Pakistan [
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Distribution of Lateral Forces
where-------------- (a)
Z = numerical coefficient dependent upon the zone as determined by Figure 1-1 to 1-5. (See Fig.1-1) (See Fig.1-2) (See Fig.1-3) (See Fig.1-4) (See Fig.1-5)
I = occupancy importance factor as set forth in Table 1. (See Table1)
K = horizontal face factor in Table 2. (See Table2)
C = base shear coefficient
S = numerical coefficient for site structure resonance
W = total dead load including the partition loading where applicable.
Exception: W shall be equal to the total dead load plus 25 percent of the floor live load in storage and warehouse occupancies. Where the design snow load is 1.44 kN/sq.m (30 lbs/sq.ft) or less, no part need be included in the value of W. where the snow load is greater than 1.44 kN/sq.m (30 lb/sq.ft), the snow load shall be included; however, where the snow load duration warrants, the building official may allow the snow load to be reduced up to 75 percent.
The value of K shall be not less than that set forth in Table2. The value of C and S are as indicated hereafter except that the product of CS need not exceed 0.14.
The value of C shall be determined in accordance with the following formula:
-------------------------
(b)T = fundamental elastic period of vibration of the building or structure in seconds in the direction under consideration.
The Value of C need not exceed 0.12.
The period T shall be established using the structural properties and deformational characteristics of the resisting elements in a properly substantiated analysis.
In the absence of a determination as indicated above, the value of T for buildings may be determined by the following formula:
---------------- (c)Where hn and D are in metres
or
---------------- (c)Where hn and D are in feet.
Or in buildings in which the lateral force-resisting system consists of ductile moment-resisting space frames capable of resisting 100 percent of the required lateral forces and such system is not enclosed by or adjoined by more rigid elements tending to prevent the frame from resisting lateral forces:
T = 0.10N ----------------- (d)
The value of S shall be determined by the following formulae, but shall be not less than 1.0:
------------ (e)
----- (f)Where:
T in Formulae (e) and (f) shall be established by a properly substantiated analysis but T shall be not less than 0.3 second.
Ts = characteristic site period.
When Ts is not properly established, the value of S shall be 1.5.
Exception: Where T has been established by a properly substantiated analysis and exceeds 2.5 seconds, the value of S may be determined by assuming a value of 2.5 seconds for Ts.
1.1 Structures having Regular Shapes or Framing Systems
The total lateral force V shall be destributed over the height of the structure in accordance with Formulas g, h and i.
-------------- (g)V = the total lateral force or shear at the base.
Ft = that portion of V considered concentrated at the top of the structure in addition to Fn.
Fi, Fn = lateral force applied to level i or n respectively.
Level i
i = i level of structure referred to by the subscript i
i = 1designates the first level above the base.
Level n = that level which is uppermost in the main portion of the structure.
The concentrated force at the top shall be determined according to the following formula:
T = fundamental elastic period of vibration of the building or structure in seconds in the direction under consideration.
V = the total lateral force or shear at the base.
Ft need not exceed 0.25V and may be considered as 0 where T is 0.7 second or less. The remaining portion of the total base shear V shall be distributed over the height of the structure including level n according to the following formula:
Fx = lateral force applied to level x respectively.
Wi, Wx = that portion of W which is lacated at or is assigned to level i or x respectively.
hi, hx = height in m ( ft ) above the base to level i or x respectively.
Level x = the level which is under design consideration.
x = 1 designates the first level above the base.
At each level designated as x, the force Fx shall be applied over the area of the building in accordance with the mass distribution on that level.
1.2 Setbacks
Buildings having setbacks where in the plan dimension of the tower in each direction is at least 75 percent of the corresponding plan dimension of the without setbacks, provided other irregularities as defined in this clause do not exist.
1.3 Structures having Irregular Shapes or Framing Systems
The distribution of the lateral forces in structures which have highly irregular shapes, large differences in lateral resistance or stiffness between adjacent stories, or other unusual structural features, shall be determined considering the dynamic characteristics of the structure.
1.4 Accidental Torsion
In addition to the horizontal torsional moments where the vertical resisting elements depend on diaphragm action for shear distribution at any level, the shear resisting elements shall be capable of resisting a torsional moment assumed to be equivalent to the storey shear acting with an eccentricity of not less than 5 percent of the maximum building dimension at that level.
1.5 Overturing
At any level the incremental changes of the design overturing moment, in the storey under consideration shall be distributed to the various resisting elements in the same proportion as the destribution of the shears in the resisting system. Where other vertical members are provided which are capable of partially resisting the overturning moments, a redistribution may be made to these members if framing members of sufficient strength and stiffness to transmit the required loads are provided.
Where a vertical resisting element is descontinuous, the overturning moment carried by the lowest storey of that element shall be carried down as loads to the foundation.
1.6 Lateral Force on Elements of Structures and Nonstructural Components
Parts or portions of structures, nonstructural components and their anchorage to the main structural system shall be designed for lateral forces in accordance with the following formula:
Fp = lateral force on a part of the structure and in the direction under consideration.
Wp = the weight of a portion of a structure or nonstructural component.
The values of Cp are setforth in Table 3 (see Table 3). The value of the I coefficient shall be the value used for the building.
EXCEPTIONS:
1. The value of I for entire connector assembly shall be 1.0.
2. The value of I for anchorage of machinery and equipment required for life safety systems shall be 1.5.
The distribution of these forces shall be according to the gravity loads pertaining thereto.
1.7 Alternate Determination and Distribution of Seismic Forces
Nothing in this section shall be seemed to prohibit the submission of properly substantiated technical data for establishing the lateral forces and distribution by dynamic analyses. In such analyses the dynamic characteristics of the structure must be considered.
1.8 Essential Facilities
Essential facilities are those structures or buildings which must be safe and usable for emergency purposes after an earthquake in order to preserve the health and safety of the general public. Such facilities shall include but not be limited to:
1. hospitals and other medical facilities having surgery or emergency treatment areas.
2. fire and police stations.
3. municipal government disaster operation and communication centres deemed to be vital in emergencies.
The design and detailing of equipment which must remain in place and be functional following a major earthquake shall be based upon the requirements of subsection 1.6 and Table3. In addition, their design and detailing shall consider effects induced by structure drifts of not less than (2.0/K) times the storey drift caused by required seismic forces nor less than the storey drift caused by wind.