RC slab.

7. If you have columns at the exterior corners of the walls, connecting your ground beam to the capping beam, this will make the whole house very robust and stable, especially against moderate earthquakes.