## Steel Design

**There are two stages in steel design:**

- Structural member design for adequate stiffness and strength
- Connection design of bolts, plates, connections, and anchors that assist to transmit forces

Frequently, it is not the self-weight of the steel that dictates the cost of the structure, but the steel fabrication and erection that govern the economical aspect of the design. Effective connection design is crucial to the cost of the structure due to the economical benefits in using heavier sections with simpler details than lighter sections with complicated connection details.

Steel members are elastic and perform elastically until its yield strength. They then deform plastically until they reach an ultimate strength.

Steel generally has low creep rates. However, steel has high creep rate in high temperature or when it is exposed to heat or fire.

Steel can also fail in fatigue.

Steel structures are designed to be ductile so they do not fail due to deflection caused by internal stresses.

Low-rise buildings are usually constructed with steel frames, either with portal frames or with bracings with walls. For portal frames, lateral wind loads are the governing loads. For braced bays, structural bracing members are usually hollow sections or angles and are usually used for vertical bracing. Single diagonal braces are designed to take both compression and tension. Cross diagonal braces inclined at 45 degrees are designed to take tension only. For horizontal bracing, triangulated floor bracing can be used in order to transmit lateral wind loads to adjacent vertical bracing systems.

## Steel: Column Design

teel structural members are designed to resist yielding, buckling, and rupture under ultimate forces. Steel columns are designed to resist direct compression where the vertical applied load is applied onto the neutral axis of the column, as well as, bending forces induced by lateral wind loads and some vertical applied loads that are applied eccentrically with respect to the neutral axis of the column.

We will be designing steel columns with two scenarios in mind, fully axial loaded columns and axial loaded columns with moments created eccentrically.

Compressive stresses depend on the slenderness ratio of the column.

We will be designing steel columns with two scenarios in mind, fully axial loaded columns and axial loaded columns with moments created eccentrically.

- Axial Loaded ColumnsSteel columns will be designed for axial compression forces and moment buckling forces. These columns can be braced, where they are supported by braced bays or core walls, or unbraced, where they are members in a portal frame. Often, bracings are used to reduce effective lengths of columns when lateral resistance for buckling is needed and when the steel column's compression resistance needs to be increased about the always weaker z-z axis. Note: It is always more economic to brace the weakest z-z axis to resist buckling.Columns can be designed for axial compressive force only when equal beams are supported on both sides of a column and when the loads are assumed to distribute evenly.
- Axially loaded Columns with Moment Created EccentricallyThis case is used when the column has been applied eccentric loads that create axial forces and bending moment forces. For scheme design purposes, we will only focus on the design of columns for axial compressive forces only.

**Scheme Design**Compressive stresses depend on the slenderness ratio of the column.

## Steel: Beam Design

Steel beams are to be designed to suit the following limit states:

The following are the typical minimum column section sizes for braced frames:

For beams, deflection is usually the most critical case for long spans and shear is usually the most critical case for beams with short spans and large loadings.

Allowable Deflection Limit = Live Load / 360

Total load = Span x Load width x Critical Load x ULS factors

Steel structural members are designed to resist yielding, buckling, and rupture under ultimate forces. Beams are designed to resist ULS bending and shear forces.

M = w x L

I = 0.5 x Ratio x K x L x M

where

M = maximum bending moment (kNm)

L = effective length of beam (m)

K = constant according to end supports and on bending moment diagrams. K values are also shown in the table below.

- Bending strength at ULS (including local buckling of flange or web, lateral torsional buckling, and plastic moment capacity)

Lateral torsional buckling is checked when extreme fibres at the top part of the neutral axis of the steel beam is undergoing compressive stresses due to downward loads onto the beam. The applied loads creates compression in the top flange of the beam, where it buckles, and tension in the bottom flange of the beam. Torsional stresses will then be created due to the web resisting the flange from buckling along its length, but during this process, inducing twisting deflections.

- Shear strength at ULS

The beam should be designed to resist shear forces parallel to the web of the beam.

- Web shear buckling

Web shear buckling needs to be checked when the beam has a slender web. In this case, the shear capacity will then be governed by web shear buckling. Stiffeners can be erected on the web to resist web shear buckling. Moreover, if transverse forces are present and distributed from flanges into webs, web buckling would need to be further checked.

- Deflection at SLS

**Sizing**

The following are the typical minimum column section sizes for braced frames:

- 203 UC = up to 2-3 storeys high
- 254 UC = up to 5 storeys high
- 305 UC = up to 8 storeys high or long spans
- 354 UC = 8-12 storeys high

**Scheme Design**

**1. Determine the live load on the beam**

**2. Find the allowable deflection based on unfactored imposed loads and check against maximum design deflections**

For beams, deflection is usually the most critical case for long spans and shear is usually the most critical case for beams with short spans and large loadings.

Allowable Deflection Limit = Live Load / 360

**3. Find total ULS loads on the beam**

Total load = Span x Load width x Critical Load x ULS factors

Steel structural members are designed to resist yielding, buckling, and rupture under ultimate forces. Beams are designed to resist ULS bending and shear forces.

**4. Determine support conditions of the beam and determine effective lengths, L, based on the support conditions**

5. Determine M, the maximum bending moment, and V, the maximum shear force5. Determine M, the maximum bending moment, and V, the maximum shear force

M = w x L

6. Determine the Second Moment of Inertia for the beam (where I has units of cm^4)6. Determine the Second Moment of Inertia for the beam (where I has units of cm^4)

I = 0.5 x Ratio x K x L x M

where

M = maximum bending moment (kNm)

L = effective length of beam (m)

K = constant according to end supports and on bending moment diagrams. K values are also shown in the table below.

7. Use the Tata Steel sections interactive 'blue book' to choose a beam section that has a second moment of inertia larger than the required second moment of inertia.