Bangladesh national building code- BNBC 2020 Load Combination
Introduction to Load Combination
Buildings, foundations and structural members shall be investigated for adequate strength to resist the most unfavorable effect resulting from the various combinations of loads provided in BNBC 2020 (Section-2.7). BNBC 2020 Load Combination may be selected using the provisions for Allowable Stress/Strength Design Method (ASD/WSD) or Strength Design Method (USD) whichever is applicable. However, the Allowable Stress/Strength Design Method (ASD/WSD) or Strength Design Method (USD) is selected for particular construction material, it must be used exclusively for proportioning elements of that material throughout the structure.
Loads such as F, H, or S shall be considered in design when their effects are significant. Floor live loads shall not be considered where their inclusion results in lower stresses in the member under consideration. The most unfavorable effects from both wind and earthquake loads shall be considered where appropriate, but they need not be assumed to act simultaneously.
BNBC IS ASSOCIATED WITH DIFFERENT CODE
– AISC 360-05
– Eurocode 8
-Alternatively in software ASCE 7-05 using Appendix C of BNBC 2020
Load Combinations effects for Allowable Strength Design Method
Provisions of this Section shall apply to all construction materials permitting their use in proportioning structural members by allowable stress/strength design method. When this method is used in designing structural members, all loads listed herein shall be considered to act in the following combinations. The combination that produces the most unfavorable effect shall be used in the design.
Basic load combinations (Sec-2.7.2)
1. D + F
2. D + H + F + L + T
3. D + H + F + (Lr or R)
4. D + H + F + 0.75(L + T ) + (Lr or R)
5. D + H + F + (W or 0.7E)
6. D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or R)
7. 0.6D + W +H
8. 0.6D + 0.7E + H
D= Dead Loads, L= Normal Live Loads, Lr= Roof Live Loads, E= Earthquake Loads, W= Wind Loads (This is the most used loads in Bangladesh. Other symbols can be found in BNBC 2020- Volume-2, Chapter-2, Section-2.1.4 )
When a structure is located in a flood zone or in a tidal surge zone, the following load combinations shall be considered –
l. In Coastal Zones vulnerable to tidal surges, l.5 Fa shall be added to other loads in combinations (5 (6); E shall be set equal to zero in (5) and (6).
2. In non-coastal Zones, 0.75Fa shall be added to combinations (5), (6) and (7); E shall be set equal to zero in (5) and (6).
Load Combinations effects for Strength Design Method
When the strength design method is used, structural members and foundations shall be designed to have the strength not less than that required to resist the most unfavorable effect of the combinations of factored loads listed in the following Sections-
Basic combinations (Sec-2.7.3)
2. l.2(D + F + T) + l.6(L + H) + 0.5(Lr or R)
3. l.2D + l.6(Lr or R) + (L or 0.8W)
4. l.2D + l.6W + L + 0.5(Lr or R)
Each relevant strength limit state shall be investigated. Effects of one or more loads not acting shall be investigated. The most unfavorable effect from both wind and earthquake loads shall be investigated, where appropriate, but they need not be considered to act simultaneously.
l. The load factor on live load L in combinations (3), (4), and (5) is permitted to be reduced to 0.5 for all occupancies in which the minimum specified uniformly distributed live load is less than or equal to 5.0 kN/m2, with the exception of garages or areas occupied as places of public assembly.
Earthquake Load Effects and Load Combinations
The seismic load effect, E, shall be determined in accordance with the following:
1. For use in load combination 5 in Section 2.7.3 or load combination 5 and 6 in Section 2.7.2, E shall be determined in accordance with the following equation,
E = Eh + Ev
2. For use in load combination 7 in Section 2.7.3 or load combination 8 in Section 2.7.2, E shall be determined in accordance with the following equation,
E = Eh – Ev
E = Total seismic load effect
Eh = Effect of horizontal seismic forces as defined in Sections 2.5.7 or 2.5.9
Ev= Effect of vertical seismic forces as defined in Section 126.96.36.199
Horizontal earthquake loading, Eh
The horizontal seismic load effect, Eh, shall be taken as the horizontal load effect of seismic base shear V (Sec- 2.5.7 or 2.5.9)
Vertical earthquake loading, Ev
The maximum vertical ground acceleration shall be taken as 50 percent of the expected horizontal peak ground acceleration (PGA). The vertical seismic load effect Ev may be determined as:
Ev = 0.50(ah)D
ah= Expected Horizontal Peak Ground Acceleration (in g) for design= (2/3) ZS
D = effect of dead load,
S = site-dependent soil factor
Z= Zone Co-efficient
Serviceability Load Combination as per BNBC 2020 (Sec-2.7.5)
Serviceability limit states of buildings and structures shall be checked for the load combinations set forth in bnbc Section as well as mentioned elsewhere in Bangladesh national building code (bnbc) 2020. For serviceability limit states involving visually objectionable deformations, repairable cracking or other damage to interior finishes, and other short term effects, the suggested load combinations for checking vertical deflection due to gravity load is-
l. D + L
For serviceability limit states involving creep, settlement, or similar long-term or permanent effects, the suggested load combination is
The dead load effect, D, used in applying combinations l and 2 above may be that portion of the dead load that occurs following the attachment of nonstructural elements. In applying combination 2 above to account for long-term creep effect, the immediate (e.g. elastic) deflection may be multiplied by a creep factor ranging from l.5 to 2.0. Serviceability against gravity loads (vertical deflections) shall be checked against the limits of BNBC as well as mentioned elsewhere in BNBC.
For serviceability limit state against lateral deflection of buildings and structures due to wind effect, the following combination shall be used
Due to its transient nature, wind load need not be considered in analyzing the effects of creep or other long-term actions.
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