Monday, 16 July 2018

Roofing: When The Roof Load Becomes An Issue - Engr. Osaz’ Enobakhare

‘Wear the cap on whom it fits’ so the saying goes. There is a lot of sense in this one and a real good line of thought too.  Although caps cannot stop you from getting wet under the rain, your roof can. Caps are better worn on whom they fit and so should roofs be. The passion for sky-scrapping roofs suddenly envelope nearly all classes of players and participants in the built environment back here and the aesthetic impression it gives to a building quickly blindfolded many to its structural implications. Now some bungalows have roofs that are five times as high as the building frame itself.  Likewise, the crave for curvy, straight-stepped and composite parapets built mostly of reinforced concrete made a mockery of traditional wooden fascia that normally flushes with external asbestos ceiling that was the norm up to the mid 90’s.

Apart from preventing direct exposure to external weather conditions, roofs are important structural elements of buildings. They constitute a significant fraction of the entire load of the structure and must be well designed so that they do not portend danger to the overall strength and stability of the building. For instance, roofs are designed to be structurally able to withstand wind loads (weight imposed on the roof from wind pressure forces) and ice load too (in country where this is applicable). Hence they have to be strong enough to perform these functions but that should not mean that a roof should become too heavy.

Some people do not take cognizance of the roof load implication on the foundation to which it is transmitted and with time, the roof load begin to take its toll on the foundation causing it to experience excessive settlement or failure. The roof carcass and parapet are very integral parts of the roof load that have heavy members. The combined load of a timber roof carcass and parapet for a residential building can reach up to 1kN/sqM. That is almost equivalent to 2 bags of 50kg cement on every square meter of the roof. Now that’s a lot!

An overweight roof is often a disaster! It is therefore generally advisable to reduce the load of the roof so as not to impede on the structural efficiency of the building. To achieve this, the use of light-weight yet rugged parapet fascia (e.g. wired polystyrene, aluminum, etc.) as well as using light-weight roof carcass (like treated timber, light steel, etc.) and fitting them in such a way as to deliberately reduce the weight on the structure. Where reinforced concrete is to be used, light-weight (aggregate-less) concrete with damp-proof membrane could be used.       

The Danger of open excavation near your foundation - Engr. Osaz' Enobakhare

There is naturally a danger of having an open excavation anywhere at all; from being a dirt trap to becoming a death trap hence protection of open excavation is a compulsory item in basic construction practice. On typical construction sites, it is often the responsibility of the Safety Engineer to enforce such vital regulation on site. They would go hard on any worker who do not obey this regulation due to the numerous potential dangers; but what happens when the site is now built up, handed over, the ‘dreaded’ safety Engineers are gone, and someone excavates close to the foundation and leaves it open? No doubt, there is equally a latent danger in having an open excavation near a foundation. Here is why –Foundations relate with the subsoil on which they are built and around them by constantly pushing against the active soil pressure in a relatively balanced manner in order to remain stable throughout their life span. If there is a substantial counter balance from either of the foundation itself or the supporting earth at any point, it will impede on the stability of the foundation and might cause it to settle (or sink) rapidly.

The rapid settlement of some defective buildings studied over the years is a result of fully or partially-open excavation at close distance to the foundation. Open excavations up to 4-metres from the foundation line can still affect your foundation depending on the soil strata. When there is an excavation near a foundation, the active soil pressure acting on the foundation at that region reduces, causing the soil to slip. Subsequently when there is natural earth movement, it drops further, making the foundation not to hold firm to the ground. If not checked on time, the vertical load of the building acting on it would cause the building to settle at that region and further apart. Although there is nothing particularly wrong in having an excavation close to a foundation but care must be taken to ensure that the walls of the excavated areas are protected or embarked to stay the adjourning earth and prevent the devastating impact on the neighboring structure especially if such structures are on shallow foundations (i.e. foundations of 5-metres or less in depth from natural ground level mostly raft, strip, pad, etc.). Such impact could also include poor resistance against ground vibrations and burrowing by small animals that may attack foundation walls.  

Reinforced Concrete can be used in protecting the walls of an excavation close to a foundation but it is important to construct them properly else they fail to serve the purpose intended. Where a tree is fell close to a foundation, it is important to refill the excavated portion and stabilize the earth firmly at that point. Leaving an open excavation close to a foundation unattended to also allows it to collect surface water which might constantly soak-away into the foundation and damage its fabric rapidly. This is particularly the case where due to low water table a traditional ‘block’ strip foundation was used. Whichever way it is advisable not to allow especially deep open excavation close to your foundations to prevent foundation cracks and the attendant regrets!

Pile Foundation: What Can Make A Pile Fail - Engr. Osaz’ ENOBAKHARE

Foundations are vital elements of earth-laid structures because they support and transmit the entire load of that structure to the earth. Of all foundation types, deep foundations are often considered the safest and most appropriate especially where cost factor cannot be allowed to take prominence. Pile foundations are the commonest of all deep foundation types and they are very critical because of their depth and size; they are relatively slender compared to the overall size of the structure and they go as far below the natural ground level as where there is suitable bearing capacity –reaching beyond 50-metres for some high-rise buildings. Piles with circular cross-section are very popular in the industry and the usual width (or diameter) lies between 300 – 600mm.  In Nigeria, reinforced concrete cast in-situ (bored) piles are common compared to driven piles because it is considered less expensive. The cost involved in driving a pre-cast reinforced concrete pile into earth is about a quarter times higher than for cast in-situ piles and then the technicalities involved.

Because piles are a sort of stand-alone foundation and are mostly designed to carry very heavy loads, it is often advisable to test their load bearing capacities before building over them. The common tests for piles are the Pile Load (PLT) and Pile Integrity Tests (PIT). These tests are not only relevant for the purpose of structural analysis; they also provide useful information about the composition of the pile throughout its length. The construction of pile foundations and testing of same are best carried out by professional structural engineers. However, due to ignorance or negligence, some project owners either outsource their piling works to random borehole drillers who are mostly skilled in water works with little or no idea on the dynamics of piles or do not carry out appropriate tests on their piles even after taking such unreasonable risk.

Shit happens and piles fail too. There are many reasons why this occurs but of all, poor construction and faulty arrangement of piles are the commonest. If a pile is not constructed properly, for instance the mix ratio or provision for reinforcement in a reinforced concrete pile is not in tandem with what was stated in the structural design; the end result might be a weaker pile. Also, if the mix ratio is adequate but the concrete was not properly loaded into the borehole, the pile may have sizable gaps and pores and lines of weaknesses may result from it. Likewise if a pile is designed as a friction pile but constructed as end-bearing, it might become surplus to requirement. There are reported cases where deliberate attempts were made by unscrupulous constructors not to reach the specified depth in a bid to cut corners. This is not a healthy practice and might not help the pile reach its desired strength. When constructing bored piles without casement and sufficient care is not taken to avoid excessive backfilling before loading concrete, the result might be a shorter or a soiled pile and that is not a good one. There are other factors like faulty designs, poor supervision, poor mixing of concrete and the use of sub-standard materials especially with regards to the tensile strength of steel rebars, dirty aggregates and foul water etc. 

For driven piles, if the blow (hit) from the pile hammer on the pile head generates excessive vibrations and stresses down the pile, the pile may lose strength.   Pile failures can be devastating to a structure as failure of a single pile can lead to a partial collapse of a building, hence the need to build it right all the time