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DEPARTMENT OF ARCHITECTURE

ABUBAKAR TAFAWA BALEWA UNIVERSITY, BAUCHI

ARC 615: ADVANCED BUILDING STRUCTURES

LESSON 5: SECTION ACTIVE STRUCTURAL SYSTEMS

5.1 Introduction

5.2 Structural concept of planar trusses

5.3 Beam structures

5.3.1 Structural concept of beam structures

5.3.2 Examples of beam structures

5.4 Frame structures

5.4.1 Structural concept of frame structures

5.4.2 Examples of frame structures

5.5 Slab structures

5.5.1 Structural concept of slab structures

5.5.2 Examples of slab structures

5.6 References

5.1 Introduction to Section Active Structural Systems

These are systems of rigid, solid, linear elements, in which redirection of forces is effected

by mobilization of sectional forces. Examples are:

1. Beam structures

2. Frame structures

3. Slab structures

5.2 Beam structures

A beam structure is a structural element that is capable of withstanding load primarily by

resisting against bending. The bending force induced into the material of the beam as a

result of the external loads, own weight, span and external reactions to these loads is called

a bending moment. Beams are characterized by their profile (shape of cross-section), their

length, and their material.

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Effect of loading on a simply supported beam.

Post and beam construction

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Post and lintel construction

One and two-bay beams

Beam grid systems

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Post and beam structure

Slab-beam-and-girder floor

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Concrete joist floor

5.2.1 Structural concept of beam structures

Classification of beams based on supports

1. Simply supported - a beam supported on the ends which are free to rotate and have

no moment resistance.

2. Fixed - a beam supported on both ends and restrained from rotation.

3. Over hanging - a simple beam extending beyond its support on one end.

4. Double overhanging - a simple beam with both ends extending beyond its supports

on both ends.

5. Continuous - a beam extending over more than two supports.

6. Cantilever - a projecting beam fixed only at one end.

7. Trussed - a beam strengthened by adding a cable or rod to form a truss

5.2.2 Precast concrete beams

Precast concrete is concrete that has been cast into the desired shape prior to placement in

a structure. There are a number of advantages obtained by removing the concrete forming,

placing, finishing, and curing operations from the construction environment.

1. Pre casting operations usually take place in a central plant where industrial

production techniques may be used.

2. Since standard shapes are commonly used, the repetitive use of formwork permits

forms to be of high quality at a low cost per unit.

3. These forms and plant finishing procedures provide better surface quality than is

usually obtained in the field.

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4. Because of controlled environment and procedures, concrete quality control is also

usually superior to that of cast-in-place concrete.

5. Forming procedures used make it relatively simple to incorporate prestressing in

structural members.

6. Upon arrival at the job site, precast structural members may be erected much more

rapidly than conventional cast-in-place components.

Common shapes of precast concrete beams

5.2.3 Prestressed concrete beams

Prestressed concrete is concrete to which an initial compression load has been applied.

Prestressing has several advantages:

1. Since concrete is quite strong in compression but weak in tension, prestressing

serves to increase the load that a beam or other flexural member can carry before

allowable tensile stresses are reached.

2. The use of prestressing in a concrete structural member permits a smaller, lighter

member to be used in supporting a given load.

3. Prestressing also reduces the amount of deflection in a beam.

4. Since the member is always kept under compression, any cracking that does occur

will remain closed up and not be apparent.

These advantages of prestressing are offset somewhat by the higher material, equipment,

and labour cost involved in the production of prestressed components. Nevertheless, the

use of prestressing, particularly in precast structural members, has become widespread.

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Stresses in a prestressed concrete beam

There are two methods for producing prestress in concrete members;

pretensioning and

posttensioning

Pretensioning

It places the prestressing material (reinforcing steel or prestressing cables) under tension

in the concrete form before the member is poured. After the concrete has hardened, the

external tensioning devices are removed. Bonding between the concrete and the

prestressing steel holds the prestressing in place and places the concrete under

compression.

Posttensioning

It places the prestressing steel (usually placed inside a metal or plastic tube cast into the

member) under tension after the concrete member has been erected. The prestressing is

then tensioned by jacks placed at each end of the member. After the prestressing load has

been applied, the prestressing steel is anchored to the concrete member by mechanical

devices at each end or by filling the prestressing tubes with a cementing agent. After the

steel has been anchored to the member, jacks are removed and the prestressing steel is cut

off flush with the ends of the member.

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5.2.4 Examples of beam structures

Beam structure (Stonehenge)

Example of post and beam structure

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5.3 Frame structures

Frame structures are the structures having the combination of beam, column and slab to

resist the lateral and gravity loads. These structures are usually used to overcome the large

moments developing due to the applied loading.

Frame structure schematic.

Parts of a frame structure.

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Parts of a frame structure.

One and two-bay frames

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5.3.1 Structural concept of frame structures

5.3.2 Types of frame structures

Frames structures can be differentiated into:

1. Rigid frame structures which are further subdivided into pin ended and fixed ended

frame structures.

2. Braced frame structure which is further subdivided into gabled frames and portal

frames.

Rigid structural frame

The word rigid means ability to resist the deformation. Rigid frame structures can be

defined as the structures in which beams and columns are made monolithically and act

collectively to resist the moments which are generating due to applied load. Rigid frame

structures provide more stability. This type of frame structures resists the shear, moment

and torsion more effectively than any other type of frame structures. This frame system

was used in the famous Burj Al-Arab.

Braced structural frames

In this frame system, bracing is usually provided between beams and columns to increase

their resistance against the lateral forces and sideways forces due to applied load. Bracing

is usually done by placing the diagonal members between the beams and columns. This

frame system provides more efficient resistance against the earthquake and wind forces.

This frame system is more effective than rigid frame system

Pin ended rigid structural frames

A pinned ended rigid frame system usually has pins as their support conditions. This frame

system is considered to be non-rigid if its support conditions are removed.

Fix ended rigid frame structure

In this type of rigid frame systems end conditions are usually fixed.

Gabled structural frame

Gabled frame structures usually have the peak at their top. These frames systems are in

use where there are possibilities of heavy rain and snow.

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Portal structural frame

Portal structural frames usually look like a door. This frame system is very much in use for

construction of industrial and commercial buildings.

Load path in frame structure

It is a path through which the load of a frame structure is transmitted to the foundations. In

frame structures, usually the load path is from slab to beams then to from beam to

columns, then from columns it transfers to the foundation.

Advantages of frame structures

1. One of the best advantages of frame structures is their ease in construction.

2. Frame structures can be constructed rapidly.

3. Frame structures have economical designs.

Disadvantages of frame structures

In frames structures, span lengths are usually restricted to 10m. Greater spans can cause

lateral deflections.

5.3.3 Examples of frame structures

Example of frame structure.

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Example of frame structure.

Example of frame structure.

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Example of frame structure.

Example of frame structure.

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Example of frame structure.

Glue laminated (glulam) timber frame structure

5.4 Slab structures

A slab is a flat piece of concrete, put on the walls or columns of a structure. It serves as a

walking surface but may also serve as a load bearing member, as in slab homes. The flat

slab is a two-way reinforced structural system that includes either drop panels or column

capitals at columns to resist heavier loads and thus permit longer spans. Construction of

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flat slabs is one of the quickest methods available. Lead times are very short as this is one

of the most common forms of construction.

Functions of a slab

1. Provide a flat surface

2. To support load

3. Sound, heat and fire insulator

4. Acts as a divider (privacy) for the occupants

5. Upper slab becomes the ceiling for the storey below

6. Space between slab and ceiling can be used to place building facilities

Types of slab structures

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Types of slab structures

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Types of slab structures

Slab structure

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Slab structure

5.4.1 Structural concept of slab structures

For a suspended slab, there are a number of designs to improve the strength-to-weight

ratio. In all cases the top surface remains flat, and the underside is modulated:

Corrugated, usually where the concrete is poured into a corrugated steel tray. This

improves strength and prevents the slab from bending under its own weight. The

corrugations run across the short dimension, from side to side.

A ribbed slab, giving considerable extra strength on one direction.

A waffle slab, giving added strength in both directions and it looks hollow from

bottom .

Reinforcement design

A one-way slab needs movement resisting reinforcement only in its short-direction

because the movement along long axes is so small that it can be neglected. When the ratio

of the length of long direction to short direction of a slab is greater than 2 it can be

considered as a one way slab.

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A two-way slab needs movement resisting reinforcement in both directions. If the ratio of

the lengths of long and short side is less than two then movement in both direction should

be considered in design.

5.4.2 Precast concrete slabs

See 5.2.2 for precast concrete.

Precast concrete slab shapes

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5.4.3 Examples of slab structures

Example of slab structure. Waffle slab structure.

Example of slab structure.

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5.5 References

King Saud University (2016). CE 417 Lecture Slides. Chapter 12, Concrete Construction

Part 1.