structural engineering basics

STRUCTURAL ENGINEERING BASICS

Worth Sharing !

Why structural engineering basics are important ?

A structural engineer should know all the basics and these learnings are the guidelines for structure stability, strength and safety point of view.

Here are the List of all the basic codes of Structural Engineering

  1. Code of practice for Reinforced cement concrete – IS456 -2000
  2. Steel structures – IS 800 -1984 – Working stress method
  3. Latest steel code – IS 800 -2007 – Limit state method
  4. Regular Loads – IS 875
  5. Wind Loads – IS 875 – part 3
  6. Seismic Loads – IS 1893 part 1 is latest in 2016
  7. Detailing Code – SP 34
  8. Design aids for Reinforced concrete – SP 16
  9. Design aids for Steel trusses – Sp 38
  10. Codes for Water retaining structures -IS 3370 part 1 and 2
  11. ACI – American concrete institute
  12. ASTM – American society for testing and Materials.
  13. AISC – American institute of steel construction

MYLEARNINGS IN STRUCTURAL DESIGN

Structural steel mostly involves Rolled Steel sections and built up sections. All steel sections confirm to IS:2062 and design rules are followed as per IS 800/AISC. Various steel sections available in the marked are

ISMB sections, Channels, angles, Jindal sections, W-sections etc. The steel sections are connected by either weld/bolts/rivets. Basic ISMB section consists of web and Flanges. Web takes shear stresses.

The slenderness ratios (L/r) of various sections are as below where ‘L’ is effective length and ‘r’ is the radius of gyration.

  • For member carrying compressive loads -180
  • Tension member for loads other than wind/seismic -180
  • Compression force member from either wind/earthquake loads -250
  • Pure Tension Member – 400

PERMISSIBLE STRESSES

  • Axial tension = 0.6 fy (fy is the yield stress of steel in N/sqmm or MPA)
  • Bending tension or compression – 0.66 fy
  • Bearing stress – 0.75 fy
  • Shear stress – 0.45 fy

All the permissible stress shall be referred from respective codes and these stresses can be increased by 33 % when effect of wind or earthquake are in consideration.

Also, the permissible stresses for connections in the table given below can be increased by 25 % for wind or seismic loads.

Connection type
(Bolts/Rivets)
Axial Tension
(Mpa)
Shear
(MPa)
Bearing
(Mpa)
Hand Driven8080250
Power Driven100100300

EFFECTIVE LENGTHS

Effective Lengths

DEFLECTION CRITERIA

  • Maximum vertical deflection permissible is l/325 where ‘l’ is the span.
  • Horizontal deflection of columns is l/325 where ‘l’ is length of column
  • For Manually operated crane – l/500
  • Electric overhead travelling crane – l/750 for upto 50t capacity
  • If the capacity of Overhead electric travelling crane is more than 50 t, then the deflection is l/1000.

Connections

Bolts are used for connections of members in either tension/compression/ bending. In addition,Washer plates are provided for bolts to enable proper pressure distribution. Also,Bolts are indicated by its class as 4.6 or 8.8 .

In Class 4.6 4 is understood as 400 N/sqmm which is the maximum tensile strength of bolt and 0.6*400 is the yield strength of the bolt. In reality High strength friction grip bolts are most commonly used bolts for all important structures. For example here are some technical terms related to bolts

  1. Pitch – Pitch of bolts is the distance between two bolts measured along the direction of force. Min pitch is 2.5 times bolt dia.
  2. Gauge – Gauge distance is the distance measured perpendicular to the direction of force.
  3. Edge distance – It is the clearance from the edge of the member face to the centre of bolt. Min edge distance is 1.5 times bolt dia.

Welds are another type of common connection used in construction. Welding is done my heating electrode and then metal creates a between two plates or members. Min weld size shall not be more than the min thickness of plate.Welding is skilled worker job.

Types of common Welds
  1. Butt Weld
  2. Fillet Weld
  3. Plug Weld
  4. Stitch Weld
  5. Slot Weld

Strength of weld is multiplication of (length of weld)*(Eff throat thickness)*(permissible stress). Miz size of weld is 3mm.

MYLEARNINGS – BASICS OF CIVIL STRUCTURAL DESIGN

Material Properties of steel and concrete

Concrete – Concrete is a mixture of cement, sand and aggregate. Concrete grades are designated as M15, M20, M25 etc.

M and 25 in M25 grade of concrete are Mix and compressive strength of concrete at the end of 28 days on a concrete cube of 150 mm side.

M25 grade means..

Steel

  1. Mild steel is indicated as Fe250 and
  2. Tor steel is indicated by Fe 415 and Fe500. These are high yield strength deformed bars. Tor mean the twisted projection on reinforced bar.
  3. 415 and 500 indicate the yield strength of steel in tension.

Concrete is good in compression and steel is good in tension.

Behavior of steel and concrete.

Commonly used methods of Design

  1. Working Stress Method
  2. Limit State Method
Working Stress Method

Working stress method is nothing but restricting the stresses for all working loads within elastic Limit.

In general Assumptions in working stress method are

  • Section Remains plane before and after bending
  • Elastic Limit is valid between concrete and steel
  • Tension in concrete certainly ignored
Limit State Method

Limit State design is Probable design of various limit states. Once we satisfy all the limit states, the structure is safe . Most important limit states are.

  1. Deflection Limits State
  2. Cracking Limit State
  3. Vibration Limit state
  4. Collapse Limit State

Nevertheless, the basic assumptions in Limit state are

  • Plane sections remain plane after bending.
  • Tensile strength of concrete is ignored
  • Maximum strain in concrete is extreme fibre is 0.0035
  • Maximum compressive stress in concrete is 0.67 *fck

In Limit state design, the partial safety factors considered in design are 1.5 for concrete and 1.15 for steel. With this in Mind the Load combination factors for various loads are in the table below.

Load combinations for limit state
Load combinations for limit states

Stress strain diagram for steel and concrete

stress strain curve

Behavior in Sections

There are three types of flexure behaviour

BALANCED SECTION

In a Balanced section, the stresses in steel and concrete reach at same time. On the positive side this is ideal case. Therefore, the stress strain diagram of balanced section is as below

Stress strain curve for balanced section
stress strain diagram for balanced section
UNDER-REINFORCED SECTION

The reinforcement provided in under reinforced section is less than balanced section causing the steel stresses reach first. The Neutral axis moves above the center.

OVER REINFORCED SECTION

Provided reinforcement here is more than that required in Balanced section. The stresses in concrete reaches first and hence causes brittleness. The Neutral axis moves lower than center.

Basic Shear force and Bending Moment diagrams

Simply supported beam with Point load in Middle
Shear force and bending moment diagram
Shear force and Bending Moment diagram for simply supported beam with point load in middle
Simply supported beam with uniformly distributed load

shear force and bending moment diagram for simply supported beam with udl
simply supported beam with UDL shear force and Bending moment diagram

At the same time here are some Snippets for Civil Engineering Interview / Competitive Exams

MYLEARNINGS – CIVIL DESIGN BASIC STRUCTURAL ENGINEERING

  • Tensile strength of concrete is 0.7*sqrt(fck) where fck is characteristic strength of concrete.
  • Modulus of elasticity of concrete is E = 5000*sqrt(fck)
  • Factor of safety against overturning is 1.4.
  • The lateral deflection under wind loads by no means shall exceed h/500 at top most point.
  • Span of simply supported beam is either center to center distance of beam or clear span+effective depth.
  • The maximum deflection should be not less than l/250
  • Nonetheless Span to depth ratios for cantilever is 7 and for simply supported is 20 and continuous beam it is 26.
  • A column or a compression member is slender if its l/b or l/d is more than 12.
ADDITIONAL STRUCTURAL ENGINEERING BASICS
  • When depth of web is more than 750mm , 0.1% of web area should be provided as side face reinforcement.
  • Min spacing of shear reinforcement should be (0.87*fy*Asv) / (0.4*b)
  • Maximum area of tension reinforcement is 0.04%*b*D
  • Minimum reinforcement in slabs is 0.15 % and 0.12 % for mild steel and HYSD steel respectively.
  • Min and Max reinforcement in columns is 0.4 % and 6 % of sectional area respectively.
  • Min dia of bar in colums is 12 mm
  • Minimum 6 nos of bars shall be provided in circular column.
  • Pitch of stirrups shall not be more than least of Least lateral dimension/ 16 times dia of bar/300 mm.
  • When ratio of long side/short side of slab is <2 , it is two way slab whereas, if the ratio is >2 it is one way slab.
  • Span of stair is either center to center distance or beams, or center to center of landing slab/1m which ever is small.
  • On the other hand moment for the footing is taken at the face of column or wall.
  • Similarly, crtical section for one way shear is at distance of eff depth from face or wall/column.
  • Likewise for two way shear, critical section is at distance of d/2 from the face of column/wall.
  • Nominal reinforcement for concrete sections greater than 1m thick shall be 360 sq.mm /meter length in each direction.
  • Design strength of steel in limit state method is 0.87*fy.
  • Torsion is not considered as limit state of serviceability.
  • Maximum redistribution of moments can be upto 30 %

MY LEARNINGS IN STEEL DESIGN BASICS

  • As per IS, rolled steel beam sections are classified into 5 series sections.
  • 0.23% is the amount of carbon in structural steel. Less % more strength.
  • Poisson ratio is 0.25 within elastic range.
  • Factor of safety in working stress design is ratio of Yield stress/working stress.
  • Preferable sections for torsional resistance is Box type sections.
  • Purlins are bending members.
  • Struts are compression members in truss.
  • To reduce span of purlins sag rods are provided.
  • All truss members are in compression or tension.
  • Max permissible vertical deflection is l/325
  • As a matter of fact Short columns fail by crushing and ,Long columns fail by buckling.
  • Angle of inclination in lacing is 40 deg and upto 60 deg.

Worth Sharing !