like

3. CONCEPT OF DEMAND AND CAPACITY

4. CONCEPT OF CAPACITY AND
DEMAND
•  Demand
•  Demand on a structure refers to all external actions.
•  Gravity, air , earthquake, snow are external actions.
•  These actions when act on the structure will induce internal
• disturbance(s) or change in the structure in the form of stresses (such
• as compression, tension, bending, shear, and torsion).
• The internal stresses are also called load effects

5. AIR
SNOW
GRAVITY
GRAVITY
EARTHQUAKE
DEAD LOADLIVE LOAD

6. CONCEPT OF CAPACITY AND
DEMAND
•  Capacity
•  The overall ability of a structure to carry an imposed
• demand.
• Beam will resist the
• applied load up to its
• capacity and will fail
• when demand exceeds
• capacity
Demand

7. • FOR EXAMPLE YOU CAN BRING 3 TON OF WEIGHT,
AND SOMEONE SYAS TO BRING 2 TON WEIGHT
FROM OUTSIDE.SO IT IS A DEMAND FOR YOU
• YOUR CAPACITY IS GREATER THAN HIS DEMAND
AND YOU CAN BRING IT EASILY.

8. CAPACITY

9. CONCEPT OF CAPACITY AND
DEMAND
•  Failure
•  Occurs when Capacity is less than Demand. C<D
•  To avoid failure, capacity to demand ratio should be kept
• greater than one (C>D or C=D), or at least equal to one.
•  It is, however, intuitive to have some margin of safety i.e., to
• have capacity to demand ratio more than one. How much?

10. CONCEPT OF CAPACITY AND
DEMAND
•  Example 1
 Calculate demand
• the given concrete block of size 10″ × 10″ with 40 Tons load on
concrete block.

11. 10” 10”
40 TONS

12. CONCEPT OF CAPACITY AND
DEMAND
•  Example 1
•  Solution: Based on convenience either the loads or the load
• effects as demand are compared to the load carrying
• capacity of the structure in the relevant units.
Demand in the form of load:
Load = 40 Tons
Demand in the form of Load effects:
The effect of load on the pad will be
a compressive stress equal to load
divided by the area of the pad.
Load Effect=(40 × 2204)/10x10
Capacity of the pad in the form
of resistance should be able to
carry a stress of 881.6psi.
In other words, the compressive
strength of concrete pad
(capacity) should be more than
881.6psi (demand).

13. CONCEPT OF CAPACITY AND
DEMAND
•  Example 1
•  Solution: Based on convenience either the loads or the load
• effects as demand are compared to the load carrying
• capacity of the structure in the relevant units.
Demand in the form of load:
Load = 40 Tons
Demand in the form of Load effects:
The effect of load on the pad will be
a compressive stress equal to load
divided by the area of the pad.
Load Effect= (40 × 2204)/10x10
=881.6psi
Capacity of the pad in the form
of resistance should be able to
carry a stress of 881.6psi.
In other words, the compressive
strength of concrete pad
(capacity) should be more than
881.6psi (demand).

14. CONCEPT OF CAPACITY AND
DEMAND
•  Example 1
•  Determine capacity to demand ratio for the pad of example
• 1 for the following capacities given in the form of
• compressive strength of concrete (i) 550 psi (ii) 881.6 psi
• (iii) 1020 psi (iv) 3000 psi. Comment on the results?

15. CONCEPT OF CAPACITY AND
DEMAND
•  Example 2
•  Solution: As calculated in example 1 , demand = 881.6psi.
• Therefore capacity to demand ratios are as under:
• i. Capacity/ Demand = 550 / 881.6= 0.623 (Failure)
• ii. 881.6/ 881.6= 1.0 (Capacity just equal to Demand)
• iii. 1020/881.6 = 1.1569 (Capacity is 1.1569 times greater than Demand)
• iv. 3000/ 881.6 = 3.402(Capacity is 3.402 times greater than Demand)
•  In (iii) and (iv), there is some margin of safety normally called as
• factor of safety.

17. SAFETY FACTOR
•  It is always better to have a factor of safety in our designs.
•  It can be achieved easily if we fix the ratio of capacity to
• demand greater than, 1.0 say 1.5, 2.0 or so, as shown in
• Example 2.