I am 1st year student of Civil Engineering so Please ignore my spilling and grammar mistakes,And inform me.If possible please appreciate me.

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.