Beginner’s Guide to Structural Analysis and Mechanics

Being an engineer, excellent comprehension is necessary on how to make structural analysis for buildings, bridges, and other structures.

Structural analysis is the calculations of the magnitudes of forces, stresses, strains and deflections or deformations of structures when LOADS, external forces are being applied and exerted on structures.

The readers of this blogpost who are not engineers may very well amaze and ask; “Where in the world did they get these Loads?” “What on earth do they think they are weighing?” That very crucial and logical questions will be answered in this blogpost.

1. Specifications, Building Codes, and Bridge codes.

Designers must look for appropriate Specification and Codes. National and Local government have published building codes, bridge and highway codes for the safety purposes of the public, which control the construction of different types of structures within their country. Actually, these codes are laws or ordinances that specify design loads, design stresses, construction types, material quality among others. Not many specifications published recommended practices for local and national use. These codes and specifications are not enforceable legally, nevertheless, unless it is embodied in their national building code, and made integral part of a particular contract of projects. Among these organizations are;

  1. ASCE -American Society of Civil Engineers
  2. AASHTO -American Association of State Highway and Transportation official
  3. AISC -American Institute of Steel Construction
  4. ACI -American Concrete Institute
  5. ASEP -Association of Structural Engineers of the Philippines

The following specifications published by the above-mentioned organizations oftenly are used to estimate the maximum load and minimum loads to which the bridges, buildings, and other structures may be subjected during their estimated lifetimes.

  1. Minimum Design Loads for Buildings and other Structures, published by ASCE 7-2005 edition;
  2. AASHTO LRFD Bridge Design Specifications, published by AASHTO;
  3. Specifications for Structural Steel Buildings- 2010, published by AISC;
  4. Steel Construction Manual, 14 edition, published by AISC;
  5. National Structural Code of the Philippines, volume 1 -Buildings, volume 2 -Bridges, published by ASEP.

Readers of this bolgpost should pay attention that reasonable and clearly written codes are really helpful to designers.

The great pyramid in Egypt, the Parthenon in Athens, and the great Roman bridges and aqueducts built by ANCIENT BUILDERS were controlled by few specifications, which precisely is true. It should be spoken that only few number of these great structures were built over many 100 of years or centuries, and were ostensibly built WITHOUT CONSIDERATION or CARE about COST OF LABOR, MATERIAL, OR HUMAN LIFE. The were built probably by intuitions, and certain RULES OF THUMBS (“SINUBOK LAMANG” at KAWALAN O walang RASYONAL na PROSESO -in local dialect), developed by seeing the minimum size or strength of members that would fail only under certain given conditions. Their NUMEROUS FAILURES are NOT RECORDED in HISTORY, only their SUCCESSES ENDURED.

For the information and guidance of all readers of this blogpost, notably the ordinary engineers in the Philippines, I would like to give emphasis to them, that the national government agencies in the Philippines (DPWH, NIA, DOTC, DSWD-Kalahi) had adopted the latest international recommended practices and codes, like the ASCE standards, ACI Codes, AREA Code, AISC standards, ASTM standards. In view of the fact that ENGINEERING EDUCATION in the Philippines is AMERICAN ORIENTED, the ASEP committee decided to recommend the adoption of the Earthquake Regulation as provided in the Uniform Building Code.

Hence, the Association of Structural Engineers of the Philippines (ASEP) published National Structural Code of the Philippines as a referral code of the National Building Code of the Philippines. The NSCP code reflects the continuing technical advances in structural engineering and the latest seismic design practice for earthquake resistant structures, viz:

  1. Reinforced concrete design conforms to the provisions of the American Concrete Institute (ACI-318) Code.
  2. Bridges and highways specifications are patterned after the provisions of the AASHTO.
  3. The ASEP recommended Earthquake Regulations are patterned after the provisions of the  Uniform Building Code (SEAOC) of the United States of America.
  4. The Minimum Design Loads for Buildings and other structures conforms to the provisions of American Society of Civil Engineers (ASCE 7-2005).
  5. Steel and Iron specifications are patterned after the provisions of the American Institute of Steel Construction (AISC) and American Standards for Testing of Materials (ASTM).

The Department of Public Works and Highways (DPWH) issued Department Order No.82-1, 1982;

“For the guidance and compliance of all concerned and pursuant to section 203 of PD 1096, the National Structural Code for Buildings a referral code of the NBC (PD 1096) to reflect the following;

  1. In Chapter 2, lateral forces, are revised to reflect the provisions of the Uniform Building Code (UBC-SEAOC)
  2. Chapter 4, Steel and Iron, conforms to the provisions of the American Institute of Steel Construction (AISC).
  3. Chapter 5, Concrete, conforms to American Concrete Institute -ACI 318 Code with the equations in SI Units.”

2. STRUCTURAL LOADS

Dead Loads: Weight of the structure under consideration, as well as any fixtures that are permanently attached to it.

Live Loads: They include occupancy loads, warehouse materials, construction loads, overhead service cranes, and equipment loads. They are gravity induced.

Environmental Loads: For Buildings, they are caused by rain, snow, wind, and earthquake.

2.1 Dead Loads

2.1.1 Weights of Common Building Materials

Reinforced Concrete -150 pcf

Concrete Hollow blocks (no plaster) -44 psf

G.I. roofing -2.5 psf

Suspended Ceiling -2 psf

Hardwood flooring -4 psf.

Minimum Uniformly Distributed Dead Loads (Source: ASCE 7 Standards)

Minimum Uniformly Distributed Dead Loads (Source: ASCE 7 Standards)

2.2 Live Loads

FLOOR LIVE LOAD

FLOOR LIVE LOAD

2.2.1 Typical Uniformly Distributed Live Loads:

Residential dwelling areas -40 psf

Classrooms in schools -40 psf

Offices in office buildings -50 psf

Retail stores -first floor -100 psf

Retail stores -upper floor -75 psf

Dance hall and ballrooms -100 psf

Library reading rooms -60 psf

ALive load2

Minimum Uniformly Distributed Live Loads (Source: ASCE 7 Standard)

Minimum Uniformly Distributed Live Loads (Source: ASCE 7 Standard)

Minimum Uniformly Distributed Live Loads (Source: ASCE 7 Standard)

2.3. Lateral Loads:

There are certain loads that are almost always applied horizontally.
Wind Loads, soil pressures, hydrostatic pressures, forces due to earthquakes, centrifugal forces, and longitudinal forces.

2.3.1 Wind Loads

WIND LOAD

WIND LOAD

A.1 The basic reference equivalent static pressure in the critical local wind speed.

Formula:

qs = 0.0000483V^2

Where:

V = wind velocity in KPH

qs = in kPa

Applicable to Duchemin formula (developed in 1829)

1. Duchemin Formula..

Pn = p (2 sinϴ/1 + sin^2ϴ) — Wind Pressure normal to an inclined roof surface.

American Society of Civil Engineers (ASCE) Recommendation:

ASCE 7-05 Wind Pressures

ASCE 7-05 Wind Pressures

Wind Pressure2

2.3.2 EARTHQUAKE LOADS or FORCES (EQ),

Seismic Load

Seismic Load

2.3.2.1 STATIC LATERAL FORCE PROCEDURE

A. Uniform Building Code (UBC)

1988-1994 UBC Formula for Base Shear

1988-1994 UBC Formula for Base Shear

1997 UBC Formula for Base Shear

1997 UBC Formula for Base Shear

3. SYSTEM LOADING:

3.1 Tributary Area Loading.

Column Tributary Area

Column Tributary Area

Girder Tributary Area

Girder Tributary Area

3.2 LOADING CONDITIONS FOR STRENGTH DESIGN:

3.2.1 LOAD COMBINATIONS,

A. ACI Code -1963 to 1971,

  • 1.5D + 1.8L
  • 1.25 [ D + L + W]
  • 1.25 [ D + L + EQ]

B. ACI Code -1977 to 1999,

  • 1.4D +1.7L
  • 0.75 [1.4D +1.7L +-1.87EQ]

C. ACI Code -2002 to 2011,

  • 1.2D + 1.6L
  • 1.2D + 1.0L + 1.0EQ
  • 1.2D + 1.0L + 1.6W

References:

  1. Structural Analysis by Aslam Kassimali, 4th edition, 2011,
  2. Structural Analysis by R. C. Hibbeler, 8th edition, 2012
  3. Structural Analysis by J. C. McCormac, 2nd edition, 1997,
  4. Structural Analysis Design of Tall Buildings, by Taranath, 2012,
  5. Structural Analysis by Venancio Besavilla, 2007 edition;
  6. Structural Analysis by Matias A. Arreola, 1992;
  7. Wind and Earthquake Resistant Building, by Taranath, 2011,
  8. ASCE 7- Minimum Design Loads for Buildings and other Vertical Structures, 2010,
  9. Standard Specifications for Highway Bridges, AASHTO 2007;
  10. Building Code Requirements for Structural Concrete, ACI 318 Code 2011 edition;
  11. Uniform Building Code (UBC) -1997,
  12. Structural Engineering handbook,
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