A brief comparison of structural steel buildings with conventional structures

highlighting the significance of Pre Engineered buildings

Advantages of structural steel construction

Steel construction is eco friendly – Structural steel can be reused or recycled. Building structural steel

buildings involves less pollution & lesser impact on the environment & surroundings. Also the dismantling

of the structure is much easier, causes less environmental impact & the scrap value is high. A steel

building, if properly designed can even be relocated & reproduced.

No curing or waiting time – Steel does not have the problems of curing or slow development of

strength, and is less affected by moisture movement and creeps. Construction process can be a

continuous process which saves time & cost.

Future expansion – Since “on-the-spot” connection of steel members is relatively easier, the frame-type

construction using structural steel can allow alteration fairly easily in case when future amendment or

expansion to the structure is required.

Speed of construction – Speed of construction using structural steel may normally be faster than in-situ

concrete due to the following reasons:

Most structural steel members can be pre-fabricated off-site, this can allow other site works such as the

construction of the foundation, the central core or the erection of other members to be done at the same

time and shorten the critical time in waiting for the completion of the other required members.

Erection of structural steel members do not require complicated in-situ formwork thus save up quite a lot

of time in the preparation, erection or striking of formwork.

Fabrication and erection of structural steel members off or on-site may be less affected by inclement

weather.

In general, maintenance cost for buildings in structural steel construction is reasonable when compare to

other forms of construction, especially when the design, workmanship and protection treatment during

construction is effective and sound. In case of maintenance is required, such as deterioration appears in

the connections, repair works can be done simply right after the exposing of the defective areas.

Disadvantages of structural steel construction

Rigidity – Owing to the flexible nature of steel and other inherited weakness in its connection, the rigidity

of a frame structure constructed by structural steel is much weaker when compare to the monolithic

reinforced concrete structure. Furthermore, such situation may become even worst when the joints of

frame get aged. To overcome this weakness in the design, additional amount of steel or very strong and

rigid connection design may be required to strengthen the performance of the structure.

Fire resistance – Though structural steel is incombustible, steel loses its strength significantly under fire.

Under normal fire situation finds in high-rise building when temperature can rise to 9000C in a short time,

strength of steel can drop by more than 60%. This may lead to disastrous result unless effective fire-

proofing treatment is provided.

Deflection – Deflection may easily occur in structural steel members especially when they are exposed to

excessive or rapidly varying loads such as under extreme temperature difference or facing sudden wind

load. The usual allowable deflection should not exceed 1/300 to 1/350of the effective span; otherwise, it

may not be acceptable from the point of providing finishes to a building. Similarly, the relative flexibility of

steel may have the problem of incompatibility when some rigid components such as cladding or curtain

walling systems.

Transportation & erection – Although structural steel members can be fabricated off-site, demands to

transport the components to and temporary storage of the members on site still incur practical difficulties.

Fabricated steel members are often made to quite a large size and heavy weight in order to minimize

unnecessary connection works on site. However, this may create problems in lining the member to the

spot of work. To overcome this, heavy hoisting equipment is required. This may at the same time increase

the loading requirement of the structure during the process of construction. Besides, the mounting,

erection and operation of the hoisting equipment may incur additional work on site.

Connections – Connection of the pre fabricated steel members requires accurate dimension

coordination. Slight discrepancy in the onsite or offsite works may result to delay in the whole operation.

Besides, the carrying out of anti-corrosion or fire proofing treatment to steel, site inspection to the steel

connections or to the encasement of the steel members may require addition work and time thus lengthen

the entire construction period. Not to mention the more specialized workers it required in the carrying out

of such works.

Generally speaking, the above points only reflect the relative characteristics in the using of structural

steel. The actual merits or demerits of this construction method should finally rely on some other local

factors such as the choice of both architectural or structural design, availability of materials and labours,

site conditions and the opportunity cost created by the speed of work etc.

Constraints in the use of structural steel construction

Lack of expertise and skill in carrying out structural steel works – Contractors or even skilled

workers qualified for structural steel construction are rare and thus resulted to relatively higher

construction cost-This situation is especially obvious for building of complicated structural steel design or

for very large scale development projects.

Lack of the utilization of latest & sophisticated technologies for fabrication & erection of steel

buildings – One of the major reasons for the delay or increased cost of fabrication is due to lack of

standardization, non utilization of technology. Also, the fabrication process is undertaken manually which

results in increased cost & time.

Availability of the heavy rolled steel sections – Either scarcity or the requirement of preordering the

heavy rolled steel members may result in cost increment & delay.

Lack of working space – To carrying out of structural steel works requires very large space for pre-

fabrication, anti-rusting treatment, or in the temporary handling and storage of the structural members.

Such working spaces are often required inside and outside the site. The completed structural members

are transported to site for connections afterward. This may in fact increase the overall cost of

construction. The completed structural members often weigh more than 10 tones. They require heavy

hoisting equipment to assist in the connection works. The positioning of such members may be difficult

especially in congested site where there are buildings and other public facilities nearby. Furthermore, the

erection, mounting, operation or dismantling of such hoisting equipment also occupy extra space, working

time and incur costs.

Existence of alternative techniques – The major advantages of using structural steel construction is its

ability to produce large-span, light-weight and space effective buildings. However, due to the introduction

of many other advance construction techniques such as the using of pre/post-stressing, flat slab

construction, high performance concrete or other effective foundation design/techniques, many of the

advantages inherit from structural steel construction can now be substituted by other relatively simpler

and more cost-effective methods of construction.

Fire proofing requirements – Fire proofing requirements is quite strict for the buildings that are mainly

high-rise in which a great number of occupants are using. Failure under the situation of fire may produce

great casualty-Though structural steel is not a combustible material, it loses most of its strength under

fire. Fire protection to steel is therefore essential for structural steel construction. Applying fire resisting

treatment to steel is costly, time consuming & the most important of all; it often involves special testing,

approval and monitoring procedures. In order to saves such extra works, engineers or designers tend to

use other methods to construct wherever alternative exists.

Process of construction

Fabrication & delivery – As for multi-storey buildings where the structure are heavy, fabrication of the

structural steel members are usually done off-site in a properly equipped fabricating yard in which the

scheduling of works, dimensional coordination, quality of welding or anti-rusting treatment of the structural

members can be done under a more accurately controlled manner The completed members or

components will then be transported to site as scheduled. In order to have more fabrication done off-site

to gain the best benefit from works, most of the completed members or components are fabricated into a

size or weight as large and heavy as possible up to the handing limits, such as the storage spaces or the

capacity of hoisting equipment of the site.

If the structure is a low rise building with comparatively light members, alternatively a temporary

fabrication shed can be set up on site if the space permits, which can considerably reduce the

transportation expenses. But again for the utilization of sophisticated equipments requires that the

fabrication to be done offsite at the shop.

Connections – Connections for structural steel sections can be classified into shop connections or site

connection in a properly equipped fabrication workshop, most of the connections are done by welding in

order to produce more rigid joints. On the other hand where connection works are done on site bolt joints

are used more frequently in conjunction with site welding for the former can be done quicker, and can

have easier dimensional tolerance, cost effectiveness and quality control. Connection to the foundation is

usually done using anchor bolts.

Fire-proofing treatment – Conventionally structural steel members are fire protected by concrete, which

is lightly reinforced and poured around the steel using suitably design formwork. However, this method

has a lot of drawbacks, such as increasing the dead load of the structure, time consuming and costly, and

is seldom used today.

The other fire protection method is to encase the steel members using some kind of non-combustible

board. Usually, a 19mm thick board can provide a fire protection up to one hour, or a 32mm board up to

two hours. Quite a lot of materials, such as gypsum board of adequate thickness or vermiculite concrete

board, both reinforced with fiber or metal mesh, can serve such purposes.

One more popular method used today to fire protect the structural steel works is by the applying of a

spray-on fire protection coating-Materials for the coating may be of cement based and mixed with mineral

wool or mineral fiber product. This kind of material can easily achieve a fire protection up to two hours

under convenient thickness. Since the material is applied using spray-on method, it can easily coat onto

most objects with irregular surfaces, or to build up its thickness in subsequent coats-Another type of

coating which developed recently can also be in a form of intumescent painting, that is, the paint surface

will expand under heat becoming spongy-like with the heat insulating ability tremendously increased.

Having discussed the advantages, disadvantages & pull backs we can come to a general conclusion that

if structural steel construction can overcome the above mentioned disadvantages & stand up to the pull

backs, then structural steel construction can be cost effective, value added, quickest & environmental

friendly option for the building construction.

Now a day’s a new trend of using PEB’s have evolved against the conventional structures. PEB stands

for PRE – ENGINEERED BUILDINGS. PEB’s are tailor-made solutions to a customer’s needs and are

custom-designed to meet exact requirements. These buildings are flexible enough to suit different

building dimensions; they are easily expandable, can withstand harsh climatic conditions and come with

maintenance-free exteriors. Pre-engineered buildings are suitable for both, industrial and commercial

operations; Warehouses, factories, aircraft hangars, cold storages, workshops, sports halls, supermarkets

or any high-rise building. Pre-engineered buildings offer modern solutions to all building constructions.

PEB help to convert complex and expensive conventional steel buildings into simpler and economical pre-

designed, pre-engineered buildings without sacrificing utility and function. In a PEB high strength

structural steel members are used for primary framing (beams, columns etc.) & cold formed steel is used

for secondary framing (Girts, purlins etc.).

Some of the advantages of a PEB which helps it to outshine the conventional structures are listed below:

Custom design – PEB are designed exactly to match the load transfer of the structure which helps to

optimize design. Tapered sections are used for beams & columns made up of structural steel plates & or

rolled sections. It reduces material wastage & the strength to weight ratio of the structure. Use of cold

form steel also reduces the strength to weight ration.

Standardization – Standardization of the layout, connections, attachments, secondary framing elements,

insulations & coverings helps to optimize the cost & time in fabrication as well as erection.

Architectural appearance

In any condition a PEB is the best option for low rise buildings considering all the factors including

construction time & cost. A comparison between the conventional structures and PEB is included below:

PRE-ENGINEERED STEEL

BUILDINGS (PEB's)

CONVENTIONAL BUILDINGS

Structure weight Pre- engineered buildings are on avg.

30% lighter through the optimum use

of steel. Primary framing members are

tapered (varying depth) built-up plate

sections with larger depths in the area

Primary steel members are selected

from standard hot rolled 'I' sections,

which are, in many segments of the

members, heavier than what is actually

required by design. Members have

constant cross-sections regardless of

of highest stress.

Secondary members are light gauge

(light weight) roll formed (low labour

cost) 'Z' or 'C' shaped members.

varying magnitude of the local stresses

along the member length.

Secondary members are selected from

standard hot rolled 'I' & 'C' sections,

which are heavier.

Design Quick and efficient, since PEB's are

mainly formed of built up sections and

connections, design time is

significantly reduced. Basic designs

are used over and over again.

Consultant’s in-house design and

drafting time is considerably reduced,

allowing more time for co-ordination

and review, and increased margins on

design fees.

Each conventional steel structure is

designed from scratch by the

consultant, with fewer design aids

available to the engineer.

Substantial engineering & detailing is

required on every project. Generalized

computer analysis requires extensive

input/output & design alterations.

Each project is a separate case;

engineers need more time to develop

the design & details of the unique

structure.

More complicated design requiring

extensive design and drafting time from

consultants.

Delivery 3 to 4 times quicker

Foundations Simple design, easy to construct. Extensive, heavy foundations required.

Erection cost

and Time

Both costs & time of erection are

accurately known, based upon

extensive experience with similar

buildings. PEB's are often erected by

specialized PEB builders with

extensive experience in the erection or

Typically, they are 20% more

expensive than PEB. In most of the

cases, the erection costs and time are

not estimated accurately.

Erection is slow and extensive since

field labour is required. Heavy

similar buildings, offering very

competitive rates. PEB builders

usually have a stock of standard

components in their camps, enabling

them to complete jobs on time should

any shortage or on site damage

occurs to materials.

The erection process is easy, fast,

step by step and with minimum

equipment requirement.

equipment is often needed.

Seismic

Resistance

The low-weight flexible frames offer

higher resistance to seismic forces.

Rigid heavy structures do not perform

well in seismic zones.

Overall Price Price per square meter may be as

much as 30% lower than conventional

building.

High price per square meter.

Architectural

Design

Outstanding architectural design can

be achieved at low cost using standard

architectural features and interface

details.

Special architectural design and

features must be developed for each

project, which often require research

and thus resulting in much higher

costs.

Sourcing &

Coordination

Building is supplied complete with

cladding and all accessories including

erection (if desired) from one single

source.

Many sources of supply, project

management time are required to

coordinate suppliers and sub-

contractors.

Responsibility Single source of supply results in total

responsibility by one supplier. PEB

manufacturers can be relied upon to

service their buildings long after they

Multiple responsibilities can result in

questions of who is responsible when

components do not fit properly,

insufficient material is supplied, or

are supplied, to protect their

reputation.

materials fail to perform, particularly at

the supplier/contractor interface. The

consultant carries total design liability.

Performance All components are designed

specifically to act together as a

system, for maximum efficiency,

precise fit, and peak performance in

the field.

Experience with similar buildings, in

actual field conditions, has resulted in

design improvements overtime, which

allow dependable prediction of

performance.

Components are custom designed for a

specific application on a specific job.

Design and detailing errors possible

when assembling the diverse

components into unique buildings.

Each building design is unique, so

prediction of how components will

perform together is uncertain. Materials

which have performed well in some

climates may not do so in other

environments.