WR1

WITHOUT MODIFIERS LESS MOMENT LESS STIFFNESS

WITH MODIFIERS (MORE INERTIA) MORE STIFFNESS SO MORE MOMENT

please explain more as i here im a little confused about your statement.

Property modifiers in etabs are used to model cracked behaviour of concrete sections. They are only applied to concrete members because of cracking. Gross moment of inertia is bd^3/12 for a rectangular section, but when you make this member of concrete, it will experience cracking when loaded after some time. This cracking will happen when concrete reaches its tensile capacity which is about 7-10% of its compressive strength. Formula to calculate cracking moments are given in ACI. For example 3000psi will have only 300psi of tensile strength. Actually the reinforcement starts its work when concrete cracks because of tension. After cracking concrete is no longer able to carry tension so steel starts taking the tension. So now if concrete cracks after 300psi the moment of inertia will be reduced because of cracking. If moment of inertia is reduced, its stiffness is reduced, taking less moment, and its deflection increases because of less stiffness. This moment which the cracked beam is not taking anymore will be re distributed to other structural members based on their stiffness. If you read ACI chapter 10, there are many sets of modifiers used for different types of analysis. ok im leaving from office, if you have more doubts i will write in later.

Well i just found out a nice presentation about the question you are asking. I would recommend you to go here http://www.slideshare.net/neikrof/membrane-plate-shell

you are right to put the same thickness in membrane and in bending. the thickness you put in membrane will give self weight and the bending will give you the bending simple. If you are using precast panels you can put different thickness in membrane and bending. You can put only the top screed thickness in membrane and slab thickness in bending. Shell: transfers loads to nodes of that shell element. Like in flat slabs. Loads will go directly to columns. You cannot design beams on shell unless you have meshed it finely. In plane as well as out of plane stiffness Membrane: transfers loads to beams. It has only inplane forces Thick plate is used where shear deformations are important but the mesh is simple. Thick plate is generally used when span over thickness is less than 15 for example if slab is 600mm and span is 6m then ratio is 6/0.6 = 10. You can use thick plate option. It has only out of plane stiffness All this is given in manual helps of CSI softwares.

so u mean lateral loads from columns is transferred through braching to shear walls?

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so then what for lateral loads? where its going?

Its the same system used in CMA Tower, Riyadh (my first project). You have mega columns for gravity loads. And sure this building must be having OUTRIGGERS....for lateral loads which attracts lateral forces from outer columns and structure to inner concrete core designed to resist lateral loads. The core walls of this tower at ground floor may be 1.8m thick with seismic time period around 5 seconds. These are my estimates. I know i am wrong most of times

thickness is governed by punching criteria. as long as your punching is okay, you dont have problem provided that the thickness is atleast more than whats mentioned in the code. I think by footing minimum thickness in code means ALL TYPES OF FOOTINGS

in my experience with normal gravity loads..here in saudi arabia! even zone 2A (UBC Seismic Zone) makes footing size approx more than double of the original (gravity loads). This is because you need self weight to resist uplift for wind pressure and earthquake. Reinforcement is also changed and often you have to do non linear uplift analysis for these cases. Footings are ofcouse desgined for earthquake and wind reactions to support structures. Otherwise the building may overturn due to uplift, soil liquefaction and other things.

take the example of column! in normal gemotery of the building, under gravity loads, a column will be under compression! and some times if there is axial tension concrete will also be in tension!

i think yasir you are talking about flexure in a simple beam where upper part (above N.A.) is in compression and below the N.A. is tension. But i think Waqar here is talking about AXIAL Tension and AXIAL compression not flexural.

Stress is directly proportion to strain so Stress=E x Strain When E is youngs modulus or modulus of elasticity if the graph is straight line...it means stress is directly proportion to strain so we have one value of E if the graph changes and stress is no longer proportion to strain ..i mean when it becomes non linear curver then we will have different value of E so plot the graph when we have tension and when we have compression..and try to see it your self

Yeah i would recommend it too to provide at least the minimum reinforcement ignoring clause 10.5.1. This is because:- Usually for usual sizes of beams and other members, minimum reinforcement is very less. If you are designing a huge member for example a beam 2m deep or a footing 1m in thickness then you dont need to (ofcouse) to satisfy the minimum reinforcement requirement because the concept will be changed and it will come in the category of deep beams or mass concrete. Anyway provide always minimum reinforcement in usual design routine ignoring ACI 10.5.1 but smartly. By this i mean, if the min reinforcement is very less then no issue, provide it. If suppose the reinforcement is much and you want to reduce it by ACI 10.5.1 then dont reduce reinforcement too much. Keep it near the minimum reinforcement calculated in usual manner. I mean lets say your minimum reinforcement is: 10 bars You want to reduce by ACI 10.5.1 and it comse to be : 6 bars then provide at least 8 bars..

yeah you can...design any beam in ETABS and see why some times its giving reinforcement less than minimum by code...check your self

yeah you are right...if it is simply supported then theoretically! no moment at top and no required reinforcement at top! but in reality we have some ethics for structures...and the CODE defines these ethics....so in reality we require bottom bars to be extended into supports....where practically there is no moment...(positve) similarly the top bars are extended upto a length equal to developement length plus d after the inflection point these minimum precautions are required to avoid failure in case of moment reversal and to provide structural integrity... generally at the discontinous ends you need one third of positive reinforcement in case of simply supported slabs but due to curling of slab edges, torsion, and temperature shirnkage and due to relative stiffness of of the beams or supports there is a need to provide top reinforcement at discontinous ends... thats y may be you have seen at some places that every other bar is bent up to give negative reinforcement area over masonry walls supports in our typical house construction in pakistan. means half of the positive moment reinforcement area is provided on discontinous egdes.. This is just a simply supported case of slabs resting on masonry walls... In case of continous slabs resting on concrete beams the negative moment is some times more than positive...

crush is a rock! and concrete is a composite material made up of different materials. Many factors are involved for example as umar quoted w/c ratio...distribution of crush and their sizes...quality of cement...there is air too in the concrete mixtures then the curing and setting temperatures...if only crush or rock is to be used alone they how they will stick together...so we add cement and water and fine crush to make it a mixture...whose properties are different that the original elements used to make that mixture...

by both sides i mean in one direction...bt the top reinf is on all four sides..... @waqar this is normal! the moment at middle span is high because of 1 span (simply supported)...so if an alternate bar is provided at top...this is perfectly normal..yeah it will take stresses as far as you have provided enough reinforcement and with proper splice and development lengths...

actually the top reinforcement is provided on both the sides..bt there are two bars.. a standard bar length is usually 40 ft or 60ft or 12m etc if the length required is greater than these standard lengths you have to make splices...there are many other reasons for which you have to split the bars and provide splices. Its normal...

i did write it simply to avoid verbiage! is anything wrong with it? cmon!

I was wondering about the people and activities of SEFP. Have a look at the following stats (date: 22.aug.2012). This is my agenda for next 3 months for SEFP. Please lets start to define some roles and targets for SEFP. You can disagree and put it down but vote for me how many of you agree with me! I wanna see the stats and numbers. Afterall this is just a suggestion. Total members: 816 This number is decent. At this point i care about participation & quality instead of just a crowd doing nothing out there. Total posts: 918 [ 66% of all the posts are by top 5 members ] Post count by top 2% members should not go above 70%. 1) Atleast 30% contribution to SEFP should be from 98% members to get a feeling the SEFP is not just run by the moderating team. Average post rate: 1.125 post per member Should reach upto 1.3 2) Ensures improved density of posts over the total population of SEFP The most liked content is by: 6 users Believe it or not. Its a measure of active members of the forum. Need to make it to 8 (not just 1 post...rather many posts by more than 8 people liking) 3) To develop the habit and to encourage people to appreciate others work and say Thanks to him/her. Users with more than 10 posts: only 16 Here i use my own self devised rule that the %age of members posting more than 1% of the total posts (9.18) should be more than 2%. In our case it is 1.96%. Lets make it 2% 4) To ensure that there is always performance. If the no of members increases so should the post count. Among the moderating team of: 11 members (if we exclude admin) the ratio is 1 moderator for every 81.6 members and 91.8 postss In my opinion these ratios should be touching 200. 5) To avoid having more controllers than needed over limited no of posts and threads and members. Moderating team acitivty: 2 members didnt even visit the site since last 1 month and the other 2 since last 2 months (pathetic) 6) Check and balance on the performance of moderators. Setting a time limit after which the moderator will lose the right to be a moderator. Voting for moderators should be done after some time in SEFP. Male/Female participation is: 99% : 1% . Additionally all that 1% posts from females is only from 1 member out of 8.. 7) Not an important issue as this is STRUCTURAL engineering not the ARCHITECTURE or INTERIOR DESIGNING but still a surprising fact. I wish if we can make it 2%???? 93% of total members have made NO POST at all! Zero Post .....0.0...93% Inactive users.....Lets put a min limit for a member to stay a member for SEFP. If a new member doesnt post, say 5 times in 2 weeks...assume...the member should be deleted automatically! 8) It will ensure more activity and realistic ratios for SEFP activities based on active members not just on inactive members. We will have nice people instead of bunch of useless spammers and trial downloaders. The list can go on! like the percentage of different threads etc but for the time being we need more members and more posts. Lets get the results asap. I think we should do it in a more organized way. Giving moderators their responsibility to do something for forum and to set targets monthly or every other month lets say! and to achieve them. I would highly recommend the ADMIN to take this suggestion seriously!

EID Mubarak (belated) from me too!

conservatively! yes you can design walls as cantilever, but this is not the case always. For example when the walls are much higher like 5 or 6m. Designing it as cantilever will not be judged based on economy when it can be designed as propped cantilever... Basically if your top slab is not monolithic with your walls as in precast panels placed on the walls for easy removal (due to cleaning, or maintenance) then ofcourse your walls are cantilever. But if your slab is monolithic solid concrete slab you must design it propped cantilever or fixed fixed based on the stiffness of slabs to walls. Additionally, water tank walls are often governed by crack width criteria (serviceability) not due to strength design.