In the past few years a considerable improvement have taken place in the understanding of structural concrete and has been incorporated in the revised codes of practice. The British Standard CP110: Code of Practice for the Structural Use of Concrete [1] , has superseded the British Standard Codes of Practice CP114 [2] , for reinforced concrete. Similarly, in America the ACI Standard ACI 318-71 [3] has replaced the previous standard ACI 318- 63 [4] . The major aspects of the revised codes is the limit state approach for designing reinforced concrete struc- tures and the separation of methods of concrete mix design procedures from that of concrete design considera- tions. However, in Nigeria, consultant structural engineers still make concrete specifications based on CP 114 [2] , which states concrete mixes of 1:2:4, 1:1½:3 and 1:1:2 produces concrete with compressive strength of 21 N/mm², 25.5 N/mm² and 30 N/mm² respectively at 28 days. However, Olanitori and Olotuah [5] show that these compressive strengths might not be attained due to the clay/silt content of sand, which negatively affects the compressive strength of concrete produced from them.

In CP110 [1] and BS8110 [6] , this portion of CP114 [2] that dealt with concrete mix prescription was ex- punge from the code and standard. This is to the superior knowledge that concrete produced from different types of sand but of same mix proportion will produce concrete with different strengths. With the publication of BS8110 in 1985 [6] , CP110 [1] was withdrawn. The publication of CP110 [1] and BS8110 [6] were accompa- nied with several BS standards which dealt with different aspects of concrete production [7] .

The quality of concrete produced depends on the quality of its constituent materials and their mix ratios, the higher the percentage of clay/silt contents of sand, the lower the characteristic strength [5] . To mitigate the effect of clay/silt content of sand on the strength of concrete produced from it, there is need to increase the cement content of the concrete, depending on the clay/silt percentage [8] . Olanitori, 2012 [9] determines the cost impli- cation of mitigating the effect of clay/silt content of sand using mathematical models. In order to improve the quality of concrete produced from locally available aggregates, there is the need to have indigenous codes of practice, which will take into consideration the quality of the available aggregates [7] .

The collapse of most reinforced concrete structures is by shear failure at the beam-column joint and sudden in nature [10] . In 1962, Joint ACI-ASCE Committee 326 [11] published a report regarding the design and behavior of beams failing due to shear and diagonal tension. To develop safe design recommendations, a database of 194 beam tests without shear reinforcement was compiled. The database consisted of 130 laboratory specimens tested under single-and double point loads and 64 beams subjected to uniformly distributed loads. Based on those data, design equation was formulated and is included in ACI 318-05 [12] and presented as Equation (1).

where is the nominal shear strength provided by concrete; is the specified compressive strength of

concrete; is the ratio; V_u is the factored shear force at section; M_u is the factored moment at section;

b_w is the web width; d is the effective depth of section; and A_s is the area of tension reinforcement.

By neglecting the term in Equation (1) a simplified but conservative version could be derived and pre- sented as Equation (2).

To include the effects of loading type and shear span to depth ratio into current code provisions, for members in which more than 1/3 of the factored shear at the critical section results from concentrated load located be- tween 2d and 6d of the face of the support, Brown et al. [13] , proposes:

Such a reduction in shear strength as indicated in Equation (3) will substantially reduce the number of tests that fall below code values.

According to Arslan [14] , the nominal shear strength provided by concrete can be estimated using Equation (4).

where: is the cracking shear strength, is the diagonal tension cracking strength and is the dowel strength.

Based on the principal shear strength V_o carried in the compression zone, considering the influence of para- meters; the slenderness ratio (a/d) and size effect (1/d), Arslan, 2012 [15] expresses the diagonal cracking strength of RC slender beams without stirrups as given in Equation (5).

where c is the depth of the neutral axis.

Other existing shear strength models for slender beams without stirrups, by Kim and Park, 1996 [16] , Khuntia and Stojadinovic, 2001 [17] , and Rebeiz, 1999 [18] are presented in Table 1.

The ACI 318-08 [19] design shear strength is a simple superposition of transverse reinforcement and concrete strength. The design strength is independent of whether flexural yield has occurred prior to shear failure. For members, design shear strength is calculated using Equation (6).

where V_c is the contribution of concrete to shear strength; V_s is the contribution of shear reinforcement to shear strength; f_c is the compressive strength of concrete; A_w is the area of shear reinforcement within a distance s and f_y is the shear reinforcement yield strength. The contribution of shear reinforcement is derived from basic equili- brium considerations on a 45-degree truss model with constant shear reinforcement spacing and an effective depth.

In their work,Arslan and Polat [20] , show that there exists a significant amount of contribution of concrete to the shear strength (18% - 69%), however, noted further experiments should be conducted with a wider range of shear reinforcement ratio, shear span-to-depth ratio, concrete strength and various loading schemes in order to obtain more reliable assessments.

Since the mid-1980s, there is an increasing amount of experimental evidence showing that the underlying concepts of the provisions of current codes (for example, BS 8110-1985 [6] and ACI 318-05 [12] ) for the shear in particular and, to a certain extent for the flexural design of reinforced concrete (RC) structures are in conflict with fundamental properties of concrete at both the material and the structural levels [21] . Also, only few of the existing shear strength models give reasonable shear strength capacity for reinforced concrete structures con- structed from concrete produced from locally available aggregates.

A lot of work has been carried out to determine the effect of the various types of pit-sand on the cube strength of concrete, however very little work from literature has been carried out to determine the effect of the various types of pit-sand on the resistance capacities of reinforced concrete space framed structures. This paper, using existing formulae from the codes, the estimated and actual resistance capacities of the collapsed building at Oba-Ile were determined, in order to determine the effect of the pit-sand used on the resistance capacities.

From Table 2, the results of the structural re-assessment show that tension reinforcements provided for the slab were adequate. Also, the longitudinal reinforcements provided for the beams were equally adequate, while that of shear reinforcement was not adequate. The reinforcements provided for the columns were adequate.

Table 3 shows the results of universal testing machine and the PUNDIT6, while Table 4 gives the characte- ristic strength of the concrete from which the collapsed building was built as 8.8 N/mm² and 8.2 N/mm² for PUNDIT6 and Universal Machine respectively, resulting in an average characteristic strength of 8.5 N/mm². The prescribed design concrete strength for the building was 20 N/mm². This shows a 57.5% reduction in the concrete strength caused by the low quality of pit-sand used for the construction.

The estimated resistance parameters, such as moment of resistance (M_R), shear capacity (V_C) and collapse load (N_C) of the building were estimated using equilibrium equation of a reinforced concrete section with the pre- scribed and actual characteristic strengths of concrete which equal 20 N/mm² and 8.5 N/mm² respectively. The analysis equations of the BS 8110-1, 1997 [25] were used to determine the actual resistance parameters of the collapsed building using the actual collapse load. The estimated and actual resistance parameters of the col- lapsed building were presented in Table 5.

From Table 5, the moments of resistance of slab (M_RS) estimated from equilibrium equation of a reinforced concrete section, using the prescribed strength of concrete (R_PCS), actual collapse load (R_ACL) and, actual con- crete characteristic strength (R_ACCS) are 20.6 kN∙m, 6.67 kN∙m and 18.2 kN∙m respectively. Also, the moment of resistance of beam (M_RB) determined using (R_PCS), (R_ACL) and, (R_ACCS) are 54.1 kN∙m, 13.6 kN∙m and 46.3 kN∙m respectively.

Also, from Table 5, partial collapse load (N_PCL) determined from the values of (R_PCS), (R_ACL) and, (R_ACCS) are 23.0 kN/m², 7.44 kN/m² and 12.6 kN/m² respectively. Also, the total collapse load (N_TCL) from (R_PCS), (R_ACL) and, (R_ACCS) values are 37.5 kN/m², 7.44 kN/m² and 32.2 kN/m² respectively, while the shear capacity (V_C) from (R_PCS), (R_ACL) and, (R_ACCS) values are 90.73 kN, 18.88 kN and 64.4 kN respectively.

From the above, the estimated moments of resistance of slab (M_RS) determined from R_PCS, and R_ACCS are 20.6 kN×m and 18.2 kN×m respectively, and greater than that determined from R_ACL, which equals 6.67 kN×m by 209% and 173% respectively. Equally, the estimated moments of resistance for beam (M_RB), determined from R_PCS and R_ACCS are 54.1 kN×m and 46.3 kN×m respectively, and greater than that determined from R_ACL, which equals 13.6 kN×m by 298% and 240% respectively.

The partial collapse loads (N_PCL) determined from R_PCS and R_ACCS are 23.0 kN/m² and 12.6 kN/m² respectively, and greater than that determined from R_ACL, which equals 7.44 kN/m² by 209% and 69% respectively. The total collapse loads (N_TCL) determined from R_PCS and R_ACCS are 37.5 kN/m², and 32.2 kN/m² respectively, and greater than R_ACL, which equals 7.44 kN/m² by 404% and 333% respectively. Also, the shear capacity, determined from R_PCS and R_ACCS are 90.73 kN and 64.4 kN respectively, and greater than that determined from R_ACL, which equals 18.88 kN, by 381% and 241% respectively.

The estimated moments of resistance of slab (M_RS) determined from R_PCS and R_ACCS are 20.6 kN×m and 18.2 kN×m respectively. This shows that, when the strength of concrete decreases by 57.5% (20 N/mm² to 8.5 N/mm²), M_RS only decreases by 12%. Also decrease by 57.5% in concrete strength, only have decrease effect of 14.4% on M_RB. Decrease in concrete strength by 57.7% only has decease effect of 14.1% on the total collapse load (N_TCL). The shear capacity determined from R_PCS and R_ACCS are 90.73 kN and 64.4 kN respectively, hence the decrease effect of 57.7% of concrete strength is 29%.

Table 6 shows shear capacities determined from existing shear strength models. Only the shear strength mod- els of Brown et al. [13] and Kim and Park [16] give shear capacities of 22.6 kN and 20.4 kN respectively, which compares favorably with the actual shear capacity of the collapsed building which equals 18.88 kN. The shear capacities from Brown et al. [13] and Kim and Park [16] , defer from that of the actual shear capacity by 8.1% and 19.7% respectively.

Table 7 shows the percentage increment of resistance parameters over that due to the actual collapse load. From Table 7, shear capacity V_C has the highest percentage of increment; hence shear failure capacity will be reached first before any others in resistance parameters of the collapsed building. Therefore, the collapse of the building is by shear failure which is sudden in nature.

From the discussion above, the following conclusions can be drawn:

1) The collapse of the building was caused by shear failure which is sudden in nature.

2) Method of analysis of section, for reinforced concrete structures, gives overestimated values of the resis- tance parameters.

3) Coefficient method gives more accurate values of the resistance parameter.

4) Shear strength models of Brown et al. (2006) and Kim and Park (1996) give the most accurate value of shear capacity of reinforced concrete at failure.

5) The estimated resistance capacities, determined using the actual concrete characteristic strengths, gives an overestimated resistance capacities when compared with the actual resistance capacities. This shows that there is the need to investigate the effect characteristic concrete strength on resistance capacities of reinforced concrete space framed structures.

1) Local Codes of Practice should be developed so as take into consideration, the quality of the locally availa- ble aggregates in formulating design formulae, which will be capable of given more realistic resistance parame- ters.

2) Effect of the reduction of the characteristic strength of concrete should be investigated, and appropriate measures should be put in place so as to produce safe reinforced concrete structures.