25A Methods of design

(1) All buildings and parts of buildings shall be designed in accordance with these Regulations and wherever possible with methods of design which will admit of a rational analysis in accordance with the established principles of mechanics and of structural design.

(2) Elements not capable of rational design shall not be considered as contributing to the basic load resisting structure of a building but the effect of the design loads on them shall be assessed and allowed for both in their design and in the design of the building as a whole.

(3) Elements, such as partitions, or panels, assumed as not contributing to the basic load resisting structure shall have the effect of the design loads on them assessed and allowed for both in their design and in the design of the building as a whole.

25B Horizontal forces

(1) The total horizontal force at any level imposed by wind or earthquake loads shall be considered to be resisted by the various resisting elements in proportion to their rigidities considering the rigidity of the horizontal bracing systems or diaphragms as well as the rigidities of the vertical resisting elements.

(2) To such a degree as the purpose and design of the building permits, and particularly for tall buildings, the main resisting system shall be located symmetrically about the centre of mass of the building.

25C Buildings to be tied

(1) All parts of a building, unless specifically designed to act otherwise, shall be tied and interconnected by adequate fixings or integral construction, designed to resist wind or earthquake loads.

(2) Parts of a building likely to damage or shatter under earthquake loads shall be tied, secured or safeguarded so as to minimise danger to life.

25D Foundations to be interconnected

Individual foundations of a building shall be interconnected by members which can carry in 2 directions at right angles in tension or compression, and in addition to any other loading effects, a horizontal force equal to 10% of the maximum vertical load on the foundation under seismic conditions, or shall be restrained by other adequate means against differential lateral movement, provided that the building surveyor may vary the requirements of this regulation in case of—

• (a)any single storey building; or
• (b)any 2 storey building whose foundations are in virgin soapstone.

25E Building separation

(1) Provision shall be made for the relative movement due to lateral forces on buildings or parts of buildings not designed and constructed to be tied and interconnected.

(2) Each building not so designed in relation to its neighbour shall have a minimum clear space from the property boundary, other than that adjoining a public space, either of 3 times its computed deflection due to lateral force or of one-fourth inch for each 10 feet of height, whichever is larger, and in any case not less than one-half inch.

(3) Parts of buildings, or buildings on the same site, not so designed in relation to each other shall have a minimum clear space from each other, either of 3 times the sum of their computed deflections due to lateral force or of one-half inch for each 10 feet of height, whichever is the larger, and in any case not less than 1 inch.

(4) The separation distances may be computed and applied at each floor level. Deflections shall be computed as for the design lateral forces set out in these Regulations.

(5) Separation spaces need not extend into the foundations except where the building surveyor may direct. Separation spaces shall be clear of debris and detailed so as to remain clear. Space coverings shall be durable and allow 3-dimension movement. Compressible space fillings shall allow the required movement at all times.

25F Stability and stress reversal

(1) All buildings and parts of building shall be designed against the adverse effects arising from uplift or overturning.

(2) Where stability is dependent on gravity forces, only such live loads shall be considered as may safely be assumed to contribute to the stabilising force.

(3) Where a building or part of a building is dependent on gravity forces for stability, it shall be designed in that part to be stable when subject to 1.5 times the imposed load other than earthquake tending to cause instability. Seismic forces need not be multiplied by a load factor for stability calculations.

(4) A building or part of a building shall be designed to resist at least 1.5 times any imposed load before failure, from reversal of stress.

25G Design loads

(1) All buildings and parts of buildings shall be designed and constructed to support the loads acting or likely to act on the building or part thereof without exceeding the working stresses or design criteria, specified in these Regulations for the materials and methods of construction.

(2) The design loads shall be not less than those specified in these Regulations as dead loads or imposed loads in their most unfavourable combination or combinations; except that wind load, earthquake load or transient dynamic effects need not be taken as acting simultaneously.

(3) Provision shall be made for the loading combinations resulting from the method or sequence of construction.

25H Test loads

(1) Buildings or parts of buildings not fully amenable to rational design, may be designed on the basis of loading tests or model experiments.

(2) Loading tests shall demonstrate that the construction has adequate strength and stiffness. Details of the tests shall be as required by American Concrete Institute Standard 318–63 or such other standard, approved by the Board.

DEAD, LIVE AND IMPOSED LOADS

25I Interpretation

(1) Dead loads shall comprise the actual or estimated weights of all walls, fixed partitions, columns, floors, roofs, finishes and other permanent construction, and permanently fixed plant and fittings.

(2) Live loads shall comprise the loads assumed or known to result from the occupancy or use of a building and shall include loads on floors, loads on roofs other than wind, loads on balustrades and loads from movable goods and machinery.

(3) Imposed loads shall comprise the load or force due to live load, wind, earthquake, water, retained materials, or any load other than dead load.

25J Minimum live loads

(1) Minimum distributed live loads for particular occupancies and uses shall be as set out in Table 1 appearing in Schedule 3. These provide for normal effects of impact and acceleration but not for unusual concentrated loads, and shall be measured on plan.

(2) Slabs, ribbed slabs, or joisted floors, shall be designed for no less a total distributed load than that appropriate to a span of 8 feet except in the case of timber construction for loads up to and including 60 lb/sq ft.

(3) Beams shall be designed for no less a total distributed load than that appropriate to an area of 64 square feet except in the case of timber construction for loads up to and including 60 lb/sq ft.

(4) Floors to carry vehicles shall be designed for the appropriate distributed load, and also for the worst combination of actual wheel loads, for the class of vehicle of the occupancy.

(5) Extra heavy loads and goods in excess of 200 lb/sq ft shall be accurately assessed and the actual weights used as the basis of live load, with an appropriate reduction with wind or earthquake where the full load is not permanently distributed over the area.

25K Partitions

(1) Fixed partitions shall be part of the dead load.

(2) Movable partitions and future partitions shall be allowed for an equivalent uniformly distributed load per square foot of not less than 10% of the weight per foot run of the finished partition, and the whole of this load shall be added to the seismic live load.

25L Corridors, balconies, stairs and landings

(1) Corridors shall be designed for the same live loading as the floor or other space to which they give access.

(2) External balconies, other than fire escapes, shall be designed for 60 lb/sq ft, or for the same live loadings as the floor or other space from which they extend, whichever is the greater.

(3) Stairs and landings shall be designed for not less than the same live loadings as the floor or other space to which they give access, but with a maximum of 100 lb/sq ft; and there need be no increase for short span slabs or beams, except that independent cantilever steps shall be designed for a concentrated load of 300 1b at the free end.

25M Balustrades and parapets

(1) Balustrades and parapets shall be designed for the following horizontal live loads:

	(a)	Light access stairs, gangways and similar .........................	15 lb/ft run
	(b)	Stairs, landings and balconies in private houses and flats .........................	25 “
	(c)	All other stairs, landings and balconies and all parapets and handrails to roofs .........................	50 “

(2) The loads in subregulation (1) and those due to wind and earthquake need not act concurrently.

25N Moving live loads

Dynamic effects on buildings or parts of buildings from moving live loads, such as cranes, lifts and machinery, shall be provided for according to accepted design rules and relative Acts and regulations.

25O Reduction in live loads

The minimum live loads of Table 1 aforesaid may be reduced as follows; except in the case of workrooms, workshops, factories, warehouses, stores and garages other than car parking buildings—

• (a)For main beams the live load on a single span may be reduced by 5% for each 500 square feet supported, up to a maximum of 25%.
• (b)For columns, walls and their foundations live loads may be reduced in accordance with Table 2 appearing in Schedule 3 the percentage reduction being applied to all floors above the columns under consideration, or in accordance with the above beam load reduction, whichever is greater.

25P Actual distribution of loads

(1) The actual live loads of an occupancy shall on no occasion be distributed less favourably than the design load assumed.

(2) Where actual live loads are excessive, the building surveyor may require the owner or occupier to redistribute or lighten the load, and any neglect or default in complying immediately with such request served under the hand of the building surveyor shall constitute an offence against these Regulations.

25Q Live load with wind or earthquake

(1) When designing for earthquake forces the live load assumed to be contributing to the total seismic load, and also to the vertical load acting concurrently, shall be the appropriate reduced values of Table 1.

(2) For stability calculations the vertical live load to be taken with wind loading shall be 0.

(3) When such reduced live loads as provided in this clause are used, no further reduction shall be made for the loading on main beams, columns, walls and foundations.

25R Worst arrangement of live load

Live load shall be considered to vary from 0 to the full design live load on any part of a building, and the worst arrangement of live loads in combination with other loads on the various parts shall be considered in design.

25S Temperature loads

(1) Provision shall be made for loadings due to temperature changes.

(2) The normal atmospheric temperature range to be considered shall be from 60°–100° F, and consideration shall be given to shading, thermal capacity, contact with the ground, and direct heating by the sun.

(3) Expansion joints to minimise temperature loads shall be constructed with due consideration for the temperature at which they are made.

25T Loads on retaining walls, basement walls and floors

(1) The imposed loads caused by retained materials and the effects of ground water pressure and uplift shall be calculated according to accepted methods and to the approval of the building surveyor.

(2) Retaining walls shall have a factor of safety of at least 1.5 against overturning or sliding instability.

(3) Retaining walls and basement walls shall be designed for additional loads due to adjacent buildings or traffic.

(4) For adjacent footways the surcharge load shall be 200 lb/sq ft and for roadways the appropriate vehicle loading to the requirements of the building surveyor.

EARTHQUAKE PROVISIONS

25U Earthquake design

(1) Every building or structure or part thereof shall be designed and constructed to withstand stresses produced by earthquake forces as set out in these Regulations.

(2) The static forces given in these Regulations are assumed to represent the dynamic response of the structure by simulating the effects in shears, moments, and direct forces of earthquake ground motion.

(3) The forces shall be applied horizontally and simultaneously at each floor and roof level and may be assumed to act non-concurrently in the direction of each of the principal axes except that, in appropriate circumstances the building surveyor may require consideration in the direction of any other vulnerable axis.

(4) A more precise form of dynamic analysis may be required for special structures and may be accepted for any structure. Account need not be taken of more than the first 3 vibrational modes. The maximum value of the dynamic shearing forces may be assumed to be the square root of the sum of the squares of the modal forces. However, the maximum values of shearing forces and overturning moments used in design shall be not less than 80% of the values computed by static analysis using the coefficients given in these Regulations.

(5) The building surveyor may require greater provision to be made for earthquake effects where buildings exceed 200 ft in height or are of unusual construction or are situated on sub-soils likely to prove unstable or to substantially alter their characteristics in earthquakes.

25V Symbols and notations

The following symbols and notations are used in the next succeeding regulations—

C	=	basic seismic coefficient specified in regulation 25W and determined from Table 4 appearing in Schedule 3
C8	=	coefficient as specified in regulation 25X and determined from Table 3 appearing in Schedule 3
D	=	the dimension of the building in feet in a direction parallel to the applied forces
Ds	=	the plan dimension of the vertical lateral force resisting system in feet
Fi, Fx	=	lateral forces applied to a level i, or x respectively, in the direction under consideration
Fs	=	lateral forces on the part of the building in the direction under consideration specified in regulation 25X and determined from Table 3 aforesaid
Ft	=	that portion of the total lateral force V considered concentrated at the top of the structure at level n
hi, hn, hx	=	height in feet above the base to level i, n or x respectively
J	=	coefficient for base moment specified in regulation 25BB
Jx	=	coefficient for overturning moment at level x
Level i	=	level of the structure referred to by the subscript I
Level n	=	that level which is uppermost in the main portion of the structure
Level x	=	that level which is under design consideration
M	=	the overturning moment at the base of the building
Mx	=	the overturning moment at level x
N	=	total number of storeys above ground level
R	=	reduction factor specified in regulation 25W(2)
T	=	fundamental period of vibration of the building or structure in seconds in the direction under consideration
V	=	total lateral load or shear at the base in the direction under consideration
	=	Ft + n Fi
	=	i = 1
	=	where i = 1 designates the first level above the base
Wt	=	total seismic load which shall be equal to the total dead load plus the seismic live load determined from Table 1 aforesaid
	=	n W
	=	i = 1
	=	where i = 1 designates the first level above the base
Ws	=	seismic live load for part of a building determined in the same manner as Wt
Wi, wx	=	that portion of Wt which is located at or is assigned to the level designated as i or x respectively

25W Total lateral seismic forces

(1) Every building shall be designed and constructed to withstand a total lateral seismic force in each direction under consideration in accordance with the following formula—

V = C.W_t

(2) Except where stated otherwise, the value of C shall be determined from Table 4 aforesaid.

For buildings of 3 or more storeys founded on virgin soapstone the building surveyor may permit the value of C to reduction factor R where—

The reduced value of C shall not be less than 0.0333 for buildings nor less than 0.06 for elevated tanks plus full contents on 4 or more cross-braced legs not supported by a building. Where properly substantiated data is not submitted for establishing the period T, its value shall be determined by the formula—

unless the building has a moment-resisting space frame (which is capable of resisting 100% of the required lateral forces and which frame is not enclosed by or adjoined by more rigid elements which would tend to prevent the frame from resisting lateral forces) in which case the determining formula shall be—

T = 0.10N

(3) Nothing in these Regulations, however, shall prevent designers from adopting higher coefficients which may be more appropriate to particular structures or unusual site conditions.

25X Lateral forces on parts or portions of buildings or other structures

Parts of buildings or other structures and their anchorages shall be designed for lateral forces in accordance with the formula—where the values of C_s and the direction of the force are given in Table 3 aforesaid.

25Y Distribution of lateral force

Except for buildings of 1 and 2 storeys, the total lateral force V shall be distributed in the height of the structure in the following manner—F_t need not exceed 0.15V and may be considered as 0 for values (hn) of 3 or less, andAll 1 and 2 storey buildings shall have the total lateral force V distributed uniformly over the height of the building.

25Z Horizontal torsional moments

Provision shall be made for the increase in shear resulting from torsion due to an eccentricity between the centre of mass and the centre of rigidity. All elements likely to affect the centre of rigidity shall be taken into account. Negative torsional shear shall not be used to offset positive translational shear. Where the vertical resisting elements depend on diaphragm action for shear distribution at any level, the elements resisting the shear shall be capable of resisting an additional torsional moment assumed to be equivalent to the storey shear acting with an eccentricity determined by adding 5% of the maximum building dimension at that level to the computed eccentricity. Towers supporting elevated tanks shall also be designed for an accidental torsion of 5%.

25AA Interstorey deflection

(1) Interstorey deflection between 2 successive floors, measured parallel to the lower floor, shall be limited to 0.0025 of the storey height the calculations being based on the maximum lateral seismic force used in design. Such deflection shall be accepted up to 0.005 of the storey height where adequate clearances are provided for all non-structural parts.

(2) When partitions or other parts are structurally isolated from the basic load resisting structure, provision shall be made for at least 3 times the deflections computed for the design lateral forces.

25BB Overturning moment

Every building or structure shall be designed to resist the overturning effects caused by the wind forces and related requirements specified in these Regulations or the earthquake forces specified in these Regulations whichever governs, except that the axial loads from earthquake forces on vertical elements and footings in every building or structure may be modified in accordance with the following provisions—

• (a)The overturning moment, M, at the base of the building or structure shall be determined in accordance with the formula—

The value of J need not be more than 1.00. J shall be taken as 1.00 for elevated tanks plus full contents on 4 or more cross-braced legs not supported by a building.

• (b)The overturning moment, M_x, at any level designated as x shall be determined in accordance with the formula—

• (c)At any level the incremental changes of the design overturning moment, in the storey under consideration, shall be distributed to the various resisting elements in the same proportion as the distribution 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.
• (d)Where a vertical resisting element is discontinuous, the overturning moment carried by the lowest storey of that element shall be carried down as loads to the foundation.

25CC Set-backs

(1) Buildings with set-backs wherein the plan dimension of the tower in each direction is at least 75% of the corresponding plan dimension of the lower part may be considered as a uniform building without set-backs for the purpose of determining seismic forces.

(2) For other conditions of set-backs the tower shall be designed as a separate building using the larger of the seismic coefficients at the base of the tower determined by considering the tower as either a separate building for its own height or as part of the overall structure. The resulting total shear from the tower shall be applied at the top of the lower part of the building which shall be otherwise considered separately for its own height.

25DD Ductility requirements

(1) All elements within the structure which resist seismic forces or movements and the building as a whole shall be designed with consideration for adequate ductility.

(2) For buildings more than 160 feet in height, a ductile moment-resisting space frame shall be provided capable of resisting at least 25% of the total lateral force. Moment-resisting space frames and ductile moment-resisting space frames may be enclosed by or adjoined by more rigid elements which would tend to prevent the space frame from resisting lateral forces where it can be shown that the action or failure of the more rigid elements will not impair the vertical and lateral load-resisting ability of the space frame.

25EE Unreinforced masonry

(1) Except as provided in subregulation (2), unreinforced masonry shall not be used for buildings of more than 2 storeys or 25 feet eaves height.

(2) (a) Infill panels may be of unreinforced masonry where supported by either a steel, reinforced concrete or reinforced masonry frame itself able to resist at normal stresses the applied loadings.

• (b)Masonry partitions may be of unreinforced masonry.

WIND DESIGN

25FF Wind loads

(1) Every building or structure or part thereof shall be designed to withstand the wind loads set out in these Regulations.

(2) The total wind load F to be applied normally to a surface such as a wall, roof or other element shall be obtained from the following formula—

F = p.Cp A

• where p = wind pressure, lb/sq ft, from
• Table 5 appearing in Schedule 3;
• C_p = total differential pressure coefficient; and
• A = area of element under consideration, sq ft.

(3) The total wind load on a building shall be determined by the summation of the wind loads for each height increment listed in Table 5 aforesaid.

25GG Pressure coefficients

(1) Pressure coefficients for various shaped structures are set out in Table 6 appearing in Schedule 3. The total differential coefficient C_p to be used in regulation 25FF(2) is the algebraic difference of the external pressure coefficient C_pe and the internal pressure coefficient C_pi. Where the pressure coefficient C_n is used as for cylinders, spheres, bill boards and free standing walls the coefficient C_p shall be replaced by C_n in the formula in regulation 25FF(2). The value of q in these cases shall be taken as 35 lb/sq ft

(2) Where structures do not conform to the dimensions given in the diagrams of Table 6 aforesaid, the most appropriate case shall apply.

25HH Local effect of wind

(1) The pressure coefficients C_pe are average only and are exceeded locally. Table 6 aforesaid sets out the maximum local external pressure coefficients, C_pe, which are to be applied to the shaded areas drawn to scale on the sketches. Fastenings for wall and roof sheetings are to be capable of resisting these local pressures, with a minimum value of C_pe of, 1.5 anywhere on the roof or walls. Local forces need not be used in calculating the total force on the structure.

(2) For chimneys, masts and unusual structures the aerodynamic effect of wind shall be considered.