Spherical tanks (Horton Sphere) are used in several applications such as water storage, nuclear cooling, and storage of liquefied gases such as liquefied natural gas (LNG) and liquefied petroleum gas. One of the most common utilization of spherical vessels in the industry is pressurized gas storage because they can withstand higher internal pressure and have fewer size limitations than cylindrical pressure vessels. Horton Spheres usually contain pressurized gas inside the steel shell. The shell is supported by heavy steel columns which transmit the load to a reinforced concrete foundation.
Fig 1: Typical LPG spherical tank
Spherical tanks have high rigidity and durability. According to Khan (2015), the performance of a 200 m3 liquefied petroleum gas (LPG) tank with a wall thickness of 24 mm under 1.7 MPa pressure was evaluated after it had been in operation for three years. The result showed high resistance to micro cracking and shell deformation, with minimal wall thinning.
Fig 2: LPG Tank farm under construction in Lagos, Nigeria
Fig 3: Author at an LPG tank farm construction site in Lagos
In this article, we are going to evaluate the potentials of Staad Pro software in the modeling and analysis of LPG sphere tanks. We are not going to deeply evaluate the design considerations for such structures but you should know that the tank shell should be able to withstand the vapour pressure from the liquefied petroleum gas.
This pressure is dependent on temperature and the design temperature is selected from the environment under consideration. For example, LPG gas cylinder pressure (LPG gas bottle pressure) is 0 kPa at -43ºC and goes up to 2482 kPa at 70ºC.
In this article, let us model and analyse an LPG sphere tank on Staad Pro with the following data;
(1) Diameter of tank = 20 m (2) Thickness of tank shell = 25 mm (3) Columns – Hollow circular steel columns of external diameter 900 mm (thickness = 40 mm) (4) Design pressure = 1700 kPa
Fig 4: Spherical LPG model on Staad Pro
Fig 5: 3D rendering of LPG tank model
When analysed on Staad Pro for a gas pressure of 1700 kPa, the following results were obtained.
Fig 6: Lateral bending moment on the tank shell due to gas pressure
Fig 7: Longitudinal bending moment on the tank shell due to gas pressure
Fig 8: Twisting moment on the tank shell due to gas pressure
Fig 9: Lateral shear stress on the tank shell due to gas pressure
Fig 10: Longitudinal shear stress on the tank shell due to gas pressure
Fig 11: Lateral axial tension on the tank shell due to gas pressure
Fig 12: Longitudinal axial tension on the tank shell due to gas pressure
The video tutorial for this modeling and analysis is available on special request by sending an e-mail to ubani@structville.com. Also free contact us for special designs related to infrastructures in oil and gas facilities and tank farms.
References Khan F.A. (2015): Spherical tanks in energy storage systems. A PhD thesis subimtted to the Department of Mechanical Engineering, WORCESTER POLYTECHNIC INSTITUTE .
In civil engineering construction works, contractors bidding for a job are always required to specify the rate they will use in executing a given item of work. In a competitive bidding, the client will review the rates supplied by the bidders, and award the contract to the person he finds most suitable.
Concrete is a common construction material that is basically made from cement, sand, gravel, and water. The main aim of this article is to teach you how to build up your rate, and quote for concrete in construction works.
The unit of concrete in construction is specified in cubic metres (m3). For instance, if a floor slab has a net area of 250 m2, and a thickness of 150 mm, the volume of concrete required will be stated as (250 x 0.15 = 37.5 m3). In the bill, a contractor is expected to state the cost of casting a cubic metre of the specified grade concrete (say grade 25), which can be used to relate the cost of casting the entire slab.
Note that the rate supplied by the contractor is expected to include the cost of materials, plant, transportation, labour, and contractor’s profits.
There are basic considerations to make while quoting for concrete because you should not bid too high or too low. It is possible for contractors to have a wide difference in their rates for the same job. For a competitive tender without bias, a company that is going to hire equipment will likely bid higher than a company that has its own equipment. The same goes with labour, transport facilities, etc.
Bidding for a job should be an intelligent process, and the contractor should know his capacity as it will likely influence his cost and profitability. The cost of casting concrete in one day is not the same with casting it for two days. Therefore, a contractor’s capacity can enable him bid higher or lower depending on the context.
To make it simpler, let us give an idea on how you can build up your rate for grade 25 concrete.
In the past, we have made a post on how you can achieve grade 25 concrete on site. We were able to show that the mix ratio of 1:2.5:3.5 can yield grade 25 concrete. Let us assume you wish to use this mix ratio in building your rate.
The total volume in the mix ratio is given by; 1 + 2.5 + 3.5 = 7
Cement Ratio of cement by volume = 1/7 Density = mass/volume Mass of cement required = (1/7) x 1440 = 205.7 kg Making allowance for shrinkage = 1.54 x 205.7 = 316.77 kg Number of bags of cement required per of concrete = 316.77/50 = 6.33 bags (use 7 bags)
Sand Ratio of sand by volume = 2.5/7 Density = mass/volume Mass of sand required = (2.5/7) x 1650 = 589.285 kg Making allowance for shrinkage = 1.54 x 589.285 = 907.498 kg Making allowance for waste = 1.2 x 907.498 = 1088.99 kg/m3
Granite Ratio of granite by volume = 3.5/7 Density = mass/volume Mass of granite required = (3.5/7) x 1650 = 825 kg Making allowance for shrinkage = 1.54 x 825 = 1270.5 kg Making allowance for waste = 1.15 x 1270.5 = 1461.075 kg/m3
Market Prices of Materials including transportation to site; Cement = ₦4100 per bag Sharp sand = ₦ 3500 per tonne Granite aggregate = ₦ 16000 per tonne (the current basic rate of granite is about NGN 9000 per tonne, but the cost of transportation is currently so high)
Cost of materials Cost of cement per cubic metre concrete = 7 x 4,100 = ₦28,700 Cost of sharp sand per cubic metre of concrete = 3500 x 1.08899 = ₦3,812 Cost of granite per cubic metre of concrete = 16000 x 1.461 = ₦23,376 Total Material Cost = ₦55,888 per cubic metre of concrete
(b) Plant Rate of Concrete mixer per cubic metre of concrete = ₦600 Rate of vibrator per cubic metre of concrete = ₦350 Operator = ₦500 Total Plant Cost = ₦1,450 per cubic metre of concrete
(c) Labour Labour output (production and placement) per cubic metre of concrete = ₦7,000
Total cost of production = ₦55,888 + ₦1,450 + ₦7,000 = ₦64,388
(d) Profit and Overhead (20%) 1.2 x ₦64,388 = ₦77,205
Therefore thecost of producing one cubic metre of grade 25 concrete is ₦64,388
The West African Standard (WAS) compaction test is a type of compaction procedure that utilises intermediate compaction energy (compactive effort) for densification of soils. It has been recommended for densification of soils for highway construction in Nigeria and some other West African Countries. The compaction energy of WAS lies between the compaction energy of BS Light (Standard Proctor) and BS Heavy (Modified Proctor).
The details of the West African Standard compaction test using BS Mould are as follows;
Volume of mould = 1000 cm3 = 0.001 m3 Number of blows = 10 Number of soil layers = 5 Weight of rammer = 4.5 kg Height of fall = 0.4575 m
Therefore, the compaction energy (compactive effort) of WAS using BS Mould is given as follows; Compactive effort = (9.81 x 10 x 5 x 4.5 x 0.4575)/0.001 = 1009816.875 N.m/m3 = 1009.816 kN.m/m3.
The details of the West African Standard compaction test using the CBR Mould are as follows;
Volume of mould = 2305 cm3 = 0.002305 m3 Number of blows = 25 Number of soil layers = 5 Weight of rammer = 4.5 kg Height of fall = 0.4575 m
Compactive effort using CBR mould = (9.81 x 25 x 5 x 4.5 x 0.4575)/0.002305 = 1094.527 kN.m/m3.
It can be observed that WAS compaction test an intermediate compaction energy when compared with BS Light (605.49 kN.m/m3) and BS Heavy (2726.5 kN.m/m3).
Applications of WAS compaction in Nigeria (1) Laboratory compaction of sub-base course according to Federal Ministry of Works Specification (1997) (2) Soaked CBR test of base course (clause 6200) (3) Soaked CBR test of sub-base course Type 2 (clause 6200)
Landing a good job soon after graduation is the dream of many young civil engineering students. On the other hand, as a professional engineer grows on the job, there might come a time when he will want to switch from one civil/structural engineering job to another. This can happen for a lot of reasons which may be personal or professional. The process of finding a new job often involves passing through interviews during which the employers will try to know as much as possible about the person interested in working for them.
The questions asked during interviews are usually related to the ideals of the organisation, and the requirements of the position they are seeking to fill. All civil engineering firms do not offer the same services. As a result, it is important to research properly on the company, know what they do, and what they stand for. Employers will also try to take a look at you, and at how your attitude and appearance will reflect their brand. Once again, it strongly depends on the area that the company will be needing your services.
Having said that, let us look at what you should expect during interviews for graduate trainee/internship roles in various civil engineering companies.
Consultancy firms Consultancy firms are usually involved in civil engineering designs, drawings, supervision, and project management. For beginner roles, they will likely be more interested in your basic knowledge of civil engineering structures, analyses, design, and drawings. They will expect the candidate to be sound, smart, and trainable. Having a basic knowledge of AUTOCAD, and other civil engineering software will be added advantage.
Small and medium scale consultancy firms usually conduct in-house interviews, and will be more interested in your technical capacity and how well you will fit into their team. For such job assessments, you might be given simple beams and slabs to analyse and design, and asked questions on the behaviour of some construction materials such as concrete. A hands-on test on AUTOCAD and other civil engineering software might be done.
On the other hand, bigger/multinational firms who have distinguished Human Resources Department might wish to conduct a larger scale interview, or might outsource the recruitment process to external consultants. In this case, the interview questions will usually extend to leadership and competency-based questions. For such bigger firms, the interview panel might consist of only one engineer and other members who might have studied sociology or business administration.
In such cases, therefore, you should widen your scope and present yourself as a general problem solver, as it will excite them more. Your ability to analyse and design complex frame structures might not really excite them as much as when you tell them how you solved a complex social problem during a volunteering activity. Apart from your technical capacity, they are very much interested in your leadership, emotional, and social credentials. Smaller design firms may not pay much attention to those aspects.
You should note that in both cases, the organisation and the interviewers understand that you are a fresh graduate with no experience, hence, they are simply looking out for a few basic things during the interview. Your ability to communicate, answer questions smartly, coordinate yourself properly, and present yourself as a quick learner will earn you serious consideration.
If you have made any significant achievement such as publications, verifiable unique designs, significant contributions, or possession of a unique skill, you should try and talk about them so that they can take you seriously from the onset. It simply gives the impression that you are an achiever. What might distinguish you from another civil engineering graduate may be your ability to write computer programs or codes, and in this digital age, any serious organisation will likely give you preferential treatment.
In summary, find the right time to talk about your special skills and experience during interviews. An organisation might wish to hire you because you did your student internship with a company they rate highly, or perhaps because you have participated in the construction of a green building, which is an area they might be interested in.
Generally, from the way you present yourself and answer questions, they will decide whether to hire you or not. As hinted earlier, your technical ability and academic records will excite smaller firms than bigger firms. This is because when an interview is done in-house, you are going to be interviewed by engineers and other technical people who will eventually become your direct colleagues. But in bigger firms, the requirements often appear broader as they seem to focus more on ‘general problem solving’ skill than specialised technical ability. Therefore, it is important to understand your interviewers based on the kind of questions they ask, and know how to answer accordingly.
Note that consultants often go to site visitation, supervision, and meeting with clients or contractors. As a result, you will be representing your firm on many occasions as an image of the organisation. Therefore, your ability to dress properly and communicate effectively is of paramount importance. It will be observed during the interview, and if your communication skills are poor, it might limit your chances.
Construction firms There are some companies that are mainly builders/contractors and rarely do designs. Interviews for civil engineering jobs with such companies are usually not focused on designs and theory of structures but on site practices. You might be asked questions like:
How do you set out a building? How do you establish levels? What concrete mix ratio will give you grade 25 concrete? How can you calculate the quantity of tiles needed to tile an area? Describe the process of constructing a flexible pavement? What is the minimum gauge of aluminium roof required to roof a steel roof building? How do you prepare bar bending schedule? etc
As you can see, these are more of practical site questions, because the firm knows that they will be sending you to a construction site. However, they know you are a fresh graduate with limited or no site experience, but you still need to impress with knowledge of basic site practices.
Bigger firms might also be interested in other things such as your knowledge of HSE, project management tools/techniques, and ability to manage people. However, for many trainee positions, most organisations will rather train you in their own way provided you are trainable.
It is important to know that highway/road construction companies will typically ask you highway-related questions, while water resources engineering companies will ask you water-related questions. But it is generally important that you exhibit good competency and knowledge of civil engineering, to the extent required of a fresh graduate.
In our previous article, we were able to evaluate the effects of temperature difference on rectangular tanks. In this article, we are going to evaluate the same effect on a cylindrical tank of the same volume, in order to obtain the internal stresses and displacements in the tank due to temperature differences. This article will serve as a comparison between the response of a rectangular and cylindrical tank to temperature actions.
In our last article, the dimensions of the rectangular tank was observed to be 3m (L) x 3m (B) x 2.5m (H), thereby giving a volume of 22.5 m3. To model an equivalent cylindrical tank of the same height of 2.5 m, the diameter of the tank was obtained as 3.38 m. The other details of the tank are as follows;
Dimensions of columns = 300 mm diameter circular column Dimension of beams = 300 x 500 mm Height of column above ground level = 3 m Diameter of tank = 3.38 m (centre to centre) Height of tank = 2.5 m (centre to centre) Thickness of tank walls and base = 250 mm Support condition = Fixed Temperature inside the tank = 120 oC Temperature outside the tank = 25 oC Maximum hydrostatic pressure from the liquid stored = 25 kPa Modulus of elasticity of concrete = 2.8 x 107 kN/m2 Coefficient of expansion of concrete = 1.0 x 10-5 /oC
Temperature change for axial elongation = Average temperature = (25 + 120)/2 = 72.5 oC Temperature difference = 25 – 120 = -95 oC
When modelled on Staad Pro using the procedure described in the video above, the configuration and results below were obtained.
Fig 1: 3D model of cylindrical water tank
Fig 2: Radial bending moment on the tank shell due to temperature load
Fig 3: Longitudinal bending moment on the tank shell due to temperature load
Fig 4: Twisting bending moment on the tank shell due to temperature load
Fig 5: Radial shear on the tank shell due to temperature load
Fig 6: Longitudinal shear on the tank shell due to temperature load
Fig 7: Hoop tension (membrane) on the tank shell due to temperature load
Fig 8: Longitudinal tension on the tank shell due to temperature load
The differences in internal stresses induced in cylindrical tanks of equal volume and height with the rectangular tank are shown in Table 1.
Table 1: Internal stresses in rectangular and cylindrical tanks due to temperature load
In some factories and industries, tanks are used for the storage of hot liquids which are used in production. In such scenarios, the temperature inside the tank and the temperature in the surrounding may not be the same. It is well known that internal forces are induced in statically indeterminate structures when there is temperature difference as the elements undergo differential thermal expansion/contraction. For simple frames, the internal forces due to temperature difference can be easily obtained using the force method of structural analysis. But for more complex structures like combination of beams and plates, software like Staad Pro can be used for evaluation of temperature difference.
For example, let us consider the reinforced concrete tank with the dimensions shown in Figure 1;
Fig 1: Structural scheme of water tank subjected to temperature difference
Dimensions of columns = 300 x 300 mm Dimension of beams = 300 x 500 mm Height of column above ground level = 3 m Length of tank = Width of tank = 3 m (centre to centre) Height of tank = 2.5 m (centre to centre) Thickness of tank walls and base = 250 mm Support condition = Fixed Temperature inside the tank = 120 oC Temperature outside the tank = 25 oC Maximum hydrostatic pressure from the liquid stored = 25 kPa Modulus of elasticity of concrete = 2.8 x 107 kN/m2 Coefficient of expansion of concrete = 1.0 x 10-5 /oC
The tank has been modelled on Staad Pro (see Figure 2) using the parameters defined above.
Fig 2: Modelling of the tank on Staad Pro
The walls of the tank were subjected to a triangular hydrostatic pressure distribution of 25 kPa. You can check how apply hydrostatic loads on Staad Pro here. The temperature difference action applied to the the tank is shown below.
Temperature change for axial elongation = Average temperature = (25 + 120)/2 = 72.5 oC Temperature difference = 25 – 120 = -95 oC
The application on Staad Pro is shown in Figure 3.
Fig 3: Application of temperature load on Staad Pro
When analysed on Staad Pro, the results shown in Figures 4-8 were obtained for the tank shells at SLS.
Fig 4: Bending moment on the tank shell due to water pressure
Fig 5: Shear stress on the tank shells due to water pressure
Fig 6: Bending moment on the tank shell due to temperature difference
Fig 7: Shear stress on the tank shell due to temperature difference
Fig 8: Displacement of tank shell and frame due to temperature difference
The internal stresses induced in the tank shell due to temperature difference is quite serious and requires detailed attention during design.
The aftermath of any building collapse is usually horrendous, and the experience is an ugly one for everybody concerned. Human lives might be lost, many might be injured, people will be traumatized, properties will be destroyed, and the environment will become a mess. The media usually responds to such events in a manner that will escalate the woes of the stakeholders involved in the construction of the building. That is why professionals must strive to get it right from start to finish.
From a technical point of view, if the recommended/standard process has been followed in construction, a lotof things will need to go wrong before a building will collapse. There are prominent issues that have been identified as the reasons for building collapse such as inadequate design, poor detailing, faulty construction, use of substandard materials, engaging non-professionals etc. In this article, we are going to focus on a special aspect of design called ‘partial factors of safety’.
The procedures for the design of buildings evolved through the years as researchers gained better understanding of the behaviour of structures and materials. This has particularly led to reductions in the factors of safety applied in the design of buildings, thereby leading to more economical designs. As a matter of fact, it is expected that the factors of safety applied in designs will continue to reduce as we gain better understanding of the behaviour of materials that we are dealing with. These factors of safety are simply associated with the fact of not being ‘so sure’. At one point in time in your life, you must have gone an extra mile in doing something, ‘just in case the worst happens’. This is exactly what the factors of safety in modern codes of practice for design offer us, but they also give us guidance on the extent of extra mile we should go, so that we would not spend too much money unnecessarily. The factors of safety were arrived at after rigourous statistical evaluations.
I have seen a cantilever roof parapet collapse due to poor placement of the reinforcement. That could have been a detailing or construction error but it has nothing to do with factor of safety which is the main topic of discussion here. Before we proceed, let us briefly look at some design principles with emphasis on factors of safety.
Design philosophies The first generally accepted principle in the design of structures is the permissible stress method. In this method;
σmax < σper ————— (1)
where σper is given by σcrit/k. In this method, the coefficient k is assessed with regard to uncertainties in the determination of local load effect σmax and of resistance σper. Therefore, the value of k may ensure with an appropriate level of security, the reliability of the structure. The main insufficiency of this method is perhaps the local verification of reliability (in the elastic range) and the impossibility to consider separately the uncertainties of basic quantities and the uncertainties of computational models for the assessment of action effects and structural resistance. In this method, the probability of failure is controlled by one quantity only, the coefficient k (Holicky, 2009).
The second widely-accepted method of structural design is the method of global safety factor. It is based on the condition;
Xresistance/Xaction > S0 ————— (2)
Accordingly, the calculated safety factor s must be greater than its specified value s0. It is a method which attempts mainly to give a truer picture of the behaviour of elements and their cross-sections, in particular through the aggregate quantities of structural resistance Xresistance and action effect Xaction. As in the case of the permissible stresses method the main insufficiency of this method remains the impossibility to consider the uncertainties of particular basic quantities and theoretical models (Holicky, 2009). The probability of failure can, again, be controlled by one quantity only, i.e. by the global safety factor s.
The last on this list is the partial factor method of design (also called the limit state method), and is currently the most advanced operational method of structural design. Partial factor method of structural design is generally characterized by the inequality shown in Equation (3);
Ed(Fd, fd, ad, θd) < Rd(Fd, fd, ad, θd) ———– (3)
This design concept is deemed satisfactory when the effects of actions Ed are less than or equal to the structural resistance Rd. The basic variables involved in modelling the relationship are the actions (Fd = ψγFFk), material properties (fd = fk/γm), dimensions (ad + Δa) and model uncertainties (θd). The reduction factors (ψ) and partial factors for actions and materials (γF and γm) are used to describe the reliability of the structure.
What defines the safety of a building? According to Section 2 of EN 1990, a structure shall be designed and executed in such a way that it will during its intended life, with appropriate degrees of reliability, and in an economic way,
– sustain all actions and influences likely to occur during execution and use; – remain fit for the use for which it is required.
The last two sentences above generally define what is referred to as ultimate limit state (ULS) and serviceability limit state (SLS), and the two key words to look out for in achieving them are ‘reliability’ and ‘economy’. Ultimate limit states are associated with collapse or other similar forms of structural failure. Serviceability limit states correspond to conditions of normal use (deflections, vibration, cracks, etc.). In general the design should include both safety and serviceability, including durability in both cases. The nature of ultimate limit states is essentially different from the nature of serviceability limit states and should be taken into account in reliability verification. ISO 2394 defined reliability as the ability of a structure to comply with given requirements under specified conditions during the intended life for which it was designed.
Can you exhaust your factors of safety? To keep what we showed in equation (3) simple, let us consider BS 8110-1:1997 code of practice (which has been withdrawn), where we normally apply partial factors of safety to loads and materials. At ultimate limit state, we factor the design loads on a building as follows;
P= 1.4gk + 1.6qk ————- (4)
Where gk and qk are the dead load (permanent actions) and imposed loads (variable actions) respectively. A little observation of Equation (4) will show that we are actually increasing the value of the dead load in the building by 40%, and the value of the live load by 60%. If we sum both actions up, we can say that we are designing the building to sustain loads that are almost in excess of 60% of what we anticipate that it will carry in its design life – which is like the collapse load.
In the Eurocodes, the design action of a building at ultimate limit state is given by;
P= 1.35gk + 1.5qk ————- (5)
As can be seen from equations (4) and (5), the partial factors of safety for loads (actions) reduced in the Eurocodes, but the partial factors for materials remained almost the same. For characteristic strength of concrete, the partial factor γc remained 1.5 in both codes of practice, while for steel, the partial factor of 1.15 was also maintained. We should note that the later releases of BS 8110 prior to its withdrawal reduced factor of safety for steel reinforcements from 1.15 (0.87fy) to 1.05 (0.95fy) after it was discovered that the reinforcements manufactured and tested in the laboratories in the UK rarely fell below the minimum strength.
Having taken note of all these partial factors, is it possible for a structure to fail due to actions and materials? The answer still remains yes. In Nigeria, most local contractors do not pay attention to the quality of materials used in construction, and some property owners may misuse the building by overloading it. The most popular design code in Nigeria is still BS 8110, which is widely applied and accepted by the government and other institutions. Let us use the standard to show how the factors of safety can be squandered.
Concrete We have seen cases where the 28 day characteristic strength of concrete used in construction was observed to be about 13 MPa. If grade 25 concrete was used in the design, and partial factor of 1.5 applied, the conservative design strength will be (25/1.5 = 16.67 MPa). In this case, the factor of safety for the concrete is gone.
Steel In most published works available online, we have seen cases where the average characteristic yield strength of reinforcements falls below 400 MPa. If the yield strength of 460 Mpa was used in the design (0.95fy = 0.95 x 460 = 437 MPa), we will discover that the factor of safety for steel is also gone.
Loads The imposed load used in the design of most residential buildings is 1.5 kPa. Sometimes, the usage of a buildings can be abused by say for example, converting a residential building to a place of worship/gathering, or storage house. If a building designed as residential building is used to stack four bags of cement spread all over the floor, we can assume that the slab has been subjected to an imposed load of about 6 kPa. At ultimate limit state, the building was designed to take imposed load of (1.6qk = 1.6 x 1.5 = 2.4 kPa), and with imposed load of 6 kPa, there will be ‘real fire on the mountain’.
Therefore we can see that through poor materials and misuse of building through overloading, the factors of safety used in a design can be exhausted, thereby placing the structure on the risk of imminent collapse. All hands must be on deck to make building collapse a thing of the past in Nigeria.
References Holicky M. (2009): Reliability Analysis for Structural Design (1st Edition). Sun Media Press Stellenbosch
Cement is one of the most popular construction materials in Nigeria, and has been extensively utilised in civil engineering works such as building construction, bridges, transport infrastructures, rigid pavements, etc. It is very important for construction professionals in Nigeria to know the current standards and different types or classes of cement we have, and their various suitability for different construction works.
There are three grades or classes of cement in Nigeria, namely grades 32.5, 42.5, and 52.5. These grades corresponds to the minimum 28th day compressive strength of the cement mortar after curing. They are also referred to as cement strength classes of 32.5 MPa, 42.5 MPa and 52.5 MPa respectively. In terms of strength, the classes of cement are 32.5N, 32.5R, 42.5N, 42.5R, 52.5N, 52.5R. The 32.5 category must have strength between 32.5 N/mm2 and 52.5N/mm2, while for the 42.5 grade must have a strength range between 42.5 N/mm2 and 52.5 N/mm2. The minimum strength of the third category is 52.5 N/mm2.
The standard document for cement specification in Nigeria is the NIS 444-1 Cement – Part 1: Composition, specifications and conformity criteria for common cements. This standard defines 27 products in the family of common cement that are grouped into 5 main types (COREN, 2017). The five groups of cement we have in Nigeria are shown in Table 1.
Table 1: Types of cement grade in Nigeria (COREN, 2017)
From Table 1, it can be seen that there are three grades of cement in Nigeria, namely grades 32.5, 42.5, and 52.5.The strength classes of cement are 32.5N, 32.5R, 42.5N, 42.5R, 52.5N, 52.5R. As described earlier, 32.5 category must have strength between 32.5 N/mm2 and 52.5N/mm2, while for the 42.5 grade must have a strength range between 42.5 N/mm2 and 52.5 N/mm2. The minimum strength of the third category is 52.5 N/mm2 (Joel and Mbapuun, 2016). These are strengths after 28 days. The appendage “N” refers to a class of cement with normal early strength, while “R” Refers to those with high early strength (Oyenuga, 2014).
It is very important to note that the most common type of cement in Nigeria is the Portland Limestone Cement (PLC) and not Ordinary Portland Cement (OPC). As a matter of fact, OPC conforming to CEM I class of cement is not available in Nigeria’s open market. The cement available in the open market of Nigeria is the Portland Limestone Cement designated as CEM II in NIS 444-1 (2003). PLC is a modified OPC which is produced by adding 6 -35 % of limestone to OPC. It has a lower clinker content range of 65 – 94 % compared with OPC’s range of 95 -100% (Joel and Mbapuun, 2016) It has lower carbon footprint than OPC and is deemed more environmentally friendly.
Fig 1: Dangote’s cement plant, Obajana
According to COREN (2017), any specified grade of concrete can be produced using any strength class of cement provided appropriate mix design procedure is followed. However, since the year 2014, Standards Organisation of Nigeria (SON) has maintained that grade 32.5 PLC cement should be limited to plastering works, block making, and light concrete works. For water retaining structures, class N cement (normal early strength) is recommended due to their low drying shrinkage that will positively affect the crackwidth of the concrete (Ubani, 2018). Also, high grade and rapid hardening cements may not be too good for plastering works to avoid plaster cracks that might arise from drying shrinkage, especially given the high temperature in Nigeria.
There are many manufacturers of cement in Nigeria, but according to United Capital’s 2019 report, Dangote Cement Plc is the undisputed largest player in the Nigerian cement market. Using installed capacity as a gauge, Dangote Cement Plc is the clear leader wielding 60.6% of the market share with Nigerian installed capacity of 29.3 MMT. Lafarge Africa Plc (10.5 MMT) and BUA Group (inclusive of CCNN) (8.0 MMT) accounts for 21.8% and 17.6% respectively. There is also a fringe player PureChem Industries Limited based in Ogun State with 900,000 MT (United Capital, 2019).
According to Dangote cement’s website, the premium cement is produced in three grades which are 32.5R, 42.5R and 52.5R. They recommended their grade 32.5R for plastering works, low rise buildings and masonry. The other higher grades can be used for high rise structures and mega infrastructures. All Dangote cements available in the open market are Portland Limestone Cement. However Dangote grade 42.5N is available in the market.
From Lafarge’s website, their premium cement brands are Elephant, UNICEM, Ashaka, Elephant Supaset, and Powermax. The exact strength grade of Elephant and Unicem cement is not explicitly stated in their website, but the grade on bag label is 32.5R. They recommended it for medium strength concrete works, suspended slabs, masonry works (block making, plastering, mortar), and reinforced concrete works. All Lafarge cement products in Nigeria’s open markets are also Portland Limestone Cement.
From the label on Elephant Supaset cement, the strength grade is 42.5N, but the information on their website recommended it for block making, precast elements such as poles, culverts, interlocking stones, etc.
BUA cement is manufactured as grade 42.5R as labelled on their bag. The cement is manufactured as Portland Limestone Cement too.
Purechem Cement is manufactured as grade 32.5R cement, and as Portland Lime Stone Cement.
Having known these few specifications, you can select the most appropriate cement grade for your construction work.
References Council for the Regulation of Engineering in Nigeria (COREN) (2017): Concrete mix design manual. Special Publication No. COREN/2017/016/RC
Joel M., and Mbapuun I. D. (2016): Comparative analysis of the properties of concrete produced with Portland Limestone Cement Grade 32.5N and 42.5R for use in rigid pavement work. Global Journal of Engineering Research (15): 17-25
NIS 444-1 Cement – Part 1: Composition, specifications and conformity criteria for common cements. Standards Organisation of Nigeria.
Oyenuga, V., 2014. Cement not Responsible for Building Collapse in Nigeria. An Editorial in This day Newspapers of 13th May 2014.
Ubani O.U. (2018): Structural Design of Swimming Pools and Underground Water Tanks. Structville Integrated Services Limited, Nigeria.
Structural failure is real, and it is the duty of structural engineers to identify all possible modes of failure in a structure, and design against them with appropriate factor of safety. When a building fails, we can identify the most probable cause of the failure by looking at the crack patterns and their location.
By looking at the image above, can you identify the cause of failure of the building? Let us know your answer in the comment section. Thank you very much.
Equivalent horizontal forces (EHF) are not strictly actions, but are forces that are applied to a frame in combination with other actions to model the effect of frame imperfections. Another alternative of doing this is to model the frame out of plumb. According to clause 5.3.2(6) of Eurocode 3, if a frame is sensitive to second order effects, member imperfection must be modelled in the analysis if the member has a moment resisting joint. In this post, we are going to show how to model the effects of imperfection for gravity actions in portal frames.
Determination of EHF According to clause 5.3.2(3) of EC3, for frames sensitive to buckling in a sway mode the effect of imperfections should be allowed for in frame analysis by means of an equivalent imperfection in the form of an initial sway imperfection and individual bow imperfections of members. The imperfections may be determined from:
ϕ = ϕ0 αh αm
Where ϕ0 = 1/200 αh = 2/√h (h is the height of the structure in metres) αm = √[0.5(1+ 1/m)] (where m is the number of columns in the row)
Solved Example Model the effect of imperfection in the frame shown in Figure 1 using the equivalent horizontal force approach Columns – UB 610 x 229 x 125 Rafters – UB 533 x 210 x 92
Fig 1: Portal frame subjected to gravity action
When analysed under the gravity action shown above, the following reactive forces and bending moment diagram was obtained.
Fig 2: Bending moment and support reaction of portal frame under gravity load
For the example above; αh = 2/√h = 2/√7 = 0.755 (h is the height of the structure in metres) αm = √[0.5(1 + 1/m)] (where m is the number of columns in the row) αm = √[0.5(1 + 1/2)]= 0.8660 Therefore; ϕ = (1/200) × 0.755 × 0.8660 = 0.0032
The equivalent horizontal forces are calculated as:
HEHF = ϕVEd
However, sway imperfections may be ignored where HEd ≥ 0.15VEd
Choosing to incorporate the effect of imperfection in our analysis, the equivalent horizontal force is given by; HEHF = 0.0032 × 114.27 = 0.366 kN
We will now model and analyse the frame for load combination 1 with the effects of imperfection included. The analysis will be done with the base pinned.
Fig 3: Portal frame with equivalent horizontal force
The resulting bending moment diagram with the effect of imperfection is given below;
Fig 4: Bending moment and support reaction with the effect of imperfection
A little consideration of the above results will show that the effects of imperfection can be safely ignored in the design of the structure. Thank you for visiting Structville today.
