concept of demand and capacity in civil engineering
TRANSCRIPT
CONCEPT OF DEMAND AND CAPACITY
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
AIR
SNOW
GRAVITY
GRAVITYEARTHQUAKE
DEAD LOADLIVE LOAD
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
• 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.
CAPACITY
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?
CONCEPT OF CAPACITY AND DEMAND
• Example 1 Calculate demand
• the given concrete block of size 10″ × 10″ with 40 Tons load on concrete block.
10” 10”
40 TONS
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 TonsDemand in the form of Load effects:The effect of load on the pad will bea compressive stress equal to loaddivided by the area of the pad.Load Effect=(40 × 2204)/10x10
Capacity of the pad in the formof resistance should be able tocarry a stress of 881.6psi.In other words, the compressivestrength of concrete pad(capacity) should be more than881.6psi (demand).
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 TonsDemand in the form of Load effects:The effect of load on the pad will bea compressive stress equal to loaddivided by the area of the pad.Load Effect= (40 × 2204)/10x10=881.6psi
Capacity of the pad in the formof resistance should be able tocarry a stress of 881.6psi.In other words, the compressivestrength of concrete pad(capacity) should be more than881.6psi (demand).
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?
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.
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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.
