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Structural Steel Construction

Introduction

There is more than a century's history of using structural steel as a method of building

construction. Even in Hong Kong, the use of structural steel in construction has a history of

more than 60 years.

Buildings like the old headquarters building of Hong Kong and Shanghai Banking Corporation

and the old Bank of China Building, are examples of buildings in structural steel which were

constructed from the 30's to50's. Recent examples of such buildings in Hong Kong are the

park Lane Hotel in Causeway Bay, the Convention and Exhibition Centre in Wanchai, the new

Bank of China Building in Central, the Time Square, Jumbo Sogo and Manulife Tower (Figure 1)

in Causeway Bay, or the Headquarter of Hong Kong Bank (Figure 2), The Center (Figure 3) and

Cheung Kong Center in Central (Figure 4).

Figure 1 – the Lee Garden Hotel Redevelopment, a typical composite structure with a RC core

and a structural steel external frame (Manulife Tower, 1997)

Figure 2 - The Headquarter Building of the HK & Shanghai Banking Corporation, a highly

prefabricated and modulated structural steel structure, 1985

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Figure 3 - The Center, a mega steel frame without the use of RC core wall, 1998

Figure 4 - The Cheung Kong Center, a composite structure using concrete-filled steel columns

as the external framel, 1999

The examples mentioned above are in fact representing two generations of structural steel

construction.

Those constructed in the 50’s or before were rivet-jointed frame with a relatively broader base

which rigidity is not an important factor in the structure. Those constructed recently are

high-rise buildings often more than 40 storeys in height. Structural performance, especially

under typhoon situation or in case of fire, which definitely fatal when human life is the main

concern, are becoming a vital consideration in the design and use of this kind of buildings.

However, due to the employing of modern materials and techniques such as the use of high

tensile steel, tension bolts, improved welding and connection techniques, more reliable

fire-proofing materials, together with the use of more effective layout concept used in building

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design, as well as the introduction of more powerful and economical equipment in construction,

the use of structural steel construction can now serve most of the strictest requirements of

modern buildings and is becoming more popularly accepted once again today.

Advantages and drawbacks in the use of structural steel in construction

Debates whether the use of structural steel in construction is better and more effective than using

reinforced concrete construction, or the vice versa, have been going on for decades. Both

arguments sound perfectly good in theories. The below are some of the advantages and

drawbacks that experts often claim in structural steel construction.

Advantages in the use of structural steel construction

1. Structural performance

Structural steel has a very high yield stress both in taking compression and tension. As a result

of this, the amount of steel used in building to produce the equivalent performance is much less

than that of using ordinary reinforced concrete. Due to the use of lesser materials, the weight

of building can be lighter and resulted to a smaller foundation, enabling the building to reduce

the size in column, achieving larger span and headroom. Similar structural advantages may

also be maximized in case where suitable beams and slab arrangement is employed, such as by

the using of one way slab on relatively closely-spaced shallow beams, or by the using of

composite beam/slab design.

Fabrication and connection of the structural steel members may be difficult to have quality

control, but as most of the members are fabricated in well-equipped fabricating yard off-site

(Figure 5), such problems are much improved under contemporary techniques.

Figure 5 – Fabrication of steel components are done in well-equipped fabricating yard

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Steel does not have the problems of curing or slow development of strength, and is less affected

by moisture movement and creeps.

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.

2. Construction

Speed of construction using structural steel may normally be faster than in-situ concrete due to

the following reasons.

2.1 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.

2.2 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.

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

inclement weather.

3. Maintenance

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 in the use of structural steel construction

1. Structural performance

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

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of steel or very strong and rigid connection design may be required to strengthen the

performance of the structure (Figure 6 and 7).

Figure 6 – the external trusses (left) used in Manulife Tower, the tie members (middle) and the

anchor frame that built inside the RC core wall for stiffening the core with the external frame

Figure 7 – Belt trusses (left) and out-rigger systems (right) used in Cheung Kong Center as a

means to increase the stiffness of the composite structure

Besides, 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 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.

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2. Construction

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

(Figure 8 & 9). Fabricated steel members are often made to quite a large size and heavy weight

in order to minimized 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 (Figure 10) 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.

Figure 8 – steel components are delivered and hoisted to work spot for erection

Figure 9 – Delivery of huge amount of steel components can cause complication in the handling

of materials on site. Some members are very massive and large in size

Figure 10 – provision of carnage facilities is critical for the erection of steel components

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Connection of the pre-fabricated steel members requires accurate dimension coordination

(Figure 11). Slight discrepancy exists in the off-site or on-site works may therefore 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.

Figure 11 – complicated frame and special design often demand very accurate dimensional

control and coordination

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.

Extent and constraint in use of structural steel construction in Hong Kong

Generally speaking, the use of structural steel construction in Hong Kong is not so popular as

compare to other countries such as in Japan, United Kingdom, United states or other western

European countries.

The main reasons for this may be due to the following factors:

1. Lack of the required resources

Workers or contractors for structural steel construction usually come from some related

industries such as shipbuilding or other heavy industries involving large-scale steel works.

However, these industries are not originated in the traditional industries of Hong Kong. In fact,

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Hong Kong lack the expertise and skill in the carrying out of such work in general-Engineers,

contractors or even skilled workers qualifies for structural steel construction are rare and thus

resulted to relatively higher construction cost-This situation is specially obvious for building of

complicated structural steel design or for very large scale development projects.

Besides, Hong Kong cannot produce most of the essential materials such as structural steel and

other required mechanical equipment for work-Such materials are to be imported and thus make

the cost of construction more expensive.

2. Lack of working spaces

The carrying out of structural steel works require 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. Temporary fabrication or

treatment yard sometimes may require up to 10,00 m2 of space to be allocated for projects using

structural steel construction under the situation of Hong Kong. The completed structural

members are transported to site for connections afterward. This may in fact increase the

overall cost of construction. Besides, the availability of extra working space is sometimes

more than a consideration of budget for land in Hong Kong is scarce.

The completed structural members often weigh more than 10 tonnes. 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. These factors are in fact unfavourable for

the use of structural steel construction as a whole in Hong Kong.

3. Existence of alternative techniques

The major advantages of using structural steel construction are 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.

4. Fire proofing requirements

Fire proofing requirements in Hong Kong is quite strict for the buildings are mainly high-rise in

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

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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.

Structural behaviour of structural steel construction for high-rise building

Ta1l and slim buildings like many of the sky-scrapers find in Hong Kong may not be a kind of

structure that are favourable in the using of structural steel construction. The main problem is

that tall buildings may bend significantly under normal wind load and produce undesirable

movements and deflection. Rigidity in the connection of the steel members is of no doubt

required to improve such situation, but of course, this incurs certain technical difficulties

especially when site connection is concerned.

To overcome such drawback, one of the common methods is to build a strong reinforced

concrete core which usually locates in the centre of a building (refer also to figure 1 & 4). This

core acts as a stiffening structure and help to take up most of the bending movement created by

wind load. The core is usually rectangular or square in section with the perimeter wall

sometimes more than 450mm thick and accommodating part of the essential utilities such as the

staircases, lift shafts, toilets or services ducts etc. Sometimes, suitably located shear wails can

act similarly to a rigid core.

The second method is by the introduction of more bracing or truss members between the main

structural steel members (refer also to figure 6 & 7). The bracing members can produce stiff

diagonal supports and help to resist wind pressure by transmitting the load from the external

faces of the building through the floors which act as rigid diaphragms also. However, this may

make the layout of the building becomes complicated, lower the space efficiency of the building

or produce additional technical difficulties to internal or external finishes.

To increase the overall performance, most of the modern floor systems for this kind of structure

are using composite floor design (Figure 12 & 13), that is, the steel floor joists are topped with a

RC slab that form a very strong and rigid composite floor membrane as part of the stiffening

provision for the entire building system.

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Figure 12 – placing GI sheet as the under deck onto the floor joists (left) and welding of the

steel studs to joists to provide the anchorage between the RC slab and the joists

Figure 13 – connecting the floor reinforcing bars to the core starters (left),

floor bars fixing and concreting (right)

Fabrication, erection, connection and fire-proofing treatment of structural steel members

1. Fabrication and Erection

Most steel embers used in structural steel works are standard hot-rolled steel sections as

specified by BS4: Part 1. The common types of section used include the universal beams,

universal columns, joists, channels, angles and T-bars. The dimension of these sections has

quite a large range to convenient various design needs and requirements. The typical steel

sections under BS4: Part 1 are shown in Figure 14.

Figure 14a – connecting steel columns to form a column base

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Figure 14b – connecting columns to columns by welding and by bolts

Figure 14c – connecting column to beams (left) and beam to beam

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 (figure 15) in which

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

(figure 16) 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.

Figure 15 – a temporary fabrication workshop set-up for the roof structure

of the new airport terminal building at Chek Lap Kok

Figure 16 – worker remove dust and grease on surface of steel before applying anti-rusting

coating (left), and the cover hood for doing sand blasting and paint curing

In order to have more fabrication done off-site to gain the best benefit from works, most of the

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completed members or components are fabricated into a size or weight as large and heavy as

possib1e up to the handing limits (figure 17), such as the storage spaces or the capacity of

hoisting equipment, of the site (refer also to figure 10).

Figure 17 – a section of circular steel column cut to module equivalent to 3-storey length (left),

each hoisting for smaller units can be up to 3 beam sections per lift.

Hoisting equipment assisting in the erection of structural steel members is usually done by one

or two tower cranes with luffing jib mounted inside the central core of the building (refer also to

figure 10). This kind of crane can handle at its effective radius up to a weight of more than 5

tonnes. Time management for the use of hoisting equipment in the erection of members is a

crucial consideration for structural steel construction. The hoisting process of a member

includes the lifting of it from the ground level up to the level of installation, maintain the

member until it can be placed in the pre-set slot or cleat where it can be initially fixed by

workers. Sometimes the entire process may engage the crane for quite along time and idling it

from other works.

2. Connection

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, rivet or 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.

By welding

Welding of steel can be done by oxy-acetylene method. A blowpipe in this case is used which

allows the heat from the burning oxygen-acetylene mixture to raise the temperature of the

surfaces of steel to be joined. A metal filler rod is held in the flame and the molten metal from

the filler rod then fuses the surfaces together.

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Figure 18 – welding equipment: transformer and power distribution frame for electric arc

welding (left) and oxy-acetylene cylinder for gas welding (right)

Another method for welding is by the use of electric arc. The electric arc is produced by a low

voltage electrical supply when in contact with an earthed steel surface. The high temperature

produced by the arc causes the metal filler rod to melt and fuse the surfaces.

Figure 19 – welding equipment: the filler rod feeding machine (left and middle)

and the treatment to the steel members before welding

The electric arc method is in fact having more advantageous performance when compare to the

oxy-acetylene method for it is more convenient and save to use, and does not produce high

temperature over a large area of steel. Such temperature may easily lower the strength of high

tensile steel due to the forming of internal stress within the section of member under differential

cooling.

By the use of bolts

Connections using bolts almost limit to site connection works nowadays. There are three kinds

of bolt used for such purposes, they are the black bolts, turned and fitted bolts and high strength

friction grip bolts.

Black bolts can be cold or hot forged with machined thread. They are the cheapest in cost but

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have a lower shear strength thus are not suitable for most modern high rise building where the

loading requirements are high.

Figure 20 – typical connection making use of bolt and splice plate, note that some bolt connections

are temporary provision and the actual joint will be welded to increase rigidity afterward

Turned and fitted bolts are turned to fit tightly into the holes of a member and secured by nuts.

Instead of being forged, the shank is machine formed so the production cost of the bolt is higher

but with a more accurate dimension and requiring smaller clearance allowance in the

connection.

High strength friction grip bolts are also known as torque bolts. They are made of high yield

steel. When the bolt is turned and secured by nut, tremendous tension is developed and

gripped the plates tightly together. This can produce great frictional resistance within the steel

plates which are to be connected and make the joints relatively quite strong and rigid. The nut

is usually tightened by using a torque wrench which measures the tightness of the connection.

3. Fire-proofing treatment

Conventionally structural steel members are fire protected by concrete, which is slightly

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

have 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.

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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.

Figure 21 – preparing the fire resistant plaster materials (left), spraying of the plaster(middle)

and the finished steel surfaces (note that the underside of floor slab does not require protection

for it is a composite slab with RC on top

Furthermore, by encasement method, say, using gypsum board, to encase the exposed surfaces

of the steel members. The encasement can serve as the surface finishes as well as to provide

additional fire protection to the steel member.