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# Building

Composite Construction

In a composite construction, a concrete slab and steel beams act together to resist the load acting on beam and thus the slab acts as a cover plate. Due to this we use a lighter steel section in composite construction

What are the Two Methods of Composite Construction in Buildings?
Method 1. In the first method, we cast the steel beam entirely in the concretion i.e steel beam encased in the concrete. The steel concrete bond is the main reason for composite action. The beam is laterally braced therefore the allowable stress in the flanges is 0.66Fy, where Fy is the yield strength, ksi (MPa), of the steel.

Fasteners in Buildings

According to AISC, the cross sectional area of of bolts and threaded parts are designed for specified allowable unit tension and shear stresses. In case of seismic loads and wind loads in combination of gravity loads, we increase the allowable stresses by 1/3. Normally in case of direct tension, rivets are not recommended.

Bearing type connections are the most commonly used connection is building construction. In buildings, the allowable bearing stress Fp, ksi (MPa), on projected area of fasteners is
Fp=1.2 Fu
where
Fuis the tensile strength of the connected part in ksi (MPa).

As per AISC guidelines, the combined effect of forces from moment and shear should be considered while designing fasteners or welds for end connections of girders, beams, and trusses. When flanges or moment connection plates for end connections of beams and girders are welded to the flange of an I- or H-shape column, a pair of column-web stiffeners having a combined cross-sectional area Ast not less than that calculated from the following equations must be provided whenever the calculated value of Astis positive:

Ast = Pbf-Fyctwc(tb+5K) / Fyst
where

Criteria for Buildings

The AISC specification for ASD for buildings places a limit on compressive stress in webs to prevent local web yielding. For a rolled beam, bearing stiffeners are required at a concentrated load if the stress f a , ksi (MPa), at the toe of the web fillet exceeds Fa= 0.66Fyw , where Fyw is the minimum specified yield stress of the web steel, ksi (MPa). In the calculation of the stressed area, the load may be assumed distributed over the distance.

For a concentrated load applied at a distance larger than the depth of the beam from the end of the beam:

Combined Axial Compression Or Tension And Bending

The AISC specification for allowable stress design for buildings includes three interaction formulas for combined axial compression and bending.

When the ratio of computed axial stress to allowable axial stress f /F a exceeds 0.15, both of the following equations must be satisfied:

( f a / F a ) + ( C m x f b x ) / (1– f a /F e x ) F b x + C m y f b y / (1 – f a / F e y ) F b y ? 1

f a / 0.60F y + f b x /F b x + f b y / F b y ? 1

when f a /F a ? 0.15, the following equation may be used instead of the preceding two:

f a / F a + f b x / F b x + f b y / F b y ? 1

In the preceding equations, subscripts x and y indicate the axis of bending about which the stress occurs, and

Deflections of Bents and Shear Walls

Horizontal deflections in the planes of bents and shear walls can be computed on the assumption that they act as cantilevers. Deflections of braced bents can be calculated
by the dummy-unit-load method or a matrix method. Deflections of rigid frames can be computed by adding the drifts of the stories, as determined by moment distribution
or a matrix method. Figure showing Building frame resists lateral forces with (a) wind bents or (g) shear walls or a combination of the two. Bents may be braced in any of several ways, including (b) X bracing, (c) K bracing, (d) inverted V bracing, (e) knee bracing, and (f) rigid connections.

Load Distribution To Bents And Shear Walls

Provision should be made for all structures to transmit lateral loads, such as those from wind, earthquakes, and traction and braking of vehicles, to foundations and their supports

Plate Girder In Buildings

For greatest resistance to bending, as much of a plate girder cross section as practicable should be concentrated in the flanges, at the greatest distance from the neutral axis. This might require, however, a web so thin that the girder would fail by web buckling before it reached its bending capacity.

To preclude this, the AISC specification limits h/t.

For an unstiffened web, this ratio should not exceed. where Fy = yield strength of compression flange,ksi (MPa).

Larger values of h/t may be used, however, if the web is stiffened at appropriate intervals.

Bearing Of Milled Surfaces

In building construction, allowable bearing stress for milled surfaces, including bearing stiffeners, and pins in reamed, drilled, or bored holes, is Fp= 0.90Fy, where Fy is the yield strength of the steel, ksi (MPa)

For expansion rollers and rockers, the allowable bearing stress, kip/linear in (kN/mm), is

Column Base Plates

The area A1, in2(mm2), required for a base plate under a column supported by concrete should be taken as the larger of the values calculated from the equation cited earlier, with R taken as the total column load, kip (kN), or

A1=R /0.70fc

Unless the projections of the plate beyond the column are small, the plate may be designed as a cantilever assumed to be fixed at the edges of a rectangle with sides equal to 0.80b and 0.95d, where b is the column flange width, in (mm), and d is the column depth, in (mm).

To minimize material requirements, the plate projections should be nearly equal. For this purpose, the plate length N, in (mm) (in the direction of d), may be taken as
N=(A1)½+0.5(0.95d–0.8b)