UNIVERSITY OF NAIROBI DEPARTMENT OF CIVIL ENGINEERING
MASTER OF SCIENCE IN CIVIL ENGINEERING (STRUCTURES)
CE 443: ADVANCED STRUCTURAL DESIGN ASSIGNMENT 1
BY
MUNYUA ADRIAN MUTHOMI F56/7509/2017
[email protected]
DECEMBER 2017
CONTENTS Assignment 1B...........................................................................................................................3 Question ................................................................................................................................. 3 Assessment and distribution of loads which the shear wall is required to support or is subjected to. ........................................................................................................................... 3 Load distribution: ................................................................................................................... 4 Permanent actions/Gravity loads: ...................................................................................... 4 Variable actions/Live loads: .............................................................................................. 4
Assignment 1B Question Do an assessment of the actions that need to be considered in a process of designing of a shear wall in a tall building and distribute these loads using a probabilistic approach. Definition A shear wall is a structural system composed of braced panels to counter the effects of lateral loads acting on a structure. In modern multi-storey buildings, lateral stability is provided by a system of frames and walls, or a combination of both. Determination of use of Shear walls/cores
Assessment and distribution of loads which the shear wall is required to support or is subjected to. Shear walls in tall building are subjected to both Primary and Secondary stresses. The primar y stresses include the gravity loads, live loads, wind loads and earthquake loads. From the above
primary stresses, secondary stresses are generated and exerted on the shear wall such as bending stresses, shear stresses and torsional stresses. Shear walls are continuous from the base so as to form a vertical cantilever. Its gravity dead load is sufficient to suppress the maximum tensile bending stress in the wall caused by lateral loads namely: wind and earthquake actions. Load distribution: Permanent actions/Gravity loads: These are attributed to the material the shear wall is made of dimensions and its unit weight. In this instance the shear wall is reinforced concrete and from EN 1991 1-1(Section 4) the unit weight is 25kN/m 3. Since the shear wall provides support to other structural members (roof slab, beams) their gravity loads will be included.
Variable actions/Live loads: They are determined by the use of the structure and are variable free actions. They are classified as fatigue actions (wind load), dynamic actions (earth quake actions).The conventional live loads o the structure will depend on its use. EN1991 Table 6.2 has the imposed loads on floors in the various building categories. In our case it is an office wit h a roof terrace. The roof terrace is used as a cafeteria thus an imposed load of 4kN/m 2 (EN1991 Table 6.1 category 3). Windload
The assessment follows EN1991-1-4-E. The wind action on the structure is dependent on several factors: terrain topography, ground roughness, exposure factor, adjacent structures, building height among others. The weight of the building has little effect on wind forces but resists uplift. When wind load is applied a full elastic response is expected. The location of our building is Nairobi. The average wind speed for a 50 year return period is 36m/s. For a probabilistic approach, 36m/s has a low probability of occurring due to the increase in new building construction that eventually lower the wind speeds. This design wind speed is conservative. Earthquakeload
The building location is in a non-seismic zone. In case an earthquake occurs the building (shear wall) ought to provide lateral stiffness to prevent failure. Dynamic actions are simplified to horizontal forces that are applied to the structure in proportion to its mass and height above ground as per Eurocode 8. The load distribution is in a triangular manner a nd is expressed as a percentage of gravity loads. Large earthquake give inelastic deformations. For probabilistic approach the partial factors of action are obtained from EN1990 Table A1.3 and Table A1.1 depending on how frequent a zone is prone to earthquakes.
Loadcombinations
Partial factors for the ultimate limit state, for the above actions on the shear wall the load combinations are obtained from (EN 1990 Table A1.2 (A) ). The recommended values when there are unfavorable conditions for the permanent actions is 1.35 while for the variable actions 1.50. There are 3 serviceability combination actions: characteris tic, frequent or quasi permanent. For the frequent and quasi permanent combination actions there are there a re factors for each of the
action. The factors are outlined in EN1991-1-1 depending on the building type. Serviceabilit y limit state the factor of the action is taken as 1.0 (EN 1990 Table A1.4)
