Structural Engineer Blogger: August 2013

Saturday, August 17, 2013

Fire Protection For Steel Element

- There are four common methods of fire protecting structural steelwork;

- Intumescent coatings

- Board based systems

- Sprayed fire protection systems

- Concrete encasement or filling

Intumescent coatings:

Intumescent coatings may be brushed or sprayed onto steelwork rather like paint. The materials expand when subjected to fire and form an insulating foam. Intumescent coatings can achieve up to 120 minutes fire resistance, and are used mostly on exposed steelwork.

Board based systems:

Board based systems are used to form rectangular encasements around steel members, such as internal beams and columns. Paint or other finishes can be applied directly to the boards. The level of fire resistance achieved depends upon the type and the thicknesses of the boards used and upon the method of attachment.

Sprayed fire protection systems:

Sprayed fire protection systems are usually based upon cementitious materials and are applied directly onto the surface of steelwork. They are generally low cost, but cannot receive finishes owing to their coarse uneven texture. Sprayed materials tend to be used where steelwork is concealed or where appearance is unimportant. Fire resistance is similar to that of board based materials.

Concrete encasement or filling:

Concrete Filled Structural Hollow Sections

Structural Hollow Sections (SHS) can be fire protected by filling with reinforced concrete. Concrete filled structural hollow sections can achieve 120 minutes fire resistance.

Slimdek®

The Slimdek® system has inherent fire resistance as the ASB section is encased in concrete with only the bottom flange exposed to fire. Without fire protection Slimdek® canachieve 60 minutes fire resistance.

Periods of fire resistance in excess of 120 minutes can be achieved if ASB is fire protected.

Multi-storey frames requiring 30-60 minutes can have 40% of the floor beams unprotected by following the recommendations of a special design guide.

Protection thicknesses

The section factor of a particular steel section is its surface area per unit length divided by its volume per unit length (A/V). This parameter defines how quickly a steel section will heat up when subjected to fire. The section factor for a member with box protection is lower than that for a member with profile protection, and hence box protected steelwork heats up more slowly and requires less protection.

Typical spray or board thicknesses for a column in a multi-storey building are as set out in the table below.

Fire resistance(minutes)	Profile Protection(mm)	Box Protection (mm)
30	10	12
60	18	15
90	24	20
120	30	25

Typical spray or board thicknesses based on 254UC x 89 kg/m column in a multi-storey building.

Connections in trusses

Truss connections are generally welded, except for site splices.

The connections between the members of a truss were traditionally bolted but welded connections are now preferred, especially in tubular construction. Nevertheless it may be necessary to introduce splices in the chord members if the complete trusses are too long for transportation. These splices provide for bolting individual lengths of trusses on site and should be located and detailed carefully if they are architecturally important.

Connections in open section trusses are lapped and welded.

Traditional truss construction based on open sections (angles and tees) used bolts and gusset plates. The position of the bolts was usually detailed to minimise eccentricities. In welded construction the internal members are typically attached directly to the top and bottom chords. These chords are usually continuous except at changes of direction or where splices are necessary to reduce the length of the assembly for transportation. Internal members lap onto the chord members and are simply connected by fillet welding.

Connections in trusses using heavier sections are welded but may require more careful preparation.

Long span or heavily loaded trusses often use I, H or channel sections. Internal members are generally cut to fit directly against the flanges of the chord members and are profile welded to form the connection.

Connections in tubular trusses are usually welded.

Tube to tube joints, such as those in tubular trusses, are usually welded, because full profile welded joints not only look better but are also cheaper than creating elaborate bolted joints. It is usually cheaper to make truss connections if the chords are made from rectangular tubes, rather than circular. This is because the ends of the diagonals can be straight cut, rather than cut to the more complex intersection profile if circular chords are used. In either case the end of the diagonal may be cut in the same plane as the chords if a fillet weld is adequate. This will normally be adequate, but if a butt weld is needed the end of the diagonal must be properly prepared. A number of fabricators use profiling machines which automatically cut the diagonal to the correct line and end preparation for welding.

Welded connections for tubular trusses

Intersecting angles should not be too small.

It is important that the truss geometry does not produce too small an intersection angle between members. The limit is about 30 degrees for rectangular hollow sections and 20 degrees for circular hollow sections, and below this value it is difficult to get the welding electrode in to make a weld.

Wednesday, August 14, 2013

The Idea Of Strengthening Reinforced Concrete Column

For reinforced concrete sections, it is well known that the concrete section resists compressive stresses while the reinforcement bars take the tensile stresses. Thus, the resistance of reinforced concrete members, especially columns, depends on the resistance of concrete to axial compression. On the other hands, the behavior of concrete elements subjected to tri-axial compression is better than that of axially loaded elements. If a Mohr-Coulomb failure criterion is considered, yield is expressed in terms of maximum and minimum principal str. For a certain value of confining stresses σ 3 , failure takes place when the circle, with a diameter equals σ3 - σ1 , touches the failure envelop as shown in Fig.

Increasing the lateral confining stress, σ3 , will consequently induce an increase in the axial stress σ1 that will produce failure. However, this idea can be implemented to increase the load carrying capacity of structural members by increasing the lateral confining pressure.

In reality, the load carrying capacity of reinforced concrete column can considerably increased by applying a reasonable amount of lateral external pressure.

The idea of the proposed method is thus to apply a lateral confining stress on the sides of the member. As high as the value of such confining stress is the expected increase in axial load capacity of the member. If the confining of the reinforced concrete element is made along all sides, the effect of strengthening is expected to be so effective. Thus, the proposed method is aimed at applying confining forces at the corner of the section to keep the whole concrete core under confining compression.

Sunday, August 11, 2013

TYPES OF CRACKS IN REINFORCED CONCRETE COLUMNS

1. Cracks due to eccentricity in concrete columns

2. Cracks due to corrosion of reinforcement in concrete columns

3. Cracks due to increased column load

Saturday, August 10, 2013

Rebar Coupler

The SimGrip®-LT System is used in applications where it is more difficult to rotate the continuation bar

GripLock-MF is a two-piece male/female (M+F) swaging system utilizing a mechanical coupler to splice deformed reinforcing bars

GripLock-PS is a position coupler with a threaded stud between two female GripLock couplers. It is used for applications where bars can not be rotated or it is not practical to rotate the bars due to bent bars, pile installations to engage the threads or when bars are too long and heavy.