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 0.66F_y, where F_y is the yield strength, ksi (MPa), of the steel.

We Assume that the full dead load is carried by steel where as the live load is carried by the composite section. Thus, the maximum unit stress, ksi (MPa), in steel is
f_s = M_D/S_s + M_L/S_tr <=0.66F_y
where
M_D = Dead load moment, in-kip (kN-mm)
M _L = Live load moment, in-kip (kN-mm)
S_s = Section modulus of steel beam in inch³(mm³)
S_tr = Section modulus of transformed composite section in inch³(mm³)

AISC also allows us to assume that both dead load and live load is carried by steel beam. In that case a higher stress in the steel is calculated as
f_s = (M_D + M_L)/ S_s<=0.76F_y

Method 2
In this method, shear connectors are used to connect the steel beam with the concrete slab. Ultimate load is the main factor considered in this method. The maximum stress in the bottom flange is
f_s = (M_D + M_L)/ S_tr<=0.66F_y

To find the transformed composite section, we have to bring in the neutral axis into consideration. We consider the concrete above the neutral axis as an equivalent steel area by dividing the concrete area by the ratio of modulus of elasticity of steel to that of the concrete( n).

A very small portion of concrete slab is considered effective in resisting the compressive flexural stresses while determining the transformed section. As per standards, the width of slab on either side of the effective beam centerline should not exceed any of the following conditions:

1. One-eighth of the beam span between centers of supports
2. Half the distance to the centerline of the adjacent beam
3. The distance from beam centerline to edge of slab

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 F_p, ksi (MPa), on projected area of fasteners is
F_p=1.2 F_u
where
F_uis the tensile strength of the connected part in ksi (MPa).

Points to be remembered
1. The distance in the line of force to the nearest edge of the connected part should be at least 1.5d.
2. The center to center spacing of fasteners should be at least 3d.
where d is the fastener diameter

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 A_st not less than that calculated from the following equations must be provided whenever the calculated value of A_stis positive:

A_st = P_bf-F_yct_wc(t_b+5K) / F_yst
where
F_yc = Column yield stress, ksi (MPa)
F_yst= Stiffener yield stress, ksi (MPa)
K= Distance, in (mm), between outer face of column flange and web toe of its fillet, if column is rolled shape, or equivalent distance if column is welded shape
P_bf= Computed force, kip (kN), delivered by flange of moment-connection plate multi plied by 5/3 , when computed force is due to live and dead load only, or by 4/3, when computed force is due to live and dead load in conjunction with wind or earthquake forces
t_wc= Thickness of column web, in (mm)
t_b= Thickness of flange or moment-connection plate delivering concentrated force, in (mm)

Notwithstanding the preceding requirements, a stiffener or a pair of stiffeners must be provided opposite the beam compression flange when the column-web depth clear of fillets d_cis greater than

d_c=( 4100t³_wc(F_yc)^½)/P_bf

and a pair of stiffeners should be provided opposite the tension flange when the thickness of the column flange t_f is less than

t_f=0.4(P_bf)^½/F_yc

Stiffeners required by the preceding equations should comply with the following additional criteria:

1. The width of each stiffener plus half the thickness of the column web should not be less than one-third the width of the flange or moment-connection plate delivering the concentrated force.

2. The thickness of stiffeners should not be less than t_b/2.

3. The weld-joining stiffeners to the column web must be sized to carry the force in the stiffener caused by unbalanced moments on opposite sides of the column.

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 F_a= 0.66F_yw , where F_yw 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:

F_a=R/f_w(N+5K)
where
R=concentrated load of reaction, kip (kN)
t_w = web thickness, in (mm)
N=length of bearing, in (mm), (for end reaction, not less than k)
K=distance, in (mm), from outer face of flange to web toe of fillet

For a concentrated load applied close to the beam end:
f_a=R/t_w(N+2.5 k)

To prevent web crippling, the AISC specification requires that bearing stiffeners be provided on webs where concentrated loads occur when the compressive force exceeds R, kip (kN), computed from the following:

For a concentrated load applied at a distance from the beam end of at least d/2, where d is the depth of beam:

R=67.5t²_w[1+3(N/d)(t_w/t_f)^1.5](F_ywt_f/t_w)^½

where
t _f=flange thickness, in (mm)

For a concentrated load applied closer than d/2 from the beam end:

R=34r²_w[1+3(N/d)(t_w/t_f)^1.5](F_ywt_f/t_w)^½

If stiffeners are provided and extend at least one-half of the web, R need not be computed.

Another consideration is prevention of sidesway web buckling. The AISC specification requires bearing stiffeners when the compressive force from a concentrated load exceeds limits that depend on the relative slenderness of web and flange r_wf and whether or not the loaded flange is restrained against rotation:

r_wf=(d_c/t_w )/(l/b_f)

where
l=largest unbraced length, in (mm), along either top or bottom flange at point of application of load
b_f=flange width, in (mm)
d_c= web depth clear of fillets = d – 2k

Stiffeners are required if the concentrated load exceeds R, kip (kN), computed from

R=6800t³_w/h(1+0.4r³_wf)
where
h=clear distance, in (mm), between flanges, and r_wf
is less than 2.3 when the loaded flange is restrained against rotation. If the loaded flange is not restrained and r_wf is less than 1.7,

R=0.4r³_wf(6800t³_w/h)
R need not be computed for larger values of r_wf

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

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

F _a = axial stress that would be permitted if axial force alone existed, ksi (MPa)

F _b = compressive bending stress that would be permitted if bending moment alone existed, ksi (MPa)

F ^‘ _e = 149,000 / ( Kl _b / r _b ) ² , ksi (MPa); as for F _a , F _b , and 0.6F _y , F ^‘ _e may be increased one-third for wind and seismic loads

Lb = actual unbraced length in plane of bending, in (mm)

r _b = radius of gyration about bending axis, in (mm)

K = effective-length factor in plane of bending

f _a = computed axial stress, ksi (MPa)

f _b = computed compressive bending stress at point under consideration, ksi (MPa)

C _m = adjustment coefficient