What are the Ponding Considerations In Buildings?

Those flat roof which face a problem of water accumulation require stability analysis under ponding conditions. For checking the stability we need to see the following equations, if both of these are fulfilled

Cp+0.9Cs<= 0.25
C p= 32 Ls L4p/107Ip
Lp= Length of primarymember or girder in ft (m)
Ls= Length of secondary member or purlin ft (m)
S= Spacing of secondary members ft (m)
Ip= Moment of inertia of primary member in in4 mm4
Is= Moment of inertia of secondary member in in4 mm4
I d= Moment of inertia of steel deck supported on secondary members, in4/ft (mm4/m)

It should be noted that Is should be decreased by 15% for trusses and other open-web members. Also the value of the total bending stress due to ponding , dead loads and live loads should not exceed 0.80Fy(the minimum specified yield stress for the steel)

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What is the Shear on Connectors?

We take the smaller value out of these two conditions to calculate the total horizontal shear which is to be to be resisted by the shear connectors in building construction
Vh= 0.85fcAc/2
Vh=As Fy/2
Vh = Total horizontal shear, kip (kN), between maximum positive moment and each end of steel beams (or between point of maximum positive moment and point of contraflexure in continuous beam)
fc= Specified compressive strength of concrete at 28 days, ksi (MPa)
Ac = Actual area of effective concrete flange in inch2(mm2)
As = Area of steel beam in inch2(mm2)

In continuous composite construction, the total horizontal shear, kip (kN), between an interior support and each adjacent point of contraflexure should calculated by

Vh = AsrFyr/2

Asr= Area of longitudinal reinforcement at support within effective area in ich2(mm2 );
Fyr= Specified minimum yield stress of longitudinal reinforcement, ksi (MPa).

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What are the Number of Connectors Required for Building Construction?

Total number of connectors required to resist Vh is given by Vh/q
where q is the allowable shear for one connector, kip (kN) and its value is given in structural design guides.

Some keypoints should be considered
1. Required number of shear connectors should be placed uniformly in the section having zero to maximum moment.
2. Shear Connectors should have a minimum of 25.4mm (1inch) of concrete cover in all the directions.
3) Stud diameter should not exceed 2.5 times the beam-flange thickness. ( May increase only if studs are located directly over the web)

In case of heavy concentrated loads, uniform spacing may not sufficient, so number of shear connectors are calculated as
N2={N1[MB/Mmax-1]}/ (B-1)
M= Moment at concentrated load, ft-kip(kN-m)
Mmax= Maximum moment in span ft-kip(kN-m)
N1= Number of shear connectors required between Mmaxand zero moment
B= Str/Ss or Seff/Ss depending on condition.
Seff= Effective section modulus for partial composite action in inch3(mm3)

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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.

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
fs = MD/Ss + ML/Str <=0.66Fy
MD = Dead load moment, in-kip (kN-mm)
M L = Live load moment, in-kip (kN-mm)
Ss = Section modulus of steel beam in inch3(mm3)
Str = 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
fs = (MD + ML)/ Ss<=0.76Fy

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
fs = (MD + ML)/ Str<=0.66Fy

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

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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
Fuis 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

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