Difference Between Bridge and Culvert

By

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June 17, 2020

It is generally said that all culverts are bridges but not all bridges are culverts. Bridges are structures that are designed to span over physical obstacles such as water bodies, valleys, or existing highways (fly over bridges). The commonest distinction that is made between bridges and culverts is that the span of culverts is not expected to exceed 6 m, otherwise it should be regarded as a small bridge.

According to the American Association of State Highway and Transportation Officials (AASHTO) the definition of bridges includes culverts with openings measuring more than 6.1 m (20 ft) along the centerline of the road, and also includes multiple pipes where the distance between openings is less than or equal to half of the pipe opening. What this means is that in some cases, multi-cell culverts can also meet the definition of a bridge. However, every site condition is unique and there can be cases where this distinction cannot be very express. But in specific terms, culverts are smaller structures with peculiar features when compared with bridges which are more elaborate structures.

The traditional definition of culvert has been based on span length instead of function.

A culvert is defined in the Standard Specifications as any structure, whether of single- or multiple-span construction, with an interior width of 6.096 m (20 ft.) or less when the measurement is made horizontally along the center line of the roadway from face-to-face of abutments or sidewalls. Structures spanning more than 6.096 m (20 ft.) along the center line of the roadway are considered bridges.

It will be interesting to look at some of the distinctions between the components of bridges and culverts, especially for a water body crossing.

A typical bridge will consist of a bridge deck and abutment. There may or may not be intermediate piers depending on the span and the structural scheme adopted. The structural component of a bridge which interacts with the soil are the mainly the abutments and wing walls only. However for culverts, all parts of the culvert interacts with the surrounding soil, or in some cases, all parts will interact with the surrounding soil except the top slab. This is mainly because culverts are tunnel-like structures and are generally referred to as buried structures.

Deep foundations or special foundations are usually required for transferring the load from bridge decks to the soil, but the loads from box culverts are usually transferred to the surrounding soils without the need for additional special foundation. The bottom slab of culverts usually acts as the medium for conveying storm water and as the foundation as the same time. In essence, spread footings are usually very adequate for box culverts. Also, the friction between the side walls of a culvert and the surrounding soil offers some shaft resistance in supporting the buried structure, that is why a backfill material must be of very high quality. When a large volume of water is anticipated with considerably deep stream bed, box culverts cannot be an option.

Typical culvert — Typical multi-cell culvert

Culverts receive less attention than bridges because they are buried structures, and when they are performing satisfactorily, they are easily forgotten. Furthermore, the failure of culverts is usually less disastrous and expensive than failure of bridges. However, failure of culverts crossing a road can pose life threatening and environmental hazards to human beings and animals.

Hydraulically, culverts are designed for peak flow with the consideration that the inlet is fully submerged. This is for proper performance and discharge of the flow but such considerations are not made for bridges. For bridges crossing a water body, the basic consideration will be to ensure that the deck doesn’t get submerged, or other considerations to ensure that boats, canoes, and floating debris can flow through freely without impacting the bridge deck. Data of the highest water level of the stream is usually very sufficient for this.

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Wheel Load Dispersal on Box Culverts

By

Ubani Obinna

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June 16, 2020

Typically, anyone who wishes to design box culverts should be very familiar with design loading for bridges. In the UK and European Union, the person should be familiar with BS 5400 Part 2 and EN 1992-2. This is because the traffic actions transferred to the members of a box culvert are very similar to that of bridges, and of course taking into account the peculiarities of box culverts due to the fact that they are usually buried under the ground. Hence, traffic actions such as wheel load is dispersed to the roof slab of the culvert as uniformly distributed load depending on the thickness of the earthfill.

The loadings on a box culvert according BS 5400:Part 2 and implemented by BD 37 are ;

(i) Permanent Loads

Dead Loads
Superimposed Dead Loads
Horizontal Earth Pressure
Hydrostatic Pressure and
Buoyancy
Differential Settlement Effects

(ii) Vertical Live Loads

HA or HB loads on the
carriageway
Footway and Cycle Track Loading
Accidental Wheel Loading
Construction Traffic

(iii) Horizontal Live Loads

Live Load Surcharge
Traction
Temperature Effects
Parapet Collision
Accidental Skidding
Centrifugal Load

For the sake of this article, our attention will be on vertical wheel load dispersal on box culverts.

HA and HB Carriageway Loading

The nominal carriageway loading shall be HA or HB Loading as described in BD 37, whichever is the more onerous.

(a) HA Loading

(i) Where the depth of cover (H) is 0.6 m or less, HA loading shall consist of the HA UDL/KEL combination. No dispersion through the fill of either the HA UDL or the HA knife edge load shall be applied.

(ii) For cover depths exceeding 0.6 m, the HA UDL/KEL combination does not adequately model traffic loading. In these circumstances the HA UDL/KEL combination shall be replaced by 30 Units of HB loading, dispersed through the fill as described in paragraph (c) below.

(iii) Account shall also be taken of the single 100 kN HA wheel load, (dispersed through the fill as described in paragraph (c) below), where this has a more severe effect on the member under consideration than the loads described in (i) or (ii) above.

(b) HB Loading

(i) 45 Units of HB loading shall be applied on structures on Trunk Roads and Motorways. On structures on other Public Highways, 30 Units shall be applied unless a higher value is specified by the Overseeing Organisation.

(ii) A minimum of 30 Units of HB loading shall be applied to all structures including those that are designated to carry HA loading only.

(c) Dispersal of Wheel and Axle loads through the Fill

(i) All wheel loads shall be assumed to be uniformly distributed at ground level over a contact area, circular or square in shape, based on an effective pressure of 1.1N/mm².

(ii) Dispersion of a wheel load through the fill may be assumed to occur both longitudinally and transversely from the limits of the contact area at ground level to the level of the top of the roof at a slope of 2 vertically to 1 horizontally as shown in Figure 2.3. Where the dispersion zones of the individual wheels overlap, they may be combined and distributed jointly as shown in Figure 2.3 (Zone 2). This applies to adjacent wheels on the same axle and to wheels on succeeding axles.

(iii) Where however any individual wheel is located close to the edge of the structure such that its 2:1 dispersal zone is curtailed by a headwall, the increase in pressure near to the headwall shall be taken into account. This may be done by assuming that the load is dispersed transversely over the curtailed width of the 2:1 dispersal zone.

(v) A wheel load not directly over the part of the structure being considered shall be included if its dispersion zone falls over the part of the structure.

Load dispersal schematic diagram — Schematic diagram of wheel load dispersion on culverts

(d) Dispersal of the Wheel and Axle Loads through the Roof Slab.

Where the dispersed width of the wheel or axle at roof level is less than the spacing between adjacent joints (L_j), a further lateral dispersal of the load may be made at 45^o down to the neutral axis of the roof slab (at depth h_na) so that:

A single wheel is dispersed over a total width of C + H + 2h_na
An axle is dispersed over a total width of C + (n-1)S + H + 2h_na

where:
n is the number of wheels on an axle
S is the wheel spacing and
h_na is the depth from the top of the roof to the neutral axis which may for convenience be approximated to half the overall roof depth.

Dispersion through the slab at 45^o cannot occur through a longitudinal joint. The above approach does not account for the distribution properties of the structure itself.

Solved Example on Wheel Load Dispersal

Let us consider a box culvert with the dimensions shown below. The cover of thickness of earthfill above the culvert is 1.5 m. Obtain the wheel load distribution at the roof of the culvert.

Since the thickness of the cover is greater than 0.6m, we shall consider 30 units of HB or single 100 kN HA load dispersed through the culvert, and then 45 units of HB. The most onerous will govern the design.

HA 100kN wheel load

Contact patch area to produce 1.1 N/mm² = √(100000/1.1) = 302 × 302 mm

Using 2:1 rule, the load will be dispersed to the roof of the culvert as follows;

Dispersed area on top of box = 302 + [2 × (1500/2)] = 1802 × 1802 mm

Let us assume that the thickness of the roof slab is 300 mm, and assuming that the neutral axis is 150 mm;

Dispersed width to neutral axis of box = C + H + 2h_na = 302 + 1500 + (2 x 150) = 2102 mm

Wheel load on dispersed area = 100/2.102² = 22.632 kN/m²

45 units of HB load

Wheel load = 45 × (10/4) = 112.5 KN

Contact patch area to produce 1.1 N/mm²= √(112500/1.1) = 320 × 320 mm

Let us assume that there are no longitudinal joints in the structure. Therefore the code allows a transverse distribution down to the neutral axis of the roof slab (estimated as half the depth of the slab).

Depth from carriageway level to neutral axis of roof slab = 1500 + (300/2) = 1650 mm

In HB loading, 4 wheels on each axle are spaced at 1 m so, at 1:2 gradient, the transverse dispersal lines will overlap at a depth = 1000 – 320 = 680 mm < 1650 mm hence overlap and need to consider 4 wheels.

An axle is dispersed over a total width of C + (n-1)S + H + 2h_na= 320 + (4 – 1) x 1000 + 1500 + 2(150) = 5120 mm
Alternatively;
Transverse dispersal of axle load = 3000 + 320 + (2 × 1500/2) + (2 × 300/2) = 5120 mm

Front and rear pair of axles are spaced at 1.8 m so, at 1:2 gradient, the longitudinal dispersal lines will overlap at a depth = 1800 – 320 = 1480 mm < 1650 mm hence we need to consider the front and rear pair of axles.

Longitudinal dispersal of axle load = C + (n-1)S + H + 2h_na= 320 + (2 – 1) x 1800 + 1500 + 2(150) = 3920 mm

A little consideration will show that the dispersed length falls outside the width of the culvert, but we still go ahead to use it.

Axle load on dispersed area = 8 × 112.5 / (5.120 × 3.92) = 44.84 kN/m²

Therefore, HB loading is critical. This shows that value of 44.84 kN/m² should be factored as appropriate, and applied on top of the culvert as a UDL for design purposes (depending on the load case of interest).

High Temperature Behaviour of Concrete Produced with Desert Sand

By

Ubani Obinna

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June 15, 2020

There has always been some issues with the use of desert sand as a construction material due to the fact that it is too rounded and uniformly graded to be suitable for construction purposes. Concrete produced from desert sand can exhibit lower strength, poor cohesion, poor slump, and poor workability when compared with River Sand. However, researchers from the College of Civil and Hydraulic Engineering, Ningxia University, China have evaluated the behaviour of concrete produced with desert sand when subjected to high temperature. The study was published in Springer – International Journal of Concrete Structures and Materials.

There have been speculations and researches on the application of desert sand in the production of ‘low carbon concrete‘, but the researchers from Ningxia suggests that desert sand has been successfully used for construction in China, citing some publications.

At higher temperature, the properties of concrete are usually altered. Therefore, the knowledge of the mechanical properties and behaviour of concrete at elevated temperature is important due to fire events that can occur in buildings. In previous research works, the compressive strength of ordinary and high-performance concrete have been observed to generally declined with temperature. However, the strength reduction of ordinary concrete was higher than that of high-performance concrete.

To carry out the experiment, the researchers used desert sand from Mu Us Sandy land in Yanchi Country as the fine aggregate, grade 42.5R cement, and 10-20 mm diamater crushed rock coarse aggregate.. They went ahead to study the effects of desert sand replacement rate (DSRR), temperature, and cooling regime on the elasticity modulus under static compression, cubic compressive strength and prismatic compressive strength of concrete. The influence of temperature on the microstructure of concrete produced with desert sand was also analyzed using X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). After curing the samples for 28 days, CSL-26-17 heating furnace with a ultimate temperature of 1600 °C was used to carry out elevated temperature test. Target temperatures were 100 °C, 300 °C, 500 °C and 700 °C.

The results showed that the cubic compressive strength of concrete produced with desert sand increased firstly, and then declined with temperature. Whereas, the prismatic compressive strength and elasticity modulus of concrete produced with desert sand under static compression declined with temperature.

Furthermore, it was observed that the elasticity modulus under static compression, cubic compressive strength and prismatic compressive strength of concrete produced with desert sand after elevated temperature increased firstly, and then declined with DSRR. When DSRR was equal to 40%, the elasticity modulus under static compression, cubic compressive strength and prismatic compressive strength of concrete produced with desert sand reached the maximum value.

SEM view of desert sanc concrete at different temperatures 1 — SEM of desert sand concrete at different temperatures (Liu et al,, 2020)

From microscopic observations, concrete produced with desert sand at room temperature was structurally-complete and had good compactness. The interface between coarse aggregate and cement mortar became more distinct after 300 °C. After 500 °C, the dimension of intact layered CH decreased and Many small pores were produced. After 700 °C, cement matrix became more shriveled and white large crystals CH were scarce.

Disclaimer
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Reference
Haifeng Liu, Xiaolong Chen, Jialing Che, Ning Liu and Minghu Zhang (2020): Mechanical Performances of Concrete Produced with Desert Sand After Elevated Temperature. International Journal of Concrete Structures and Materials. (2020) 14:26 https://doi.org/10.1186/s40069-020-00402-3

Vanity Walls in Toilets as an Alternative Route for Waste Pipes

By

Ubani Obinna

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June 15, 2020

For a building to be useful, the structural integrity and building services such as plumbing, electrical fittings, HVAC, etc must be functional and efficient. Unfortunately, it is common to see MEP services and pipes clash with structural members such as beams and columns in buildings. However, this can be prevented or handled in the design stage through the use of Building Information Modelling (BIM). One of the commonest solutions to avoid the clash of columns and water closet (WC) waste pipes is through the use of vanity walls behind the WC.

Traditionally, vanities are fittings that are provided in rooms or water closets to aid storage, provide counter top and improve aesthetics. They can be wall mounted or supported on the floor, but they are typically placed next to the wall of a building, hence the name vanity walls.

In our own context, the aim of the vanity is to conceal the waste pipe of the WC, and to also avoid clashing of the waste pipe with the columns of the building. This is usually dependent on how the architect positioned the WC. Some contractors observe the very bad practice of chiseling structural members to allow pipes and services pass through. This is completely unacceptable and should be discouraged.

Some modern designs of WC require vanity walls by default (to accommodate the water cistern), but in some cases vanities can be constructed using properly finished sandcrete walls to provide an alternative route for waste pipes, especially when the position of the WC is not very favourable. To be favourable to plumbers, WC’s are usually positioned to back an external wall. However, when a WC is backing an internal wall, the waste pipe may clash with some sensitive structural members, and vanities may be needed. The height of the vanity can be properly raised to accommodate the flush cistern, and the top can also provide some storage as shown in the image below.

wc 1 — Vanity accommodating WC waste pipe, cistern tank, and providing counter top storage

We are going to use an example to show how vanities can be used to avoid waste pipe conflict with columns.

Floor plan 1 — Original drawing showing route of waste pipes

In the floor plan shown above, a WC is provided adjacent to an internal wall, and the location of the duct is as shown in the plan layout. A little observation of the preliminary drawing shows that the route of the discharge pipe (in green lines) is conflicting with the column close to the duct (all columns are highlighted in red). To solve this problem, a vanity wall can be provided adjacent to the main wall as shown below. The new route of the waste pipes as they pass through non-structural members directly to the duct can also be observed in the new layout.

Floor plan 2 — Introduction of vanity walls as an alternative route for waste pipe

The only disadvantage of this method is that the size of the toilet will be reduced by size of the vanity wall introduced, but it is usually a viable solution.

vanity wall 2 — Toilet with vanity walls

Building construction requires good technical coordination among all the consultants at the design stage. By so doing, all potential conflicts and difficulties that can be encountered during construction will be minimised.

The Two Aspects of Civil Engineering – How Do You Hire the Right Person?

By

Ubani Obinna

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June 12, 2020

The civil engineering industry is wide and unique. There are different fields in civil engineering such as structures, highway and transportation, water resources, environmental, and geotechnical engineering. Each of these fields appear unique in their own way, but one common thing among them is that there will always be design, and then execution (construction). Therefore, I will describe the two aspects of civil engineering as design and construction. Having experienced both faces of the industry, I can confidently talk about the characteristics of each aspect.

A design engineer/consultant

A design engineer deals with lines, shapes, figures, numbers, calculations, regulations, and codes of practice. This is usually done on a paper or on a computer. The final output of a design engineer is a construction drawing or model which is taken to site for execution. Therefore, a design engineer must be technically skilled with good analytical capacity. He should be computer literate, and good with numbers. A knowledge of the requirements of the building code of practice is a must, and he should have the basic ones on his fingertips without consulting any literature. His knowledge in his area of expertise should be vast.

For instance, a structural engineering consultant should be able to talk about the stresses he is expecting in any part of a structure, and the acceptable disposition of reinforcements. A geotechnical engineer on looking at soil report and diameter of a pile should be able to say something about the expected range of safe working load of the pile. A water resources and environmental engineer should be able to stand on his feet and talk about effluent water treatment design of an industry and so many more.

These people are the consultants, and they do not do the dirty job in civil engineering. Since they are mainly office guys, they should always dress smart, and be able to communicate eloquently. Whenever they visit a construction site, it is usually for inspection, supervision, sign-off, or site meetings. Anybody who is not intellectually smart has no business being in consultancy. This is because they are the people that the man on site will call once there is any challenge, and he should be able to say something meaningful while standing on his feet. Construction projects are usually time-bound, and there should be no room for delay from any stakeholder. If the challenge on site is a major one, he should be able to issue the right instruction and then provide the solution in the shortest possible time. This is because he made the design.

The expected qualities and characteristics of a civil engineering consultant are as follows;

Good communication skills (written and oral)
Strong analytical skills
Sound theoretical knowledge of civil engineering
Good knowledge of latest industry codes and standards
Good computer skills
Ability to make good engineering judgement
Ability to provide technical leadership
Confidence and integrity

Civil engineers who will end up in the design office as consultants usually have a minimum CGPA of 3.5 on a 5.0 scale. This does not mean that people will lower class of degree cannot be consultants if they develop themselves. They are usually mathematically sound, and find topics in structural engineering interesting. They can be software enthusiast, always looking out for the latest engineering software that will make their work easier. Typical consultants may have a hard time adjusting to full time site conditions, especially if they have not had a prior site experience. Furthermore, they can be distracted by designs and other unrelated activities when given a site to manage.

Site engineers/construction managers

A contractor takes over a site after receiving the approved drawings and contractual terms to go ahead and build what the consultant has designed. A site manager deals with a lot of issues that are wider than what a design engineer faces. He is interested in understanding ALL the technicalities in the drawings handed over to him. Building construction drawings comprise of architectural drawings, structural drawings, mechanical drawings, and electrical drawings. He is also expected to have construction methodology document and standard safety policy documents. As a matter of fact, all the ‘madness’ experienced by people in different professions is experienced by the man on site.

A site manager is always dealing with workers, sub-contractors, logistics, quality control, safety, time, technical accuracy, and general management. While technical ability is the key characteristic of a design engineer, managing people is the key characteristic of a site manager. This is because his success or failure depends on people.

As a result of this, people from other related professions such as architecture, building, and quantity surveying can function satisfactorily as site managers provided they follow instructions. The fundamental ability needed by a site manager is the capacity to read and understand construction drawings properly. Once he can do that satisfactorily, the rest has to do with his leadership and managerial skills. A site engineer is expected to meet his targets, and must always have a working plan of achieving them.

The expected qualities of a site manager are as follows;

Focus and result oriented
Human multitasking
Good people management skills
Integrity and resolve to always do the right thing and not cut corners
Good technical ability
Resources management
Good communications skills (written and oral)
Being proactive
Team player
Ability to think ahead
Good organisation and planning skills
Positive vibes and energy
Possession of relevant construction skills and use of common site equipment/tools

The highest technical skill required of a site manager is the ability to produce as accurately as possible, what the consultant has designed. With experience, he can however make suggestions on how the construction can be made better. This however is a function of training. A site manager with background in building or architecture will be more natural with the shell of a structure such as masonry works and finishes, while someone with structural engineering background will be more natural with the core of the structure such as reinforcements, concrete mixes, and formwork. What an architect will call the structural engineering consultant for might not bother someone with civil engineering background.

A site manager must always have an open communication line, and makes a lot of phone calls in a typical day at work. He is meant to coordinate the activities of sub-contractors such as electricians, plumbers, HVAC contractors, carpenters, interior decorators, etc. He is also meant to coordinate logistics and delivery of materials to site. During site meetings and inspections, he is supposed to lead the team around the site, and offer report on the progress of work, his targets, and the challenges they are facing. Typically, a good site manager should see challenges ahead of time and work against them from the onset. However, with the dynamics of a typical construction site, whenever an unexpected challenge comes up during construction, he should rise to the challenge and solve it immediately.

With all these, it is therefore obvious that site managers do not work alone. It is basically about people, and he is meant to offer leadership and direction at all levels. A site manager may not necessarily be a high flying academic guru. The key thing in hiring a site manager should be the person’s passion for construction, and his basic training.

A typical site manager doesn’t like to do designs, and will prefer to work with someone else’s drawing. Making designs during an ongoing project is a distraction for a site manager. A site engineer should be focused on the project a hand, and typically thinks about the project and makes plans for the following day before sleeping. His first activity in the morning could be calling a sub-contractor or supplier to know if he is already on his way to site.

Therefore, recruiters should watch out for the passion of the person they are hiring. Being passionate for either design or construction does not make you less an engineer. Do not fall for the stereotype that you should excel in both; it is not necessarily the true. Design and construction are two special but closely related aspects of engineering. From experience however, people with site experience tend to make better and more efficient design than people without site experience.

Find your passion, monetize it properly, work hard, and live your dream life. God bless civil engineers all over the world.

Building Construction Industry Reacts to Weird Slab Collapse

By

Ivy Ugorji

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June 5, 2020

A lot of engineers and builders from all over the world have reacted to the weird partial collapse of a concrete slab in a residential building. The failure was shown in a picture that has been circulating across several civil engineering social media platforms since this week.

In the picture, some section of a concrete slab of an apparently live bedroom spalled off and crashed to the floor of the room in what could have been a life threatening situation. The failure showed no reinforcements, and evidence from the rubble suggested that no reinforcement was provided in the zone that gave way. While half of the section collapsed, the remaining section stayed in place, leaving engineers to wonder what could have led to that pattern of failure.

On reacting to the failure, some people said that it is due to poor concrete mixture.

Poor concrete mixture

Segregation caused by poor materials and Inadequate proper mixture of concrete

Some others attributed the failure to lack of reinforcement.

I can’t see any reinforcement

Is there bottom reinforcement there?

Another person suggested that the ceiling fan could have been part of the cause of the failure. According to him;

If we critically look at the image, I will say the failure is as a result of weak thick sand-mortar which must have cracked due to vibration force generated from the ceiling fan. I am seeing mortar of up to 100 mm thick without mesh reinforcement. I will recommend that the ceiling fan should be removed.

Someone else attributed it to excess concrete cover as he said;

This is critical the reinforcement cover seems to be 5 inches thick

While there may not be enough data to make proper conclusion, what is your opinion on the cause of the failure?

Analysis and Design of Arch Bridges

By

Ubani Obinna

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May 31, 2020

The use of arches as a structural form can be dated back to antiquity. Arch structures are unique structural forms which resists forces majorly by converting them to compressive forces, in a process popularly referred to as arch action. By transferring the compressive forces through the arch rib or barrels, they are transferred to the base of the arch as outward thrusts, which implies that the final support of the arch must be stiff and stable. According to [1], the economic viability of any arch bridge depends on the suitability of the site’s geology and soil condition. This article aims to explore the analysis and design of a concrete arch bridges subjected to Load Model 1 of Eurocode, using Staad Pro software.

Historically, most arch bridges were associated with stone masonry, which later gave way to the use of bricks in the nineteenth century. These structures were designed to minimise the development of tensile stresses in the members, and hence often gave rise to very massive structures. However, with advances in materials such as concrete and steel, more slender and aesthetically pleasing structural forms can be achieved.

Arch bridges can be classified according to the following;

Materials of construction
Structural scheme, and
Shape of arch

As far as materials is concerned, arch bridges can be constructed from timber, stone masonry, bricks, concrete, or steel. However, in recent times, timber bridges are usually restricted to small spans. An example of a timber arch bridge is the Mur River Bridge in Austria (also called the Holzeuropabrücke wood bridge) which is made of three-hinged parabolic timber. It is considered to be the largest cantilevered timber bridge in Europe. Also, the Eagle River Timber Bridge in Michigan is another example of three-hinged arch timber bridge with a span of about 23-24 metres. The Tynset Bridge in Norway built in the year 2001 is considered the longest timber bridge in the world designed for full highway loading with a span of 70 m. The structural form consists of tied timber truss arches supporting the bridge deck with the use of suspension cables.

Another timber bridge — Mur River Bridge (Holzeuropabrücke wood bridge) in Austria

Reinforced concrete and steel arches are altogether much lighter structures than masonry arch bridges. The structure consists basically of the arch, the deck and usually some supports from the arch to the deck – in that order of importance. The basic parts of an arch bridge are;

The deck
The crown
The spandrel
The arch rib or barrel
The springings
The extrados or back
The intrados or soffit
The skewback or abutment
The rise, and
The span

These components are shown in the figure below;

Parts of an arch bridge — Components of an arch bridge [1]

Concrete arches can be made of full width curved arch, or series of ribs. Steel is usually made of series of ribs.

Full width arch rib — Full width curved arch bridge under construction

concrete arch with series of ribs — Concrete arch bridge with two ribs

Steel arch bridges can be lower arch bridge or through trussed arch bridge.

Through arch bridge 2 — Through-trussed steel arch bridge

The structural scheme adopted in any arch bridge can be influenced by a lot of factors such as the type of deck, environmental conditions, cost, and feasibility. However in terms of structural form, arches can be broadly classified as hinged or fixed. A hinged arch can be two-hinged arch or three-hinged arch. While the former is statically indeterminate, the later is statically determinate. Statically determinate arch structures are free from secondary stresses from indirect actions such as differential settlement and temperature difference. An example of a three-hinged arch bridge is the Rossgraben Bridge in Switzerland.

In terms of shape, an arch bridge can be segmental (circular), parabolic, or elliptical. However, the parabolic arch is the most popular shape for arch bridges. According to [1], the ratio of span to rise should generally be in the range of 2:1 to 10:1. The flatter the arch the greater the horizontal thrust and this may affect the structural form selected, i.e. whether or not a tie should be introduced, or the stiffness of the deck relative to the arch.

Analysis of an arch bridge on Staad Pro

It is possible to model and analyse arch bridges using Staad Pro software. We are going to demonstrate this using a parabolic arch bridge. The general structural form of the bridge is shown below. The structural form can be said to be a bit similar to that of Krka River Bridge in Croatia.

free body diagram of arch bridge — Schematic of the arch bridge

The equation of the parabolic arch bridge is given by;

y = 4x/5 – x²/50 ———– (1)

Using equation (1), the nodes for the vertical coordinates of the arch were established at 1m interval along the horizontal axis, and connected using linear line elements.

The arch is made of two ribs connected to each other with rigid reinforced concrete members along the axis of the arch and at the deck level. The arch ribs are made of concrete members 1500 mm deep, and 750 mm wide. The spandrels are made of concrete columns of dimensions 600 mm x 600 mm transferring the load of the bridge deck to the arch. The rendered structural form of the bridge deck is shown below;

Rendered skeleton of the arch bridge — Structural scheme of the arch bridge

The longitudinal girders of the bridge deck are 5 in number with dimensions of 1000 mm x 400 mm, spaced at 2 m centre to centre. They are supported by transverse girders of the same dimension, which transfer the deck load to the columns. The columns (spandrels) ultimately transfer the deck load to the arch ribs. The deck slab is 200 mm thick and has a total width of 10.4 m. The vehicle carriage way is 8.0 m wide, with 1.2m cantilever on either side with raised kerbs for pedestrian walkway. The overall dimensions of the arch bridge is shown below.

staad model arch — Dimensions of the arch bridge

The bridge deck has been subjected to Load Model 1 on 2 notional lanes, and a remaining area of 2 m. The tandem load system on the bridge was modelled as a moving load on Staad Pro. Hence, the loads considered on the arch bridge are the self weight, UDL traffic action, and wheel load traffic action.

The analysis results are as follows;

(1) Self weight

support reation — Support reactions under self weight

bending moment under self weight — Bending moment under self weight

Maximum sagging moment in arch rib = 259 kNm
Maximum hogging moment in arch rib = 901 kNm
Maximum Bending moment in column (spandrel) = 138 kNm (4th column from the left)

SHEAR FORCE UNDER DEAD LOAD — Shear force under self weight

Maximum shear force in arch rib = 359 kN
Maximum shear force in column (spandrel) = 101 kN (4th column from the left)

AXIAL FORCE UNDER DEAD LOAD — Axial force under self weight

Maximum axial force in arch rib = 3276 kN (compression)
Maximum axial force in column (spandrel) = 501 kN (1st column from the left)

(2) Traffic UDL

Bending moment traffic UDL — Bending moment under UDL traffic action

Maximum sagging moment in arch rib = 82 kNm
Maximum hogging moment in arch rib = 222 kNm
Maximum Bending moment in column (spandrel) = 39.7 kNm (4th column from the left)

SHEAR FORCE TRAFFIC UDL — *Shear force under UDL traffic action*

Maximum shear force in arch rib = 93 kN
Maximum shear force in column (spandrel) = 28.8 kN (4th column from the left)

axial force traffic UDL — *Axial force diagram under UDL traffic action*

Maximum axial force in arch rib = 855 kN (compression)
Maximum axial force in column (spandrel) = 160 kN (1st column from the left)

(3) Traffic Wheel Load

The variation of bending moment as wheel load travels through the bridge deck is shown below;

Variation of bending moment as wheel load travels through bridge deck

For moving traffic action;
Maximum hogging moment = 1593.285 kNm
Maximum sagging moment = 900.343 kNm
Maximum shear force = 646.233 kN
Maximum axial compression = 1414 kN
Maximum axial tension = 804.699 kN

There are other forces such as torsion that should be checked in the analysis result. For design purposes, the self weight and traffic actions can be combined using 1.35Gk + 1.5Qk. Other actions on bridges should also be considered.

Thank you for visiting Structville today, and God bless you. Remember to contact us for your structural designs, detailing, and project management and training. You can send an e-mail to ubani@structville.com or a whatsapp message to +2347053638996.

References
Melbourne C. (2008): Design of Arch Bridges. In ICE Manual of Bridge Design, Institution of Civil Engineers, UK

Analysis of Moving Load on Cable-Stayed Bridges Using Staad Pro

By

Ubani Obinna

-

May 30, 2020

Stays can be used to support orthotropic bridge decks which consists of continuous girders. The stays which are inclined cables passing over or pinned to the piers are attached to the girders and forms part of the supporting system of the bridge deck. Wide application of cable stayed bridges have been achieved recently due to the development of high strength steel, improved bridge deck systems, and analytical tools/software. In this article, we intend to explore the applicability of Staad Pro software (v8i) in the non-linear analysis of cable-stayed bridges under moving traffic load. This will be evaluated using a simple bridge deck model.

In cable-stayed bridges, the tension members (inclined cables) should be able to develop high degree of stiffness due to prestress under the dead load of the brdige deck, with additional capacity to take loads from the live load. In the example adopted in this article, the radial or converging cable stayed system was adopted, which is deemed the most efficient arrangement due to its ability to carry the maximum component of the dead and live load, while keeping the axial load in the bridge deck to a minimum. An example of this arrangement is the Ikoyi-Lekki link bridge in Lagos, which is the first cable-stayed bridge in West Africa.

Lekki Ikoyi Link Bridge — Ikoy-Lekki Link Bridge, Lagos, Nigeria

In Staad Pro, the simplified arrangement adopted is shown below.

CABLE STAYED BRIDGE STAAD 1 — Model of a cable stayed bridge on Staad Pro

The properties of the sections used in the analysis are as follows;
Pylons – Concrete (1000 x 500)mm
Main girders – Steel (UB 762 x 267 x 147)
Cross girders – Steel (UB 457 x 152 x 60)
Stays – 50 mm diameter steel cables

The pylons were placed on a fixed support, while the two ends of the bridge deck were placed on pinned and roller support respectively. The geometry and dimensions of the bridge components are shown below;

Cable bridge dimensions — Dimensions of the bridge

The load cases that were considered in the model are the self weight of the bridge and moving traffic wheel load. Since the deck slab was not modelled, it was not accounted for in the self weight of the bridge. Also, the UDL components of traffic action and full wheel load recommendations according to EN 1991-2 were not considered. Therefore, the results from this analysis are only representative of the assumptions made, and may not fully reflect the behaviour of cable-stayed bridges.

A tandem load of 4 wheels (300 kN each) with a width of 3 m and longitudinal spacing of 1.2 m was used in the model, and applied at the centre of the bridge.

Vehiche definition — Definition of vehicle wheel load on Staad Pro

Linear Analysis Results

When analysed on Staad Pro using linear elastic analysis, the bending moment, shear forces, axial force, and displacements were obtained due to the self weight and the moving load.

The variation of the bending moment as the wheel load travels on the bridge deck is shown in the figures below;

The deflection and bending moment of the structure under self weight is shown below. Note that this linear analysis was carried out under zero initial tension in the cable.

kb1 — Deflection of the bridge under self weight – Linear Analysis

bmd 9 — *Bending moment of the bridge under self weight – Linear Analysis*

The tension in the cables are;
Cable 1 (closest to the piers) = 19 kN
Cable 2 (intermediate) = 22.3 kN
Cable 3 (farthest from the pier) = 11.2 kN

Non-Linear Cable Analysis

A preliminary attempt to run a non-linear cable analysis for moving load using Staad Pro was not successful. However, a non-linear analysis result was obtained for the self weight of the bridge members. The non-linear cable analysis command used is shown below. You can click HERE to see the definition of the terms. An initial tension force of 0.5 kN was applied to the cables.

non linear command — Non-linear cable analysis command on Staad Pro

Using the parameters above, the self weight load case converged 100% with zero errors and zero warnings. The non-linear displacement of the structure under self-weight is shown below.

non linear displacement of cable supported bridge — *Deflection of the bridge under self weight – Non-Linear Analysis*

The value of deflection was observed to be higher than the deflection obtained from linear analysis. The tension in the cables are shown below, but were observed to be lesser than the result obtained from static linear analysis.

Cable 1 (closest to the piers) = 17.3 kN
Cable 2 (intermediate) = 16.5 kN
Cable 3 (farthest from the pier) = 7.7 kN

The maximum moment under the self weight for non-linear analysis is shown below;

BMD 4 — *Bending moment of the bridge under self weight – Non-Linear Analysis*

Further analysis showed that the result of non-linear cable analysis is heavily influenced by ‘Sag Minimum’. The closer ‘Sag Minimum’ is to 1.0, the closer the non-linear result is to linear analysis and vice versa. ‘Sag minimum’ is a factor used to account for sagging in the cable when the tension is low. This is achieved is Staad Pro by modification of the modulus of elasticity of the cable. However, if the value is too low, the analysis will not converge properly.

Since the non-linear cable analysis of the structure was not successful for moving load, I recommend you run linear analysis of the moving load and obtain the critical load locations. Later, you can apply the loads statically at the critical location, and run the non-linear cable analysis under a single load case. A example of this process is shown below.

hgt — Application of wheel load statically for non-linear cable analysis

The wheel load shown in the figure above has been applied statically, and the load case was combined with the self weight of the structure. Using the same non-linear parameters used above, the analysis results are as follows;

dd1 — *Deflection of the bridge under self weight and wheel load – Non-Linear Analysis*

dd2 — *Bending moment of the bridge under self weight and wheel load – Non-Linear Analysis*

The tension in the cables at the left hand side of the piers are;
Cable 1 (closest to the piers) = 259 kN
Cable 2 (intermediate) = 185 kN
Cable 3 (farthest from the pier) = 0

The tension in the cables at the right hand side of the piers are;
Cable 1 (closest to the piers) = 14.8 kN
Cable 2 (intermediate) = 76 kN
Cable 3 (farthest from the pier) = 195 kN

A better option for handling this can as well be recommended.

You can book a zoom meeting request for training or discussion with the author by sending an e-mail to ubani@structville.com or Whatsapp message to +2347053638996

Structville Announces Webinar on Design of Box Culverts – 6th June, 2020

By

Ubani Obinna

-

May 29, 2020

In our core commitment to provide a flexible platform for learning, improvement, and disseminating civil engineering knowledge, we are delighted to announce that we will be holding our webinar for the month of June, 2020. Details are as follows;

Theme: Structural Design of Box Culverts
Date: Saturday, 6th June, 2020
Time: 6:00 pm (WAT)
Platform: Zoom
Fee: NGN 1,500 only ($5.00 USD)

Features:

Comparison of box culvert loading and design to BS 5400 and Eurocodes
Comparative methods on analysis of box Culverts (Manual analysis and computer aided finite element analysis)
Design calculation sheets on design of box culverts
Design drawings
Interactive question and answer sessions
Certificate of participation

To book a space for this webinar, click HERE.

For more information, contact:
WhatsApp: +2347053638996
E-mail: info@structville.com

Eurocode 2Btraffic 0A 0A 2Bload 2Bon 2Bculvert

Load 2BDispersal 2Bon 0A 0A 2BBox 2BCulvert

Over a time of about 4 years, www.structville.com has published over 200 free unique articles on different topics in civil engineering, and continues to get better. We sincerely appreciate the patronage and support we have received over the years, as more and more people all over the world continue to benefit from the services we offer. We look forward to an exciting future together in the civil engineering community. God bless us all.

IABSE Invites Participants and Calls for Abstracts for Ghent 2021

By

Peace Ikpotokin

-

May 28, 2020

The International Association of Bridge and Structural Engineering (IABSE) has announced that the IABSE Congress Ghent 2021 will be held from 22 to 24 September 2021. The International Association of Bridge and Structural Engineering (IABSE) is a scientific / technical association with members including renowned and top-level engineers in more than 100 countries. The upcoming Congress will be held at International Convention Centre (ICC) Ghent, and is organised by Belgian and Dutch Groups of IABSE in co-operation with Ghent University. The theme of the Congress is ‘Structural Engineering for Future Societal Needs‘.

Download the preliminary invitation card and call for abstracts HERE

According to the information on IABSE website, the Congress will be an excellent forum to discuss the latest innovations on structural and bridge engineering, especially regarding the future needs of society. Scientists, students, designers, contractors, owners and experts from international organisations around the world are given the opportunity to share their latest experiences. In addition to the congress, plenty of technical and social events are organised in order to expand your knowledge and network. All of this is organised with the historical city centre of Ghent as the background.

Based on the theme, the scientific committee of the congress stated that future societal needs comprise building and maintaining safe and reliable buildings and infrastructures while coping with the effects of climate change in a world with scarcer resources and satisfying the ambition to reduce mankind’s CO₂ footprint. Anticipated sub-themes are therefore amongst others ‘Structural safety and reliability with respect to climate change’ and ‘Circularity, re-use and sustainability of structures’.

Important Dates for the congress are as follows;

February 28, 2020
September 15, 2020
November 30, 2020
January 21, 2021
March 1, 2021
April 30, 2021

June 15, 2021
June 15, 2021
September 20-21, 2021
September 22-24, 2021

Call for abstracts
Deadline for abstract submission
Notification of acceptance abstract
Submission deadline for full papers
Final Invitation and registration opens
Notification of acceptance of full papers and
announcement of presentation types
Registration deadline for all presenting authors
Deadline for early-bird registration
IABSE Annual Meetings
IABSE Congress Ghent 2021

Anticipated sub-themes amongst others are:

  • Structural safety and reliability with respect to climate change
  • Circularity, re-use and sustainability of structures
  • Emission free building of structures

Special sub-themes are:

Enhancing resilience of civil infrastructure to hurricane and thunderstorm hazards under changing climate
Structural bearings and anti-seismic devices: innovation, standards and testing requirements
Towards extending the service life of existing concrete infrastructure through advanced assessment methods

However, also more traditional sub-themes may be addressed, if the abstract and paper refer to the main conference theme, e.g.:
  • All types of bridges
  • Large span structures
  • Light-weight structures
  • High-rise buildings
  • All structural materials
  • Structural health monitoring
  • Design for earthquakes
  • Case studies
  • Failures and forensic engineering
  • Strengthening and retrofitting
  • Dynamics of structures
  • Innovative structures
  • Fatigue and fracture
  • Structural analysis and optimisation
  • Parametric design
  • Structural behaviour under fire conditions
  • Soil-structure Interaction
  • Exceptional loads on structures
  • Safety, reliability and risk
  • Architecture of structures
  • Additive manufacturing

According to the scientific committee, all written papers submitted will be peer-reviewed. The large majority of the presentations will be delivered orally in normal presentation sessions but also poster sessions, poster ‘elevator pitch’ sessions, discussion sessions, a Pecha Kucha session and special sessions will take place. In order to accommodate all sessions and speakers, parallel sessions run simultaneously in 6 to 8 breakout rooms on the three main congress days. Keynote speakers will introduce relevant topics to the congress theme and/or give a state-of-the-art overview on these topics. The technical programme will attract all those involved and interested in the state of the art and the future of bridge and structural engineering. You are hereby invited to contribute to this IABSE Congress and join IABSE community in Ghent in 2021!

Disclaimer:
www.structville.com is neither an agent, staff, nor representative of IABSE. This information obtained from IABSE website has been shared here as news in our commitment to spread qualitative information that will beneficial to the civil engineering community for their general development and career enhancement. Therefore all further inquiries regarding this congress should be directed to IABSE official website.

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